Blowback vs Recoil operated: A tutorial

John Travis

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Over the years I've spent on the gun boards, I've seen many a discussion/debate/flame war over straight blowback vs recoil operated designs, mostly centered around how "different" they are.

But, are they...really...that much different?

First, let's take a look at straight blowback operation from a physics standpoint...specifically Newton's 3rd Law of action and reaction and how it relates.

Bang! Expanding gases and pressure accelerate the bullet. Assuming a condition of zero headspace, the slide is accelerated in the opposite direction at the same instant...because force forward is force backward. As long as there are two interacting objects and two forces, you have an action-reaction system. In other words, the straight blowback is also recoil operated.
For this reason, I like to think of the straight blowback as Unlocked Breech Recoil Operated.

Now, for the other one.

Bang! Gases accelerate the bullet, and assuming the same condition of zero headspace, the slide starts to accelerate in the opposite direction at the same instant. So far, the two are exactly the same in operation. The difference lies in the method used to delay the opening of the breech until pressures have fallen to a safe level. At this point, please note that "Safe Pressure Level" doesn't necessarily mean atmospheric. It means the level below which the case won't blow out.

The unlocked breech uses slide mass and spring tension to achieve this delay. The locked breech example uses a mechanical connection between slide and barrel with the slide pulling the barrel backward with it for a brief instant, giving the bullet time to exit and pressures to fall off...usually to atmospheric levels...or so close that it can be called.

The main delaying force in the locked breech example isn't the slide's mass nor the spring, nor even the barrel staying with the slide for the 1/10th inch of travel needed to give the bullet time to exit. The strongest delaying force is the bullet itself as it moves through the barrel on its way to freedom. Though all those things play a part, the bullet's influence outweighs all of them combined.

So, both are recoil operated...and both are blown back...and both slides continue their journey rearward on momentum generated by the initial, brief period of acceleration that ends with bullet exit.
 
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While the bullet is in the barrel being pushed downrange by expanding gasses it is itself grabbing the rifling in the barrel trying to drag it in that direction.
 
Can you explain this statement?
Several years ago, I was having this conversation with a guy who was able to wrap his head around it, and we got curious as to just how much frictional force/resistance existed between the bullet and the barrel, so we got a bathroom scale, an unfired 230 grain bullet...round nose...and a rod and proceeded to find out.

Starting the bullet into the rifling required 103 pounds. Keeping it moving...94 pounds.

Going on the understanding that whatever resistance the barrel offers to the bullet, the bullet also offers to the barrel, and...

Friction doesn't dissipate with speed, and...

Because the barrel is being hauled backward by the slide at the same time that the bullet is being driven forward...

Whatever resists the barrel's rearward movement, resists the slide's rearward movement, and...

I can prove by demonstration that I can cycle the slide manually as quickly as it moves in normal operation, therefore...

The bullet's influence outweighs all other resistive/delaying forces combined.

And, that's why I often refer to it as "Delayed Blowback" action...and it's fun to see the confused looks that I get from that description.

It also explains why a locked breech pistol can be fired without a recoil spring without ill effects.
 
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While the bullet is in the barrel being pushed downrange by expanding gasses it is itself grabbing the rifling in the barrel trying to drag it in that direction.
Bingo.

Back in 2007, a custom pistolsmith from Monroe, Louisiana came to stay with me for a few days while he took an engraving class at Montgomery Community College. We talked a lot about pistols and pistol building at length, and this came up. He got it.

When he returned to Monroe, he visited with Jim Clark, Jr. He'd apprenticed under Jim's father, who was an early name in the custom pistol industry and had pioneered several innovations with Bullseye pistols. He ran these points by Jim, Jr who was at the bench, working on a project.

He listened, nodding...mm hm...mm hmm...and got quiet. Then, he stopped working, laid his file down, and said, verbatim:

"I'll be damned. That's how it works. I never thought about it, but that's exactly how it works. I'll be damned!"
 
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When I applied to teach at SDI as a Faculty, they had each new hire write an essay on the four types of firearm operation and such. Here is the paper I wrote and it directly speaks to this topic.

Blowback-Operated
  • Blowback-operating system uses step five (Obturation) of the nine steps of semi-automatic firearm operation to cycle the firearm. Once the gun is fired, the internal ballistic process creates gas pressure directed to both barrel ends (breach and crown), causing the bullet and the bolt/slide to move simultaneously. Due to the mass and velocity difference, the bullet moves first and faster due to its lighter weight (mass). Once the bullet exits the barrel, the pressures of gas drop drastically, allowing the cartridge to contract. This shrinking causes the gas seal to be broken, and the bolt/slide is now released to start its rearward travel and complete the cycle of operation. The main difference between a blowback and a gas-operated system is the gas pressure on a blowback system is directed to the bolt face and not to a piston or gas chamber within a bolt carrier.

  • Tradeoffs: No physical locking breach; blowback-operated firearms work best with low-power cartridges, not higher-powered cartridges, due to mass balance. All gas pressure is focused onto the bolt face.

Recoil-Operated
  • In general, we have two recoil-operated designs, short and long. With both systems, the barrel and the bolt are locked together. Once step five (Obturation) begins, the reward pressure pushes against the locked bolt-barrel combo. Both parts travel rearward for a measured distance and remain locked. The bolt unlocks from the barrel's breach at the end of the travel distance. The bolt continues to retract and is accomplishing more steps in the cycle of operation. The difference between short and long recoil-operated systems is the distance traveled. Typically, short travels for about half the cartridge length, while long recoil-operated travels the entire length.

  • Tradeoffs: Recoil-Operated systems are not as accurate as other operating systems due to the movement of the barrel. The short recoil-operated system is light and has few parts, perfect for lower-powered cartridges and fast-firing systems. Long recoil-operated systems are complex and generate stiff felt recoil to the shooter.

Gas-Operated
  • The two primary gas-operated systems are direct impingement and piston, both of which work with the same mechanical principles. First, a gas port is drilled into the barrel and bleeds off gas into a gas block. Next, the gas block directs the gas to a gas tube or a chamber. In the next step, be it a piston or direct impingement, the bolt carrier travel is activated by the gas pushing the bolt carrier that allows the bolt to unlock from the breach. The main difference between a blowback and a gas-operated system is the gas pressure places force on a piston or gas chamber that operates the bolt carrier, and no gas impacts the bolt face.

  • Tradeoffs: Gas-Operated systems have more parts than blowback and recoil-operated systems. Because of this, not many small firearms use the gas-operated design. Heat is also a factor. With the re-routing of expended gas, heat is generated faster than the other operating systems. Lastly, the typical locking system in a gas-used firearm is more complex than other operating systems.

The Cycle of Operation of semi-automatic firearms is done in nine steps. First, each operating system is uniquely designed; broadly, the below steps are the standard process.
  1. Feeding - Bolt strips the next round of ammunition out of the magazine.
  2. Chambering - The bolt pushes or controls the round of ammunition into the chamber.
  3. Locking - The operating system is locked upon the bolt, impacting the breach.
  4. Firing - Depressing the trigger releases the hammer, the hammer strikes the firing pin, the firing pin impacts the ammunition's primer, and the primer ignites the gunpowder.
  5. Obturation - The bullet is forced down the barrel and creates a seal while the rifling lands and grooves impart spin.
  6. Unlocking - The firearm’s bolt-to-breach mechanism unlocks.
  7. Cocking - the unlocked bolt/carrier travels rearward. The cocking ramp on the bolt/carrier cocks the hammer and re-sets the trigger mechanism.
  8. Extraction - the rearward travel of the bolt allows the extractor to pull on the ammunition case head and extract the case from the chamber
  9. Ejecting - As the bolt approaches the end of its travel path, the expended casing aligns with the ejection port, and the ejector can now impart force to eject the case.
Based on the design of the firearm/operational system, it can incorporate some steps in more than one sequence. But all nine of these steps are still being accomplished as individual operations.

John
 
Several years ago, I was having this conversation with a guy who was able to wrap his head around it, and we got curious as to just how much frictional force/resistance existed between the bullet and the barrel, so we got a bathroom scale, an unfired 230 grain bullet...round nose...and a rod and proceeded to find out.

Starting the bullet into the rifling required 103 pounds. Keeping it moving...94 pounds.

Going on the understanding that whatever resistance the barrel offers to the bullet, the bullet also offers to the barrel, and...

Friction doesn't dissipate with speed, and...

Because the barrel is being hauled backward by the slide at the same time that the bullet is being driven forward...

Whatever resists the barrel's rearward movement, resists the slide's rearward movement, and...

I can prove by demonstration that I can cycle the slide manually as quickly as it moves in normal operation, therefore...

The bullet's influence outweighs all other resistive/delaying forces combined.

And, that's why I often refer to it as "Delayed Blowback" action...and it's fun to see the confused looks that I get from that description.

It also explains why a locked breech pistol can be fired without a recoil spring without ill effects.
One thing though is that the same forces exist in a direct blowback gun. So the answer is more nuanced and just "the bullets influence delays the opening". I mean I agree with you it does. But it becomes confusing when you start trying to grasp what the difference is between blowback and recoil operated.
 
Sorry Jon, I wasn't really clear...I meant the Importance of the mention of the Extractor in it's Initial pickup within the feeding cycle. When it leaves the mag and is picked up by the extractor for it's journey forward into the Locking portion.
Fully agree with you.

I always struggle with push feed (AR-15, Rem 700) and control round feed (M-14, Winchester 70) as to what's the best design.

Even semi-auto pistols are push feed until the ammunition is in alignment to be chambered.

It's a interesting topic for one to ponder on when thinking of malfunctions too, because that's a stoppage in one of the 9-steps
 
One thing though is that the same forces exist in a direct blowback gun. So the answer is more nuanced and just "the bullets influence delays the opening". I mean I agree with you it does. But it becomes confusing when you start trying to grasp what the difference is between blowback and recoil operated.
Well, not really. It's a simple matter of resistance within a closed system. Whatever happens to one end...of a closed system...happens to the other end.
But it becomes confusing when you start trying to grasp what the difference is between blowback and recoil operated.
I don't know why it would be confusing. With the unlocked/direct blowback system, the slide moves independently of the barrel. It becomes a simple action and reaction event...like pushing a heavy medicine ball away from your body and releasing it the instant your arms are straight...whereas, if you gripped the ball, when your arms straightened, the mass of the ball would pull you forward with it.

With the locked breech operation, the bullet is the only "other" outside force affecting the movement of the slide.
 
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Well, not really. It's a simple matter of resistance within a closed system. Whatever happens to one end...of a closed system...happens to the other end.

I don't know why it would be confusing. With the unlocked/direct blowback system, the slide moves independently of the barrel. It becomes a simple action and reaction event...like pushing a heavy medicine ball away from your body and releasing it the instant your arms are straight...whereas, if you gripped the ball, when your arms straightened, the mass of the ball would pull you forward with it.
Unlocked / direct blowback is a system of weights and springs.

The weights are the weight of the bolt, bolt carrier in relation to the bullet weight.

Springs with elastic potential energy and the restoring force.

John
 
Springs with elastic potential energy and the restoring force.
The spring's loading must also be considered because it plays a role.

I call your attention to our old friend and favorite whipping boy...the High Point.

A few people had dabbled with a straight blowback 9mm with little notable success, and to my knowledge nobody had ever attempted one in .45 Auto caliber.

Enter the High Point. In the two mentioned calibers, the slide alone is an ounce heavier than an unloaded LW Commander, and the spring...particularly in the .45 caliber pistol...is beyond belief. By a conservative estimate, I'd say that at full slide travel, it exerts 35 pounds of force. Manually locking the slide open is a struggle for me, and I have large, fairly strong hands.

So, as the man said...Nothing means everything, but everything means something.
 
@John Travis,

The below is not directly posted to you, but to the readers in general.

The mechanical terms for the energy to compress a spring are potential energy and elastic potential energy.

When a spring is compressed, it stores potential energy, which is the energy stored in an object due to its position or configuration. This potential energy is also referred to as elastic potential energy because it is associated with the distortion or deformation of the spring.

When the compressed spring is released, it exerts a force, pushing or extending itself back to its original shape. The mechanical term for this force is the restoring force. The restoring force is the force exerted by the spring to return to its equilibrium position and is a result of the stored elastic potential energy being released.

John
 
After thinking about this, I don't know that its accurate to say the bullet plays the "most important" role in delaying the opening of the breach in a recoil operated firearm. Its better to say that the movement of the bullet is proportional and opposite to the movement of the slide.

Its not the bullet gripping the rifling that prevents the opening of the breach. A gun with no rifling would still operate the same.

What prevents the opening of the breach is simply that, without bullet movement, there would be net 0 force operating on the system. Merely having pressure in a container does not cause force or movement. The pressure has to perform some work inorder to cause movement. The pressure performs work on the bullet, the bullet moves, newtons 3rd law, the slide must move proportionally opposite.
 
Over the years I've spent on the gun boards, I've seen many a discussion/debate/flame war over straight blowback vs recoil operated designs, mostly centered around how "different" they are.

But, are they...really...that much different?
Many many moons ago I owned and briefly pocket carried an NAA Guardian in .380. It sort of resembled a Seecamp on steriods. Bigger, heavier....

It was blowback operated. It was also UNPLEASANT to shoot. I've owned and shot harshly recoiling big bore revolvers and never had one hurt like that gun did. After a magazine or two I was done. My palm was sore and I was often left with a bleeding trigger finger from the trigger guard whacking it.

I sold it and picked up a Ruger LCP when they first came out. I was expecting it to be more unpleasant due to the significant difference in weight. It was polymer framed where the NAA was all steel. I forget the figures but the NAA was around double the weight of the LCP if memory serves. I was pleasantly surprised that the LCP was MUCH easier to shoot. So much so that I've probably put close to 2k rounds through that gun in the 14 years I've owned it. It's on it's second cerakote application, third trigger return spring, and second recoil spring.

I've always attributed the difference in felt recoil to be the NAA Guardian's blowback operation in contrast to the LCP's locked breech. Am I wrong there?
 
Very interesting discussion!


Side note, slightly off topic, on Jim Clark (amazing gunsmith and shooter, worth some reading about):
His daughter is Kay Miculek, Jerrys wife. Kay is also a multi-time IPSC and USPSA Champion. Lena's grandad.



Bingo.

Back in 2007, a custom pistolsmith from Momroe, Louisiana came to stay with me for a few days while he took an engraving class at Montgomery Community College. We talked a lot about pistols and pistol building at length, and this came up. He got it.

When he returned to Monroe, he visited with Jim Clark, Jr. He'd apprenticed under Jim, father, who was an early name in the custom pistol industry and had pioneered several innovations with Bullseye pistols. He ran these points by Jim, Jr who was at the bench, working on a project.

He listened, nodding...mm hm...mm hmm...and got quiet. Then, he stopped working, laid his file down, and said, verbatim:

"I'll be damned. That's how it works. I never thought about it, but that's exactly how it works. I'll be damned!"
 
After thinking about this, I don't know that its accurate to say the bullet plays the "most important" role in delaying the opening of the breach in a recoil operated firearm.
The bullet offers the greatest resistance to the barrel's rearward movement of all the outside forces acting on it, and whatever resists the barrel resists the slide.
What prevents the opening of the breach is simply that, without bullet movement, there would be net 0 force operating on the system.
If you don't believe that force without bullet movement would result in no movement, block the bullet's path in an unlocked breech/blowback system and fire the gun and get back to me. I suggetst triggering it from a safe distance, because when the breech opens under full pressure, there will be shards of brass exiting at high speeds.

Because, the slide moves independently of the barrel.
A gun with no rifling would still operate the same.
As long as a frictional resistance between bullet and barrel exists, the bullet would act to delay the slide in a locked breech system. It may not offer as much resistance with a smooth bore, but it would be there.
One thing though is that the same forces exist in a direct blowback gun.
In a direct blowback gun, the slide moves independently of the barrel, and thus the frictional forces between barrel and bullet have no influence on the slide. When the barrel is mechanically connected to the slide, and must be carried rearward by the slide...they do.
 
The bullet offers the greatest resistance to the barrel's rearward movement of all the outside forces acting on it, and whatever resists the barrel resists the slide.
No. In fact, I contend that a barrel without rifling would cause the slide to move back less. Probably you would have slightly less recoil and possibly malfunction.


If you don't believe that force without bullet movement would result in no movement, block the bullet's path in an unlocked breech/blowback system and fire the gun and get back to me. I suggetst triggering it from a safe distance, because when the breech opens under full pressure, there will be shards of brass exiting at high speeds.
Well, nothing is gonna work if your barrel can't handle the pressure you are working with.

Here: assume the barrel and breach can handle infinite pressure and seal perfectly. Assume that when you pull the trigger the bullet gets stuck almost immediately.

There will be almost no recoil and the slide will not come back.

As long as a frictional resistance between bullet and barrel exists, the bullet would act to delay the slide in a locked breech system. It may not offer as much resistance with a smooth bore, but it would be there.
Not true. There could be zero friction and it would still work. As long as the bullet seals the barrel (and frankly even if the bullet does not seal perfectly) a recoil system will work fine. The mass of the bullet and the slide system are what matters. Friction is secondary.


In a direct blowback gun, the slide moves independently of the barrel, and thus the frictional forces between barrel and bullet have no influence on the slide. When the barrel is mechanically connected to the slide, and must be carried rearward by the slide...they do.
I would also argue this is not true but its kindof a tangential issue so I will leave it.
 
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As long as a frictional resistance between bullet and barrel exists, the bullet would act to delay the slide in a locked breech system. It may not offer as much resistance with a smooth bore, but it would be there.

Complete nonsense! The only thing governing the unlocking process is the linkage Kinematics.
So many obvious flaws in this statement it hard to know where to begin. imagine if you will a 1911 with the barrel link missing. What would happen? Slide would not open at all. Why? The link physically moves the barrel lugs out of alignment with the grooves in the slide. What does that? The rearward movement of the slide. What causes that? Conservation of energy-you know equal and opposite. The energy imparted on the bullet is balanced out by the energy moving the slide back.
 
No. In fact, I contend that a barrel without rifling would cause the slide to move back less.
There will be almost no recoil and the slide will not come back.
And, you'd be wrong on both counts.

I addressed this very question some years ago when another guy made pretty much the same observations as you just did. I think he wondered if a bullet fired in an oversized bore would even allow enough pressure to develop to make it to a 25 yard target.

So...being the mad scientist that I am...I drilled the rifling out of an old, worn out barrel, opening it up to a couple thousandths over bullet diameter...and fired the gun. Not only did the slide cycle, the bullet was driven deep enough into a Maple tree that you couldn't see the base.

Some years later, I had a brainfart while playing with my single action revolvers. I inadvertently loaded a .41 Magnum round...mid range loading...into a .44 Special Cimarron revolver.

The gun recoiled normally for .44 Special. The bullet knocked over 6 falling plates in a row at 15 yards...and I didn't realize that anything was wrong until I punched out the empties and saw the bulges.

We can have beliefs and form all sorts of theories...but we don't know until we know.
 
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I addressed this very question some years ago when another guy made pretty much the same observations as you just did. I think he wondered if a bullet fired in an oversized bore would even allow enough pressure to develop to make it to a 25 yard target.

So...being the mad scientist that I am...I drilled the rifling out of an old, worn out barrel, opening it up to a couple thousandths over bullet diameter...and fired the gun. Not only did the slide cycle, the bullet was driven deep enough into a Maple tree that you couldn't see the base.
Ok but what does that prove? You'd need to measure how much the slide moves before and after you did the modification. I still say it would move less. I didnt say it would move none (this is the barrel without rifling, not stuck bullet)


Some years later, I had a brainfart while playing with my single action revolvers. I inadvertently loaded a .41 Magnum round...mid range loading...into a .44 Special Cimarron revolver.

The gun recoiled normally for .44 Special. The bullet knocked over 6 falling plates in a row at 15 yards...and I didn't realize that anything was wrong until I punched out the empties and saw the bulges.

Yea, not surprised at all.

I thought we were talking about semi-auto recoil-operated firearms though?

And, you'd be wrong on both counts.
Which statement addresses the "2nd" comment?
There will be almost no recoil and the slide will not come back.
 
I have to wonder if he's thought about shotguns. They have smooth bores, and they seem to recoil just fine.
Where did I say that a smoothbore doesn't recoil?

I'm trying to disprove that somehow the "bullet gripping the rifling" is the main thing that makes a short recoil-operated firearm work.
 
I am still chewing on the "The bullet pulls the gun forward" bit...as the bullet itself wants to stay still. It has zero potential energy in and of itself. The only energy applied to it is the pressure from the expanding gasses attempting to push it forward. So, in the example of pushing a round through a bore the pounds of pressure to push it forward is actually being pushed backwards through resistance.

Maybe I am misunderstanding that bit. I could see it more if there was something pulling it from the muzzle end, but until it exits the barrel it is being pushed the whole time. I would think the resistance would all be directed backwards.
 
I am still chewing on the "The bullet pulls the gun forward" bit...as the bullet itself wants to stay still. It has zero potential energy in and of itself. The only energy applied to it is the pressure from the expanding gasses attempting to push it forward. So, in the example of pushing a round through a bore the pounds of pressure to push it forward is actually being pushed backwards through resistance.

Maybe I am misunderstanding that bit. I could see it more if there was something pulling it from the muzzle end, but until it exits the barrel it is being pushed the whole time. I would think the resistance would all be directed backwards.

Nevermind...I think I am getting it...its like holding onto an umbrella in a windstorm, the umbrella wants to pull away because of the wind behind it. The gasses expanding is basically the wind.
 
I am still chewing on the "The bullet pulls the gun forward" bit...as the bullet itself wants to stay still. It has zero potential energy in and of itself. The only energy applied to it is the pressure from the expanding gasses attempting to push it forward. So, in the example of pushing a round through a bore the pounds of pressure to push it forward is actually being pushed backwards through resistance.

Maybe I am misunderstanding that bit. I could see it more if there was something pulling it from the muzzle end, but until it exits the barrel it is being pushed the whole time. I would think the resistance would all be directed backwards.
The bullet is in fwd motion and through friction is attempting to take the barrel with it. At least that was my takeaway
 
The bullet is in fwd motion and through friction is attempting to take the barrel with it. At least that was my takeaway
Yeah, I think I am getting it. Takin'some chewin'with ma mind meat.
 
Where did I say that a smoothbore doesn't recoil?
"There will be almost no recoil and the slide will not come back."

I'm trying to disprove that somehow the "bullet gripping the rifling" is the main thing that makes a short recoil-operated firearm work.
I never said it was the main thing that makes it work. I said it was the largest restive force in play. It outweighs all others...springs, hammer mass, friction...combined.

You can easily prove that to yourself if you can apply the principle of explosive movement. You can easily hand cycle the slide as fast as it moves in operation.

But, it you attach a cable to a bullet by drilling through...drop it into the chamber...attach the cable to a wall...and try to pull the barrel backward, you'll have to apply 103 pounds of force to start it, and 94 to keep it moving until you pull the barrel off the bullet.

And, whatever resists...delays...the barrel's rearward movement resists/delays the slide's rearward movement. It's very simple.
 
I could see it more if there was something pulling it from the muzzle end, but until it exits the barrel it is being pushed the whole time.
Yes, and the whole time, force backward is pushing the slide. The slide recoils and grabs the barrel by the upper lugs and hauls it backward with it AGAINST the resistance offered to the barrel by the bullet.
I would think the resistance would all be directed backwards.
The resistance imposed by bullet to barrel friction is on the bullet and barrel. Whatever forces the barrel offers to the bullet, the bullet offers to the barrel...and whatever forces resist the barrel resist the slide.
 
The resistance imposed by bullet to barrel friction is on the bullet and barrel. Whatever forces the barrel offers to the bullet, the bullet offers to the barrel...and whatever forces resist the barrel resist the slide.
The result is friction which will increase the barrel temperature but in the end the energy equations will equal out.
Imagine a hydraulic cylinder- does the piston “restrain” the body of the cylinder? Does a champagne cork “restrain” the bottle from recoiling back into your hand?
 
Guys,
In my list of the steps of cycle is #5

Obturation - The bullet is forced down the barrel and creates a seal while the rifling lands and grooves impart spin.

Google

Obturation
&
Obturation in ballistics

And come back and let's talk a bit
 
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Obturation - The bullet is forced down the barrel and creates a seal while the rifling lands and grooves impart spin.
And obturation/seal isn't immediate, nor is it ever perfect, evidenced by the gases escaping ahead of the bullet.

Lead bullets obturate earlier and seal better than jacketed bullets. That's why a given powder charge produce higher velocities with lead bullets than with jacketed bullets of the same shape and mass. Not...as many believe...due to reduced friction. Though friction is part of the equation, earlier and better sealing is the main factor. The simple reason is that less gas/pressure is wasted, thus more is available to accelerate the bullet.
 
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