I gleaned this from another forum. I found it very informative, and thought it would be a good idea to share it here. I appreciate the efforts of the original OP for taking the time to put this together and hope he is not too peeved with my re-posting it here. There is a graphic that was part of the original posting, showing the pressure curves, but I think most will be able to understand the article without it.
Reloading for the M1 Garand
One of the problems with the internet is that erroneous or incomplete information sometimes gets circulated and accepted to the point that it overwhelms 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 (technically, mass and temperature), 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 we may begin to see risk of damage to the operating rod. Of course powders must be chosen that will also yield acceptable peak pressure and velocity. (50 grains of 4198 will still make a mess - thanks to excessive peak pressure - but the port pressure would be near normal.)
50-grain charges of fast and slow powders yield vastly different peak pressure and velocity, but M1 port pressure is virtually the same even with these extreme examples.
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.