Plainfield Upper Hand Guard

[Harlan (Deceased)]

Rocky,
Loose hand guards are common on M1 carbines, and as imarangemaster said, they don't effect accuracy.
(I cannot stand for them to be loose either)

I've also used IMR's technique, except I reform (bend) the corners inward slightly, as if to reduce the circumference of the upside down 'U' slightly.
Hold the flange with a small pair of regular pliers and 'slightly' bend the corners inward with another small pair of pliers or needle nosed pliers.
He's correct - Do not put any force on the rivets, as they're easy to pull out, or make loose themselves.
It only takes a slight reform to tighten the hand guard right up. (Quick and easy 'fix')