I have never had headspace issues. I’ve only used .136” face depth bolts with the same lug lengths, with barrels from manufacturers that cut chambers for the standard. I test my chambers with a set of dummy cartridges I made, with 4 different bullet types. If the cartridges don’t chamber and unseat with ease, I don’t use a barrel until it can be corrected or replaced. I do that with 5.56/.224 Wylde as well. I eliminate the likelihood of that by ordering from known barrel manufacturers and not rolling the dice with the lower price points or unproven brands.
I have moving boxes full of Lapua and Hornady brass (not even counting Federal and PPU), much of it with multiple firings, so I know my round counts are quite voluminous.
I don’t chase pressure though when hand-loading and reloading, outside of doing my initial ladder tests to see where the trend line and excursion is.
Bill Alexander engineered the Grendel bolts very well, based on decades of experience from the UK MoD, the work on .50 Beowulf, and then all the fleet sample size testing he did. He was used to dealing more with Canadian-built M16/AR-15 variants for the UK MoD, and the Canadians use a different set of processes for their bolt manufacturing that is demonstrably-superior to how they were made for US DoD. Biggest problem with the US TDP is the requirement for High Pressure Test (HPT) on every bolt, where failure nodes are introduced in the bolts with the HPT loads.
I had a discussion with Colt Canada’s engineers about this several years ago, and they said they don’t do that because it’s stupid. KAC said the same thing. US Army demanded that KAC do HPT on M110 bolts, so you would see SR-25s run hard, suppressed for decades with no bolt failures, then see M110s breaking bolts quite regularly. KAC strongly recommended against it, but big Army insisted. The units who have been using SR-25s from the start made sure they kept getting SR-25s and not M110s.
Some of the initial after-market attempts to make Grendel bolts were very weak, and experienced regular failures. Model 1 Sales comes to mind. I haven’t seen anything from them in ages though.
Another thing about my shoot schedule is that I shoot a lot during the winter months, so we take rifles from sub-freezing ambient temperatures up to whatever they reach shooting throughout the day, with an up-and-down temp gradient that is much harder on guns than temperate weather shooting.
Those are the hardest aspects of the Mil-Std compliance, and what really drives a lot of the metallurgy and processes for Military type classification/standardization. US Army used to test up at Fort Greeley in Alaska for that. USAF uses a climatic chamber for aerospace systems down at Eglin AFB, Florida, that they can set the temps and conditions to whatever they want and not have to wait for weather.
An interesting development is that there’s a new Low Plasticity Burnishing process that doubles bolt life, tested by ARDEC (now called RDECOM).
Army got tired of replacing bolts, which started to show up in the budget once regular units started shooting more high volume CQM range sessions. There’s an interesting paper on it:
Low Plasticity Burnishing for Fatigue Life Extension of the M4A1 Carbine Bolt
Live Fire Testing
– Single Weapon Test (M855)
• LPB Bolt ~ 26,000 rds fired before failure
• Standard Bolt ~ 13,000 rds fired before failure
– Multiple Weapon Test: Ongoing (ATC)
• 6 LPB treated bolts
• 6 standard bolts
There have been a lot of 10.3” guns that broke bolts at 4,000rds, since they’re almost exclusively run suppressed without any back pressure mitigation. 14.5” M4A1s with the KAC NT4 were also notorious for running the cyclic rate too high and shearing bolt lugs off.
Anytime I have broken or seen others break a 5.56 bolt, we just replaced it and kept shooting. I’ve been shooting 5.56 since 1987, and 6.5 Grendel since 2009. This year (2023) marks the 20th year of 6.5 Grendel history.
