by Bmt85 » Sat Sep 23, 2017 8:47 am
You are not wrong in your thinking, bolt thrust is a legitimate concern. However, there are a couple things to look at. If I remember correctly, bolt thrust is usually calculated with no concern to case geometry. 5.56 has a pretty good body taper and is shouldered. Compare a 7.62x39 to 5.56. 7.62x39 is again shouldered, has a bigger case head, and also has a lot more body taper. It runs at lower pressure, so calculated bolt thrust is a little more than 5.56, but they break more bolts (good ones not crap) than 5.56. Then look at 6.8 SPC II, bigger case head than 5.56, similar body shape to 5.56, can be ran up to the same pressure as 5.56, yet good bolts hold up. Now taking a 450 case that is straight walled and has less body taper leads to less bolt thrust. If you keep your chamber clean and dry, and shoot loads that completely seal to the chamber, you will have less bolt thrust, to a point anyway. So to say that the bolt thrust calculation is an absolute, is wrong.
Here's another example, I've had a couple factory rounds that had some serious leakage between the brass and chamber. They actually showed very slight imprinting. Those rounds probably equalled the equivalent of 5.56 bolt thrust. Now my 275 reloads using Hornady new brass are way over what would be considered max book loads, yet, my mid level loads show no signs of over pressure. My max loads show very slight imprinting. So if you clean and dry your chamber, and keep a close eye on everything, you can take it above rated pressure.