Barrel Strain............
What in the world is "Barrel Strain".............. Barrel strain is defined as the amount of strain, stress, or stretch of a particular bullet as it passes down the barrel. For most common purposes it has very little meaning to most of us. The biggest factor that barrel strain can have is direct pressure with a bullet. An oversized bullet that is tight in the barrel will cause more pressure, common sense. But to others, barrel strain can have other meanings, and more far reaching factors.
Double rifle shooters are very concerned about barrel strain, and most do not even understand or know the term itself. Double rifles present a different problem. Barrels are soldered together and it is an extremely tedious and expensive process to get two barrels to shoot together, and then lock them down after you achieve this. Any bullet going down a barrel stretches and expands the barrel the entire time it is going down. The more that stretches those barrels, the more strain put on the solder that is holding it together. When modern CNC Monolithic bullets came on the market, double rifle shooters blamed these bullets for the very infrequent occasions that the barrels would separate. Soon myths were perpetuated that you could not shoot monolithic bullets in a double rifle, it would cause damage to the barrels, separate barrels and even cause OSR. OSR? Over Stressed Rifling. Many years ago, a double rifle shooter noticed strips on his barrels when held up to the light in a certain way. The stripping matched the rifling in the barrel, and from this a total MYTH was started that monolithic bullets would press the rifling through the steel of the barrel and show on the outside!
OSR is a total myth and rumor, and is still perpetuated by ignorance even today. In the next part of this discussion in Pressure Works, we will address OSR, and bust the myth completely.
Barrel strain is very important however with these double rifle guys. I am not a double rifle type of chap, I don't care for them in the least, they are a real pain in the ass. The crowd that I used to run with however are sometimes nuts about the damn things, and it is all about "Nostalgia" of hunting in Africa. They try to justify all the reasons for a double rifle, but in the end its all about Nostalgia and hunting like they did in the old days.... Blah Blah Blah............To me, double rifles are far inferior, and far less versatile than your Winchester M70 big bore........ You just can't shoot a variety of ammo or bullets in one, its normally regulated at 50 yards for 1-2 loads and bullets, and that is all you get.
I have a very good friend from NC, Sam Rose. Sam has been a test partner and friend for nearly 15 years now. We have tested everything you can imagine, Sam was instrumental in working with me to develop the CEB and North Fork bullets we all shoot today, we have tested new cartridges, bullets, load data and all manner of things, and one of the most important works in which we have embarked upon is the double rifle work we have done together. Sam has the double rifle bug, and he loves them. So all the double rifle work we did was instigated by Sam. I love discovery, and firearms science, so it did not matter to me what we were working on, it was very exciting and a great learning experience, and not only that, what we did was decisive, proven, and had never been done before, or since........... Much of the work we did on this stands today as accepted science and used by various manufacturers.
Sam and I first met we were working with different bullets, mostly solids for big bores. One day he wanted to test one of his mighty 577 NE loads, 750 gr Round Nose Woodleigh. Sam left very disappointed that day, as the bullet failed miserably in penetration. In fact, it turned sideways and was only able to penetrate 14 inches straight. This set him on fire, and from that point forward we were concentrating efforts to design a better solid and making them work in double rifles as well. Long story short, we developed the right size meplat and nose profile, and now it was time to test this in double rifles...........
We started working with a 470 NE to begin with, set up a gage to do Chamber pressure. While doing this, we decided we could put a stain gage 4 inches back from the muzzle, and measure how much the barrel expanded at that point by the passage of a given bullet..................This would measure what we came to define as Barrel Strain.
Our first work in this area we used full power loads. At 4 inches back from the muzzle we were still getting some effects from the chamber pressures. In later tests, in other calibers, we reduced the loads down to where chamber pressure did not make it to that point, and all we received was the passage of the bullet at the strain gage. It was a learning process............
In the world of Double Rifles, Woodleigh used to rule. Everyone considered the Woodleigh Soft Point a safe double rifle bullet, and because of its design, Round Nose, it held nostalgic value to those guys as it resembled the old original Kynoch bullets. In all tests, we considered the Woodleigh Soft as the "BenchMark Bullet"............... We test the Woodleigh and all strains below the Woodleigh Soft would be considered SAFE, and all strains ABOVE the Woodleigh Soft considered a danger or cause for possible damage to the barrels.
There are a few ways to REDUCE barrel strain. The common one used by bullet manufacturers is by reducing the diameter of the bullet. The other is reducing the bearing surface that is actually engraved by the rifling. Materials also can be a factor as well.
I ranked the bullets tested from LOW to HIGH....... here is the work done on the 470 Nitro, reminded that these were full power loads, all at 106/IMR 4831 regardless of bullet. Also note that in this data bullet diameter was not recorded, we were still learning.
Sam and I concentrated our efforts at reducing barrel strain with the bands, and by reducing bearing surface. While keeping the bullets at caliber diameter. Here is the data, with measured bullets ranked from low to high. Keeping the Woodleigh Soft as the BenchMark to go by. As you note, Woodleigh and a few others reduced diameter of their bullets.
The CEB and Or #13 BBW bullets are the bullets that Sam and I had direct design of the bands, some of these ere only 2 band bullets, others 4 band.............Of course the BBW #13 Nose profile and meplat size are the same as todays CEB Safari Solid line...........
The lower the barrel strain the better. And in all the 470 Nitro tests we were still getting some effects from full power loads and chamber pressures, as you will see in later tests with 458 and 510 calibers, in which we reduced the load to eliminate that factor.......... In the end, the various ranking of different bullets was much the same as what you see above............just lower numbers..........
To be Continued.........................