I know it sounds crazy but its true, only the unlimited LG doesnt help much because it does 0 damage lol.

Go to any ctf server that has syringe use item. collect it, but before using it, get your health over 125, prefferably up to 200 or soemthing, collect a mega.
Hold down use item ... note that it cannot be used unless your health is under 125 so it will appear to do nothing, but whilst holding down use item, hold down shoot also. (use lg) you can see your ammo is not going down and your invincible. Strange bug. but fun :P