RKG,
The reason the hard brass is preferable in this instance has nothing to do with pressure, just durability. Federal's commercial brass is exceptionally soft, as commercial brass goes. Look around and you'll find complaints about their higher pressure factory magnum loads having primers fall out on extraction because the head has expanded in just that one firing. Other problems, like sticky bolt lift with loads that work fine in other, similar capacity brass, or excessive pressure ring expansion also are periodically mentioned. Board member Bart B. has suggested this is because they get better accuracy results with less springy brass. Whatever the rationale, some consider Federal commercial brass unsuitable for reloading (Dan Newberry of OCW fame, for one), while others are perfectly happy with it, but at moderate pressures.
I expect part of Federal's thinking behind using mil-spec brass for the Garand loads was simple authenticity, as that mimics M2 Ball, and a lot of Garand shooters are CMP as-issued match shooters, and they make other mil spec brass anyway, so this wasn’t really a special challenge for them. But I also suspect they wanted to avoid concerns and complaints from users finding the Garand had bent the rims or who reloaded and couldn’t get good case life who didn’t have that happen with other makes of brass. Hornady's Garand ammo, for example, doesn’t need different brass, but their regular brass is harder than Federal’s.
TimSr,
If you look up John Clarke's Garand load data from the 80's, you'll find 4350 used with very heavy bullets. That's because, in order not to produce excessive pressure, heavy bullets require charges small enough that the total gas made is not great and doesn’t produce excessive near-muzzle pressure. But with charges large enough to get normal Garand velocities from lighter bullets, it just shoves the op-rod too hard.
BTW, Reloader 17 is not really a 4350 equivalent. It's a slower burning, higher energy double-base powder with higher bulk density. In QuickLOAD, if I load both powders into .30-06 behind the 175 grain MatchKing to the same peak pressure, I get:
55,000 psi peak
4350: 9102 psi muzzle, 2740 fps muzzle, 100.3% case fill (slightly compressed)
RE17: 9683 psi muzzle, 2858 fps muzzle, 96.4% case fill (QuickLOAD default RL17 file)
RE17: 9175 psi muzzle, 2832 fps muzzle, 92.1% case fill (QuickLOAD adjusted RL17 file)
The QuickLOAD model and reality don't always track exactly, but it usually gives you a good general idea of relative performance. The match to Hodgdon's data with the default IMR 4350 is good enough for the error to be in the normal noise (within. For Reloader 17 I put up two sets of numbers, the first being QL defaults and the second with the burn rate adjusted to produce Alliant's published load results. Chances are the difference is due to lot changes or supplier changes over the time between when a sample was tested for QL's database and the last time Alliant did measurements.
What actually affects the op-rod is the size of the gas impulse. It is pressure on the barrel side of the gas port multiplied by how long that pressure is present. That's what determines how much gas is driven into the cylinder. There isn't actually a fixed pressure number because if the bullet moves fast enough, the pressure can be higher and the impulse still comes out about right because the faster bullet's base moves from the gas port to the muzzle in a shorter time, allowing the impulse to end sooner and thereby limiting how much gas the higher pressure moves into the cylinder.
If you bought the
Garand Gear cylinder plug, rather than venting gas to limit the impulse, as the others do, it is hollowed out. That extra volume drops the entering gas pressure and requires a larger amount of gas to reach a given piston pressure. That's why it lets you use commercial loads. Gas impulse measurements are posted at that link for commercial loads, which I know do include 4350-like powders. Write Eric and ask, and he probably has some 4350 test results. See what he says.
