Like @ShooterG said above. Anytime you are running direct thread (fixed barrel) you will need a spacer to go over the piston instead of the spring. The spacer keeps the can from moving when firing. If you don't, you risk ruining the barrel and the piston/mount as the can will hammer both without the spacer from the spring motion. Also, the spacer will often leave a little gap in the housing when installed to show it has been installed. It will all make much more sense once you go shoot it and see what everything does. General rule is that if the barrel moves, spring. If the barrel is fixed (AR9 for example), install the spacer instead of the spring. I don't have any 3 lug set ups, so I can't help with those.