M-1 Garand Loads

[SFG(A)]

I have been reading several posts, concerning loads for the M-1 Garand rifle. There is alot of discussion of slow burning powders which causes high pressures which could damage the gas port.

My load for the Garand is:

150 FMJ BT bullet
Military brass
IMR 4350 53.0 Grains
Remington 91/2 primers

Should I purchase a adjustable gas plug when shooting this loads? Second question is where do you purchase this adjustable plug? Thanks in advance.

Roy

Information

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[Bruce McAskill]

YES you need the gas plug with IMR 4350. It is too slow for the M1 and your operating rod is in danger of being bent from high port pressure.

[Parashooter]

My load for the Garand is:

150 FMJ BT bullet
Military brass
IMR 4350 53.0 Grains
Remington 91/2 primers

Should I purchase a adjustable gas plug when shooting this loads? Second question is where do you purchase this adjustable plug?

If you're going to make a habit of using that load in your M1, the adjustable plug would be a good investment. More practical, however, might be to use an appropriate powder. Here's a long-winded explanation of the factors involved -

One of the problems with the internet is that erroneous or incomplete information sometimes gets circulated and accepted to the point that it overpowers the truth.

In the case of M1 rifle port pressure, the erroneous information is that port pressure is primarily related to powder burning rate and bullet weight. The truth is that these are merely secondary factors. M1 port pressure is most closely related to gas volume, which is directly related to powder charge weight. Burning rate and bullet weight of course have a direct influence on PEAK pressure, but this occurs long before the bullet gets to the gas port.

With light bullets, we normally use faster powders for best performance since the relative ease with which the bullet starts to move means we can use a fairly large charge of fast powder without excessive peak pressure. With heavy bullets that take longer to accelerate, charge tables tell us the slower powders will give the highest velocity with the lowest peak pressure.

The M1 rifle's gas system was designed for the port pressures generated by the volume of gas produced by a charge of about 44 to 50 grains of powder behind a 173-grain bullet at 2640 fps (M1 Ball). It also happened to work just fine with about the same charge using 150-grain bullets at about 2800 fps (M2 Ball). The burning rate that gives these velocities to these bullets is about that we find in IMR 4895 and 4064. If we use a slower powder, say 4350, we find the appropriate charge for these velocities is heavier - about 55 grains for the 173 and 58 for the 150. Such heavier charges naturally generate a larger volume of gas, but at a slower rate that keeps peak pressure in normal limits. Given that the volume of the cartridge case and bore (up to the gas port) is a fixed quantity, the larger volume of gas necessarily translates to higher pressure at the gas port.

Conversely, if we stick with 4895 but change to a 110-grain bullet, we can stuff in some 54 grains of powder at normal pressure, for a much higher velocity. Again, the heavier charge generates more volume of gas and gives high port pressure. With 200-grain bullets, on the other hand, we can get good performance with 45-50 grain charges of slightly slower powders like 4320 or 4350, giving the same gas volume and consequently appropriate port pressure.

A lot of people who haven't well understood the role of gas volume have focused on burning rate or bullet weight instead - and that's what gets them into logical difficulty. It's very true that an optimum load of the slow powders with 150-180 grain bullets will give excessive M1 port pressure, and also true that the usual best bolt-gun loads of the really slow numbers (like 4831) with 200+ grain bullets will also give excess port pressure. What's missing in the logic is that it's neither the powder burning rate nor the bullet weight that's the problem's root cause - but rather the charge weight (mass, to be more accurate) and consequent gas volume.

It's unfortunate this mistaken (or just incomplete) logic has been so widely publicized, since knowing the whole story really makes powder selection much easier. Regardless of bullet weight, powder charges below 50 grains will generally give appropriate M1 port pressure (or less). Between 50 and 52 grains is marginal. Over 52 grains risks damage to the operating rod. Of course powders must be chosen that will also yield acceptable peak pressure and velocity. (50 grains of 4227 will still make a mess - thanks to excessive peak pressure - but the port pressure would be near normal.)

There are certainly exceptions to this basic rule. Different powder compositions give off different volumes of gas for a given charge weight. But if we stick to the commonly-available rifle powders now on the market, there is surprisingly little variation in the mass/gas relationship and we're not likely to get in trouble with excess port pressure if we choose a published load using less than 50 grains of a powder that gives acceptable performance with our chosen bullet weight.

I urge anyone finding this concept difficult to stick to their existing guidelines. After all, there's little to be lost by limiting one's bullet and powder choices to the accepted standards - 150-180 grain bullets and powders close to 4895's burning rate.

[burgie]

Hi when I started loading for my m1 I read this one thing many times, "do not use any powder with a burn rate slower than IMR 4320 or a faster than IMR 3031." This works for me, just my 2 cents............

http://carnival.saysuncle.com/002449.html

[MEHavey]

Parashoot is exactly right, and brings up the real issue that many (including me) have ignored:

Op Rod velocity is directly proportional to the Impulse imparted to it. Conservation of momentum then immediately makes relative gas mass at the port every bit as much an issue as the port pressure. (!) As you can see from the pic comparing between the two "good" powders and the "no-no" 4350 necessary for equal velocities, port pressure is only 7% different for the slow 4350 powder from that of the IMR4064 "standard," but the resulting OpRod momentum is three times that. (!) (again)

Good on`ya Parashooter.

(On the other hand, 4064 is 98.5% burnt at the port, while the 4350 is barely 91% burned -- so the mass difference might well be pretty much offset as far as the port sees the pressure impulse. I'll have to go back my slurry/fluid mechanics books on that one.)

Anyway -- Don't use 4350 w/ any regularity. Sooner or later your OpRod willl complain.

[Larry Gibson]

I have been reading several posts, concerning loads for the M-1 Garand rifle. There is alot of discussion of slow burning powders which causes high pressures which could damage the gas port.

My load for the Garand is:

150 FMJ BT bullet
Military brass
IMR 4350 53.0 Grains
Remington 91/2 primers

Should I purchase a adjustable gas plug when shooting this loads? Second question is where do you purchase this adjustable plug? Thanks in advance.

Roy

Roy

Switch to 4895 and work up in 1/2 gr increments starting at 46.5 gr up throught 48 gr. Work up until you've a velocity between 2750 and 2800 fps. 4895 was one of the 2 original powders for the 30-06 as used in the M1 Garand. The velocity of 2750-2800 fps is in the range of what M2 ammunition and the original '06 ammuntion was loaded to. This makes for a perfectly safe load for the m1 that meets original specs.

A velocity of 2750 to 2800 fps will calibrate to the elevations on the rear sight. Those calibrations are for M1 ammuntion BTW, not M2 ammuntion. Thus the lighter 147 gr FMJBT at 2750-2800 fps has close to the same trajectory as the 174 gr M1 bullet at 2640 fps. Zero poa to equal poi at 200 yards and adjust the elevation drum so it is set at "2" with the rear sight the same number of clicks up from the bottom as with the 200 yards zero. You can then adjust the elevation drum to the range being shot and be right on or as close as it will get.

Larry Gibson

PS; I spent 27 years in SF (18B, 18F, 18Z).

[Gun Surfer]

A technician at Hodgdon Powder Company, when asked specifically about this issue, told me to use no powder slower than H380 in an M1. Not going slower than IMR 4320 would leave out Varget and IMR 4064 based upon Hodgdon's burn rate chart currently published on line. I do not believe that IMR 4064 is too slow for the M1 gas system, but occasionally someone will state adamantly that it is. (I understand thet burn rate charts are relative in nature.)

[cary m2a]

NO NO NO NO please listen to everyone, H4895 IMR4895 IMR4064 works great. I've been loading for M1 for 10 years with no wear on anything but have seen a Garand with slow powder lose an oprod and batter some other parts.

Good luck

Cary

[slamfire1]

Op Rod velocity is directly proportional to the Impulse imparted to it. Conservation of momentum then immediately makes relative gas mass at the port every bit as much an issue as the port pressure. (!) As you can see from the pic comparing between the two "good" powders and the "no-no" 4350 necessary for equal velocities, port pressure is only 7% different for the slow 4350 powder from that of the IMR4064 "standard," but the resulting OpRod momentum is three times that. (!) (again)

I have read Parashooter's explanation and am puzzling over it. .

The formula’s I have seen in AMCP 706-260 “Automatic Weapons” and in Technical Notes by John Rocha all reduce to equations of pressure. They start out with mass equations, but end up with equations that are primarily dependant on pressure.

The gas cylinder is a chamber. It has a port to the barrel. This chamber has to be filled in milliseconds. If you don’t have the pressure, you can’t stuff the mass through the port. Also, PV=nRT (idea gas law). P= nRT/V. If you could stuff more mass in that chamber than the pressure would go up. But you have a time limit. So given the IMR 4350 port pressure is about the same how are you getting the mass in there faster than IMR 4895?

[Parashooter]

The part you have quoted was posted by MEHarvey, not me.

The information I posted is, of course, an oversimplification. Trying to avoid confusing the average reader, I write of "gas volume" varying with charge weight. More precise would be "gas volume at a given pressure and temperature" or maybe "gas volume potential." The basic idea is that when we burn 55 grains of nitrocellulose we get "more" gas than when we burn 45 grains. In reality, the volume of gas present at the moment the bullet passes the gas port is a constant defined by the volume of the cartridge case and the portion of the bore behind the bullet. If the gas contained in that volume has greater mass and/or temperature, its pressure will be higher. Unless the powder's burning rate is inappropriately slow, there will be little or no unburned powder at this point. Consequently, there's a reasonably direct correlation between powder charge weight and port pressure - enough that I feel fairly confident a safe load using less than 52 grains of powder probably won't harm my M1's operating rod.

This approach is useful when selecting loads for bullets outside the "normal" M1 range and when dealing with powders having an unknown burning rate (but for which we know a charge giving acceptable peak pressure). For example, suppose I wanted to use the 100-grain Lapua bullet and VV powder. I look at the VV data book -



I find maximum charges of 51.8 grains with N130 and 56.1 grains of N133, both giving about the same velocity. I choose N130 as a more appropriate powder for the M1 with this light bullet (presuming I really wanted to drive it that fast) and proceed to work up from the starting load, keeping a careful eye on both the usual (peak) pressure signs and the M1's operation.

Again, this process is useful primarily when I have some need to go "outside the envelope" - using unusually light or heavy bullets and/or powders of uncertain burning rate. It's also helpful for determining if loaded ammo is OK for the M1 - pull one down and weigh the powder charge. If it's over 50-52 grains, it's probably not suitable for the M1 without an adjustable gas plug.

Anyone uncomfortable with this approach should stick to the traditional limits of powder speed and bullet weight - safe, effective and proven.