The story goes like this. After spending years developing a fancy new 30-06 cartridge (the M1) for their fancy new semi-automatic rifle (the “U.S. Rifle, Caliber .30”), the U.S. Army made an unfortunate discovery: the M1 cartridge could shoot well past the safe zones built into exiting Army practice ranges.
Going back to the drawing board, the military developed the M2 cartridge, a throttled-back version of the M1 that would keep errant rounds within bounds.
The rifle, commonly known today as the M1 Garand, was designed to function with the M2, which means that modern commercial 30-06 ammunition often produces higher-than-safe pressures. These higher pressures can bend the rifle’s operating rod, destroying its functionality and seriously decreasing its value.
Now, let me be clear: the safest way to avoid a bent op rod is to use either the diminishing supply of military surplus M2 ball or one of the commercial loads specifically designed for the M1 Garand. But if you’re looking to use a heavier bullet or hotter load for hunting or competition, that’s not going to work. You need a way to decrease pressures, and that’s where an adjustable gas plug comes in.
Adjustable gas plugs allow users to control how much gas from a fired cartridge is redirected back into the piston and operating rod, which in turn cycles the action and loads another cartridge. If all the gas is allowed to escape, the action won’t cycle; if too much gas is redirected, the action will cycle too quickly or too forcefully. Adjusting the gas flow to fit a specific cartridge ensures that the action cycles reliably with minimal wear and tear on the bolt, op-rod, etc.
There are several options on the market, but the most common adjustable plug seems to be the Schuster DCM Adjustable Gas Plug. For less than $40, this adjustable plug is easy to install and tune, and can be readjusted for whichever 30-06 loads you want to shoot.
1. Remove existing gas plug.
First things first. Make sure your Garand is unloaded!
Next, you need to get the stock gas plug unscrewed and out of the way. The plug on my rifle wasn’t very tight, but I’ve heard of units that require some elbow grease. There are lots of tools specifically designed to remove M1 Garand gas plugs, but I just used the tool that came in the buttstock cleaning kit. If you have a wide, thick flathead screwdriver, that’ll also work.
2. Install Schuster adjustable plug.
Once you have the stock plug out, simply screw in the new plug. Before you do this, be careful not to adjust the screw inside the plug. It comes from the factory at the correct setting and tightening it could damage the action.
Also, be sure to tighten the Schuster plug before you begin firing (the entire unit, not just the internal screw). The M1 wrench doesn’t work well for this, but a flathead will get the job done. If you don’t tighten the plug, loosening the screw inside the plug will loosen the entire unit.
3. Tune the plug until the action cycles reliably.
The first round you shoot shouldn’t move the action, so you’ll have to extract the spent casing by pulling back the bolt. With the action locked to the rear and without any cartridges in the gun, reach forward and turn the gas plug ¼ turn clockwise with the provided Allen wrench.
Repeat this process until the action extracts the spent casing and the bolt locks to the rear. I started with 180g Winchester Powerpoint loads, and I turned the plug one full rotation until the action ejected the spent casing on its own. Once you think you have the correct setting, load multiple cartridges and check for reliable cycling.
After trying the process with the Winchester loads, I turned the screw back to its original position and repeated the steps with the 150g Remington cartridges I plan to use most often. With that load, I only had to turn the screw three-quarters of a turn before the gun started to cycle reliably.
4. Find the sweet spot.
This can mean one of two things. If you’re wanting to make the action cycle as slowly as possible (and thus, less wear and tear), turn the screw counterclockwise by small increments (I used 1/8 of a turn) to find the exact spot at which the action still functions.
Or, if you’re planning to use this rifle for competition or hunting, you can also tune the gas plug to achieve optimal accuracy. To do this, turn the screw in 1/8 turn increments, shooting a group after each adjustment. Repeat this process until you find the best group.
Now that your mind is at ease about damaging your M1 Garand, take it out to the range and start shooting! If you ever want to switch cartridges, you can simply loosen the screw to its original position and start the process from the beginning.
Also, for your viewing pleasure, check out this great “How the M1 Garand Works” video from the 1940s.