Staking is the correct method. TM 9-1276 is very thorough in its discussion of carbine gas cylinder, piston, and nut maintenance, but provides very little guidance on staking the nut: "Make certain to stake the cylinder lightly into the nut, in one of three places provided or the nut may work loose during firing of the carbine." I have never done it, but if I needed to, I would use a sharpened and hardened small diameter punch. I'm surprised that Loctite doesn't work.