A loose gas cylinder on a Garand can cause poor accuracy. Before trying to peen your barrel first see if you can just tighten up the gas plug. Use the fancy wench numrich sells here they are handy to have around.

IMPORTANT: do not grip your rifle by the front or top hand guards as they can easily break when twisted hard.

If that doesn’t do the trick you will need to peen the barrel splines. Before you take my advice you should read CMPs article here
I picked up a beat up rack grade Winchester “grenadier” that I intend to fix up a bit. I’m working on cleaning up the stock (more on that later) and while I still may get it re-barreled I thought I would see how it shot after peening.

I already had my rifle completely stripped so I clamped the barrel between a couple of boards in my vice. Don’t do anything crazy like sticking the stock in the vice you will break something. A vice might not be required you could probably get away with just laying it on the bench and have someone hold the receiver but be careful as there is hammering involved.

You will need:

– a hammer
– a socket … I used a 1/2″
– a beer (recommended)

Note the holes in the stock for a grenade sight disk.

Line up the socket with the spline. Leave a bit at the front so later you can get the gas tube started.

Give the socket several wacks with the hammer. You should eventually see the marks.

I did it just until the cylinder doesn’t slide down with gravity. Like so…

I used a board and a hammer and just gently tapped the gas cylinder down. There should be some resistance but don’t bring it down so far that it makes contact with the front hand guard as there should be some play. What I did was leave it out just a little and tightened the lock down until it pushed the cylinder down until it I was at the right spot. Then it is just as simple as tightening the gas plug. Again be careful not to twist anything wooden while doing this.

Alternatively you can use ball-peen hammer place the hammer on the splines and give the hammer a whack with a rubber or wooden mallet (not another hardened steel hammer hammer).