[Legbreaker]

There's a couple of different methods actually, all officially frowned upon (and attracting a charge).
The problem with the methods I know is that unless judicious application of a file is involved (or a replacement part installed), the weapon is only capable of automatic fire and looses the abilty to be placed on "safe" (at least quickly).
The better option, if possible, is to lay your hands on the automatic support version (DAMN RARE!) known as the AR (Automatic Rifle), the L2A1 I think. Equipped with heavier barrel and bipod it uses larger 30 round magazines (instead of the standard 20) which unfortunately empty far, far too quickly.

Give me something belt fed anyday. Yes, they're almost always heavier, but you don't need to reload as often and recoil is much less of a factor.

Regarding the FAL and SLR, there are a few differences, but mainly in the fact that one is build to metric and the other imperial (millimetres and inches). They are still basically the same weapon although don't try swapping parts, or even magazines from one to the other and expect it to work properly!