You know I appreciate this guys, been reading this since page one and been shooting/building Ar's since 1991. Interesting about the pressure it can handle being determined by what the Bolt can handle while Cycling. So is that saying with an adjustable Gas Block/ Gas Key/ of even an A5 Buffer system with H1-H4 Buffers to slow the Bolt down it can be loaded hotter and still be within the realm of acceptable due to it not stressing the Bolt ? (Until the Brass itself starts showing pressure of course.)
We may be thinking/saying the same thing, but for clarity I prefer to explain it this way:
There is a limit to what the AR15 bolt can handle, not accounting for other influences that decrease that limit. This base upper limit is generally somewhat less than a bolt action, but IMO somewhat closer than many think. Consider this the upper limit that the bolt itself can handle if the rest of the rifle is set up perfectly.
Common influences that decrease bolt capacity are:
- Uneven lug contact
- Early unlocking (under pressure)
Most "off the shelf" or DIY AR15 rifles have one or often both of these issues, and that significantly degrades the capacity of the bolt; IMO this is a big factor in the lower pressure "AR15 loads" published in some load data. Also note that early unlocking adds stress to the extractor as well and can result in failures there, regardless whether it's a Grendel or x39 extractor.
Any time you add load, you can expect life to decrease, but in my own experience I've seen that the combination of a good chamber (no excessive bevel to damage brass), even bolt lug contact, and careful gas tuning can run at or close to published bolt gun data without issues. Hornady's published bolt gun load for the 105gr BTHP and Lever is basically what I settled on several years before Hornady published that, and I use it as the primary load in all three of my 243 LBC AR15 rifles. Of course a caveat to this is that I'm not expecting a bolt in this cartridge to last 30K-50K rounds; that just doesn't really match my application or needs.
edit for clarity - the three 243 LBC rifles mentioned above are all AR15 (no bolt guns) with trued and shimmed or thermo-fit receivers and careful gas system sizing and tuning. Gas system lengths are rifle for the 24" and 19", and carbine for the 12.5".