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Old August 29, 2011, 11:02 AM   #26
Unclenick
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The 150's were long, themselves. 7 caliber radius tangent ogive. Square base, with no boattail subtracting from the bearing surface length. Then they were crimped in the middle of the bearing surface with something resembling a Lee Factory Crimp, except down little way from the mouth.

Varget and IMR4064 go almost grain for grain in a lot of .30-06 loads. They fill the case a little better than 4895. If you look at old (pre-Varget) data for match loads you find most match shooters preferred 4064. Indeed, it was used in Federal GMM .308 loads until they switched to Reloader 15 about ten years ago.

IMR4320 is usually named as the slowest of the old powders that was safe to use in Garands, and it is slower than any of the above three. Still slower is WC852 (packaged as H380 in canister grade), and that is what was used in the LC 72 M2 that I have. So the Garand can handle more than some folks expect.

Also note that the heavier a bullet is, the more completely the powder will burn in the early portion of bullet travel down the bore. This is why you use lower charges of any given powder with heavier bullets to reach the same peak pressure. That lower charge makes less total gas, but is doing it in a smaller space because the heavier bullet has not got as far down the tube when it peaks. So, if you get a heavy enough bullet, you can actually use slower powders than are normal for the gun. Clark had a 4350 load under a 180 grain bullet in the Garand. If you were using, say, the 185 grain Berger VLD's or even the 208 grain A-max for single-loading at the 600 yard line, then a powder that slow becomes op-rod safe, whereas with 150 grain bullets, using enough of it to get to normal M2 velocities would make so much gas the op-rod could be bent.

In QuickLOAD, both 4064 and Varget actually generate lower gas port pressure than 4895. This is a tricky element of the calculation, so I won't swear by that without measuring. But it needs to be remembered that burn rates are determined under just one set of standard conditions. Their order can reverse at some pressures. For this reason I prefer the burn rate charts that group powders in horizontal rows rather than a list. Take a look at the lists and you'll find Bullseye, for example, ranked anywhere from fastest to fifteenth fastest, so even the standard conditions are hard to keep consistent enough for reliable absolute readings. Throw in that some powder burn rates vary up to 10% lot-to-lot, and you can see why there is a lot of confusion about this. Burn rate charts provide a kind of vague general suggestion for what's appropriate, but they're not be taken absolutely. John Feamster, in the Precision Shooting Reloading Guide, has an illustration of 4895 and 4064 (both IMR) changing burn rate order with load level in the the .308.

Then there is another factor that is a complete surprise to most people, because it is contrary to what seems "obvious". In the Garand, the different suitable powders all have a load level at which the gas port pressure peaks, and it is NOT at the maximum load level. Here's what happens: As the charge weight goes up, the bullet is driven faster. This produces two effects. One is that the time between the bullet base exposing the gas port to pressure and the time it exits the muzzle gets shorter, so the time available to drive gas into the port decreases. Second, the faster the bullet goes, the more powder energy is consumed by accelerating the mass of the gas itself chasing the bullet. The result is a pressure gradient of several thousand psi difference from breech to bullet base in the typical rifle barrel. The faster the bullet goes, the bigger that difference gets. Thus, though you'd expect muzzle pressure to roughly reflect average pressure in the bore (that's what determines bullet velocity, not peak pressure) in practice it lags behind average pressure as velocity goes up.

The result of all that is you actually have gas impulse drop with increase in loads above a certain load level. I worked out an example of an increased powder charge at the CMP forums which drove the bullet 7% faster, but the muzzle pressure only increased 6%. Thus, you had the gas port exposed to 6% higher pressure for 7% less time, with the net op-rod impulse actually decreasing with that powder charge increase.

CMP forum member Ericc has gas port impulse measuring equipment and verifies this effect. It showed up best with a relatively quick burning load of Benchmark in the example he gives. The plot is in post #49 on page 5 of this thread. As the charge gets bigger, increasing velocity, the gas port impulse gets lower. In the particular case of Varget, in another thread he shows the gas impulse basically the same with 46.5 or 51.0 grains of Varget behind a 155 grain Palma match bullet. The peak pressure in the gas cylinder is actually lower with 51 grains, as my description suggests would occur, but the impulse total (what the op-rod experiences as drive force multiplied by the time the force is applied) also depends on how fast the rod moves back, and a higher peak pressure at the breech usually makes the case slightly harder to extract, which delays that. The general point is the same though. Warmer loads of powders in the correct burning rate range, which Hatcher shows the receiver handles just fine, won't damage the op-rod because the impulse doesn't grow in proportion to the powder charge.

If you take that too far, using really low loads, like some of those starting loads in the Hornady manual, the powder burn efficiency declines, so the port pressure eventually goes down again as you lower charge further below that middling range. In that same Varget example above, Ericc has a 41 grain charge of Varget generating a lower gas system peak pressure and total impulse.

The main point I wanted to make is that if you shoot middling loads of a powder successfully in the Garand, you really don't need to worry about working them up within normal limits, as your gun is already experiencing maximum gas system impulse. Hornady's super-cautious maximum Garand loads aren't really any safer for the op-rod than the old standby Garand match loads John Clark published in the mid-1980's, and those are mostly a couple of grains warmer. Indeed, published maximum loads for bolt guns with the same powders are op-rod safe, too.
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Old August 29, 2011, 11:10 AM   #27
dmazur
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That was a great explanation of gas port pressure. Thanks.
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