I have read the a flash suppressor has to be indexed. What exactly does that mean? Are there marks on the barrel and FS that need to be lined up?
thanksfor all y'alls help.

An A2 flash hider has a closed bottom, so that you can shoot prone without raising a ton of dust due to the blast from your muzzle, the closed bottom of the FH reduces that.  Therefore, when installing one, you have to have it tight so that the closed area faces down.  


This is accomplished with a peel washer, which is a thick washer made of many thin washers that you peel enough off of so it is just thick enough to have the FH tight in just the right spot.  IMO, a giant pain in the A--.  

Then some lazy person invented the crush washer.  This washer gives some when you tighten the FH and allow you to tighten the FH to just the right spot.  This is the way to go.

I installed one of the A2s with a crush washer and it was a piece of cake. Definitely take that route.

I'm sorry I didn't specify which FS i would be using. I will be adding a Phantom FS.

IIRC phantoms do not need to be indexed, as they do not have a closed bottom?

Quoted: IIRC phantoms do not need to be indexed, as they do not have a closed bottom?

Some do... you can usually pick which one you want when you order one.