Bullet RPM?

Sasquatch

Sasquatch

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Should I care about it? I've got nearly a metric ton of 125 grain sinterfire bullets in 308 (ok, 1500 of them).

For my 300 bolt, I've tried different powders (H110, AA1680, AA5744, 322) and can't get decent groups. I've tried 'high' velocities (~2100) and low (~1400) and when I shoot 5, I get a 2" group of a cluster of 3, and two flyers.

Since I've tried powders, I'm trying to see what other levers I can modify, and I'm wondering if I go for RPM. Maybe these things are really RPM sensitive? the only published loads I could find were for 308 Win. Which had speeds of ~2800-3000, with a barrel twist of 1:12.

So if it's RPM, the formula to calculate it is V(muzzle) * 720 / Bullet Twist in Inches. Looking at their established loads, the bullet RPM is 175,000. Doing the math, my 1:7 300BO rifle would need to be going at 1700 FPS.

I don't know if I'm just grasping at straws, because some of my test loads above had speeds near that velocity and I didn't get any amazing groups.

Does RPM matter?
 
Have you played with seating depth?
 
RPM does matter…
It is weight sensitive.
Heavier will need more stabilization
What ogive are you running?
 
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Tim said:
Have you played with seating depth?
Click to expand...
A little, I've been using Gordon's reloading tool to help predict loads, and I'm usually going for filling the case up, which I can do by modifying depth.

Suggestions as to go longer or shorter?
 
Sasquatch said:
Should I care about it? I've got nearly a metric ton of 125 grain sinterfire bullets in 308 (ok, 1500 of them).

For my 300 bolt, I've tried different powders (H110, AA1680, AA5744, 322) and can't get decent groups. I've tried 'high' velocities (~2100) and low (~1400) and when I shoot 5, I get a 2" group of a cluster of 3, and two flyers.

Since I've tried powders, I'm trying to see what other levers I can modify, and I'm wondering if I go for RPM. Maybe these things are really RPM sensitive? the only published loads I could find were for 308 Win. Which had speeds of ~2800-3000, with a barrel twist of 1:12.

So if it's RPM, the formula to calculate it is V(muzzle) * 720 / Bullet Twist in Inches. Looking at their established loads, the bullet RPM is 175,000. Doing the math, my 1:7 300BO rifle would need to be going at 1700 FPS.

I don't know if I'm just grasping at straws, because some of my test loads above had speeds near that velocity and I didn't get any amazing groups.

Does RPM matter?
Click to expand...

yes but in this instance, you barrel just plain might not like those projectiles
 
Spartan01 said:
RPM does matter…
It is weight sensitive.
Heavier will need more stabilization
What ogive are you running?
Click to expand...
I believe technically it's length not weight/mass that directly relates to requiring a minimum RPM for stabilization. But for a given caliber, weight directly relates to length, so...

To the OP, I have gone round and round (see what at I did there? :)) trying different loads in my Ruger American 300 BLK bolt to find an accurate subsonic loading. Problem is I run out of a particular projectile about the time I think I'm zeroing in on it, lol. But yes I think for some reason 300BLK is just not as inherently as accurate as 308 for example, but I couldn't tell you why.

I've been using Lil'gun powder because I hear is is good for this application, haven't tried any others.
 
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I wouldn’t set seating depth to fill the case, I’d seat 2thou back from the rifling and see what happens.
 
Flashpoint said:
I believe technically it's length not weight/mass that directly relates to requiring a minimum RPM for stabilization. But for a given caliber, weight directly relates to length, so...
Click to expand...
Well, actually, it is both.

The weight affects the gyroscopic force that stabilizes the bullet, the length affects the aerodynamic forces on the bullet.

You are correct about length directly related to weight, but only as long as the bullet makeup (density) remains the same. For a given material, there are only two ways to make the weight increase - increase diameter (not on option for a given caliber) or increase the length (or to a lesser extent, change the profile). However, when switching to a different material, such as an all copper bullet, then the length/weight ratio changes. An all copper bullet may be less stable than a cup-n-core bullet of the same weight.

@backwoodsshooter may be right - your barrel/gun just may not like that particular bullet.
 
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Many years ago I had a hunting rifle (Ruger 77 in 270) that was a tack driver straight out of the box. Didn't matter what I feed it. It would easily put a 3 shot group in a dime at 100 yards. Unfortunately it was stolen. I replaced it with another Ruger 77. Unfortunately Ruger had started farming out the making of their barrels. This rifle was lucky to put 3 rounds on the paper at 100 yards.

I talked to a friend who worked at a gun store and he gave me the specs for a load they had developed at the store for their rifles. It was cci 250 large magnum rifle primers with 55.5 grains of IMR4350 and 130 grain Nosler balistic tip 130 grain boat tails. I tried it but quickly found it was too much pressure. Bolt had to cycle after a shot and primer cratering. Plus it was still putting shots all over the paper. I started backing down the powder by .2 grains. Loaded 5 rounds each and tried it. Still all over the paper.

When I got down to 54.5 grains I could then put 3 rounds in a dime again. anything more even .2 grains and it was all over the paper. Point of the story is what works well in one rifle may not in another. Even as little a .2 grains can make all the difference.
 
@Tim I am now a believer. I watched Eric cortana's video about don't chase the lands, instead chase the jam. set five different depths starting at $20,000 and backing off and $5,000 increments and it was amazing the difference in grouping I finally have something that I haven't even perfected that groups at 1.5 MOA

The lengths ended up being more than a hundred thousands longer than what I had before
 
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Sasquatch said:
@Tim I am now a believer. I watched Eric cortana's video about don't chase the lands, instead chase the jam. set five different depths starting at $20,000 and backing off and $5,000 increments and it was amazing the difference in grouping I finally have something that I haven't even perfected that groups at 1.5 MOA

The lengths ended up being more than a hundred thousands longer than what I had before
Click to expand...
Seating depth was a HUGE factor in my precision load.
 
@Tim @Sasquatch

So I agree seating depth can be important but in a PRS rifle that gets 100 rounds at a time and used at multiple matches how do adjust the seating depth to compensate for bore erosion? I never bother with seating depth studies. I shoot bergers and they like some jump so I measure the distance to the lands with the Hornaday tool I have and then seat the bullets 15 thou off the lands and never look back. Always get 1/4" moa performance.

For instance I measured the distance to the lands on a new 6cm and on my 6cm same barrel manufacturer so they should be close from the factory. My barrel had 900 rounds through it and the new barrel had 200. The distance to the lands on my barrel was over 50 thou more than in the new barrel. So again are you adjusting your seating depth every hundred rounds? I guarantee you the throat is eroding 5 thou every hundred on a PRS rifle like a cm or a dasher. On a 308 or 556 not so much.
 
Hi @TSConver

Yes, for the measurements above I was doing COAL, which you're right- is not as good as ogive. I am going to use the Hornady bullet comparator going forward- and just got a precision seating die to help as well.

The reason I chose Erik's video is that he doesn't measure to the lands- you're right- the lands can and do erode. He measure to jam, which is where the bullet cannot physically go farther. Theoretically it should be close to the lands, but when I was using the tool, I was getting different results as to where that 'touch' point is. Jam (with decent neck tension, plus lube on the bullet) was far more repeatable. He goes onto say by the time the barrel wears enough that his jam has moved, it's time to throw it away. He also talks about over time seeing if you need to make it longer as the lands wear. It all made sense to me- and the proof to me was the above. I'm going to try some other lengths to make sure I am really in a node this weekend.

The result from this is that bullets I was going to throw away are now at least plinkers, and maybe more.
 
Sasquatch said:
Hi @TSConver

Yes, for the measurements above I was doing COAL, which you're right- is not as good as ogive. I am going to use the Hornady bullet comparator going forward- and just got a precision seating die to help as well.

The reason I chose Erik's video is that he doesn't measure to the lands- you're right- the lands can and do erode. He measure to jam, which is where the bullet cannot physically go farther. Theoretically it should be close to the lands, but when I was using the tool, I was getting different results as to where that 'touch' point is. Jam (with decent neck tension, plus lube on the bullet) was far more repeatable. He goes onto say by the time the barrel wears enough that his jam has moved, it's time to throw it away. He also talks about over time seeing if you need to make it longer as the lands wear. It all made sense to me- and the proof to me was the above. I'm going to try some other lengths to make sure I am really in a node this weekend.

The result from this is that bullets I was going to throw away are now at least plinkers, and maybe more.
Click to expand...


Will watch the video, but I like I said I have always just back it 15 thou off the lands and got 1/4" moa groups with just OCW powder charge method for finding the load.
 
Sasquatch said:
Hi @TSConver

Yes, for the measurements above I was doing COAL, which you're right- is not as good as ogive. I am going to use the Hornady bullet comparator going forward- and just got a precision seating die to help as well.

The reason I chose Erik's video is that he doesn't measure to the lands- you're right- the lands can and do erode. He measure to jam, which is where the bullet cannot physically go farther. Theoretically it should be close to the lands, but when I was using the tool, I was getting different results as to where that 'touch' point is. Jam (with decent neck tension, plus lube on the bullet) was far more repeatable. He goes onto say by the time the barrel wears enough that his jam has moved, it's time to throw it away. He also talks about over time seeing if you need to make it longer as the lands wear. It all made sense to me- and the proof to me was the above. I'm going to try some other lengths to make sure I am really in a node this weekend.

The result from this is that bullets I was going to throw away are now at least plinkers, and maybe more.
Click to expand...

Just watched it. I use the hornady chamber & cartridge measuring tool. Measure the distance until to the ogive hits the end of the chamber. Pull it back 15 thou and get good results. Not sure what he is saying, the jam is moving with each round fired. The lands are what causes the jam effect. Jam is just forcing the bullet past the end of the lands a couple of thou.
 
I'd worry about the length of the projectiles first. How long are the projectiles? Longer ones need more twist to stabilize, no matter what velocity.
 
Not really. You don't shoot a match and burn 10tho of throat out and have to re-test and rebuild your load after every match.

If you follow Cortinas method, or nearly any other, you will find a seating depth window that the bullet likes. Let's say that window is 6tho wide. (Think of that big sine wave he drew on the whiteboard in his examples) So a load from 2.220 to 2.226 BTO (random number as an example) that printed well on paper downstream. So knowing that throat will erode with every shot, whatever that value is, load to the shorter end of that window, so you erode into or through the window that the bullet likes to jump. So you effectively have 6 tho of jump tolerance. Adjust as needed when accuracy falls off. 400 rounds? Whatever that is. You'll see it when you confirm zero, and shoot a shitty string. Hopefully off the clock!

Same philosophy and principal in powder charging in finding a velocity node. For example, if you find a plateau and very similar velocity patters at 41.4, 41.5 and 4.6. you have a bit of leeway or forgiveness in not having to charge to the exact kernel of powder. Set it to 41.5 and go. Your load won't go out the window with a slightly under or over weight charge. By slightly, I mean a half of a tenth.

Typos? Yep. Sitting on my phone. This post probably has them. :)
 
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regarding RPM.... a few things play into the calculation, length being key to gyroscopic stability.
Berger's site has an excellent reference and calculator to look at bullet weight, size, and twist rate stabilization. Enjoy!

Personally, I've never been concerned with what actual RPM my bullet is spinning, but more so is it stable or not from the gun, twist rate, weight, etc. Whatever that RPM number is never mattered. Does it shoot? That's all I care about.

bergerbullets.com

Twist Rate Stability Calculator

Our newly updated Twist Rate Stability Calculator not only tells you what your SG is, but it also lets you know if you are reaching your optimized BC.
bergerbullets.com bergerbullets.com
 
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