What is the best way to fix this?

Tighten the screws?
I'm sorry , it had to be done


Seriouly though, whole thing loose? wood or plastic?

Try tightening it ... LOL, I couldn't resist ...

If it's loose from the screws like they're wallowed out, just put some glue in the hole and hammer a tapered piece of wood in it, let it dry and redrill it. If it doesn't fit tight in the receiver get some veneer and glue it to the sides to build it out. I've done both and it works fine, I use Gorilla Glue but if you've never used it I'd glue something else with it first, so you can see how it works, a little goes a long way ...

Is this a synthetic butt stock or a wood butt stock.  The method of repair will vary depending on which material it is.

Yours,
One Rusty Nut.

OK I deserved that! I did atempt to tighten the screws before I posted this. Its a wood stock and it is tight side to side but verticaly it moves about 1/16". I was thinking about some glue and some shims any one else tried it?