Re: Turn gas off on M1A for Superformance use?
I think the highest estimated port pressures QuickLOAD has predicted for me were 16-18KPSI, piezo.
The M14's White Gas System is, per my sources, designed for 12,500 KPSI, CUP. +/- 2KPSI (max 14,500 CUP). The port is about 14-1/8 inches from the bolt face, or about 12.5 inches of bullet travel down-bore.
Cross-referencing the only known actually-tested published info on reloads I could find, (it was either NRA or Rifle or Handloading magazine, mid-1980s, no I don't want to dig it up any time soon!), two things remain firmly in my memory:
1. Loads like the classic "substitute" M852 load of 43.0 W-W 748 in USGI brass pushing any of the 168-gr SMKs or clones, reported CUP port pressures ran almost exactly 2KPSI less than what QuickLOAD predicts for those very same loads (the other I think is 40.5 or so of IMR-4895, but I'm a 4064 guy from my .30-06 days...).
2. Lighter bullets tended to run higher port pressures.
My own personal experiences have been this:
3. Excessive port pressure loads move attempted extraction too early in the firing cycle, resulting in rim lift and occasional failures to extract (the empty gets withdrawn and then put back in the chamber). The bolt almost always got far back enough to cock the hammer.
4. The op rod on an M14 type is almost impossible to "bend" as WAS legendary with the M1 Garand. No one I know has ever seen it happen with that short, stiff, M14 rod, at least from shooting heavier loads.
5. I am *told* from those who shot hundreds and even thousands of 190-grain bullets at 2400 or so FPS from the M14-type that you can get excessive wear and battering on the rear of the op rod and the bolt impact area in the receiver. There are second-hand reports of receivers cracking from bolt impact, but these also at least hint or openly admit hot-rodding the loads with heavier bullets.
But, no measurements of port pressure, which *tends* to be lower with heavier bullets compared to lighter...in the days of IMR and W-W powders at least.
6. The one thing of ErrorNet legend that does universally hold true is that "slower" powders do run higher port pressures.
7. My alternate RL-15 load (42.5 behind 175 SMKs/178 A-Max in GI cases or FCs of almost identical capacity) models within spec for port pressure, with mild chamber pressures. However, QuickLOAD says that if I try to follow the bolt gunners up past 2600 FPS from that 22-inch barrel, my chamber pressures are fine but port pressures get more than 1,000 PSI above spec. So, I stick with this apparent accuracy node with my rifle.
8. Since the pressure in the bore peaks only a few inches past the leade, and drops to less than 18KPSI 12.5 inches down the bore for slower "gas gun" powders, and the UN-numbered pressure/time curves from Hornady show an extended PEAK pressure that then drops to match the curve of more conventional powders at what looks like 6-8 inches down the bore, I *suspect* that what Hornady speaks truth when it says that SuPerformance ammo is safe in normal-length gas systems.
And finally,
9. Since no one has ever checked the hardness of the pretty Hornady Match ammo brass that pops primers, and that phenomenon appears with only a few rifles whose velocities reported are NOT notably above advertised (if at all), I am suspecting that the reported problems with high pressure reports in that ammo are a combined artifact of a longer pressure pulse above .357 Magnum pressures combined with soft brass.
So, if Hornady would ever PUBLISH what port pressure its
SuPerformance ammo generates in .308 loads, I could have more confidence in their declarations of its safe-ness in gas guns.
I cannot speak to the AR systems, since I still have not found any port pressure figures, with tolerances, for the 7.62NATO/.308 systems. One guy gave me just the nominal for 5.56/.223 systems, but I have no idea whether it's the same for the bigger caliber.
