I'm working on a project (further down on a P522) and need to know the difference Sig 556 handguard and 552? or the difference in length between receiver and gas block if not the specific handguard info.

Trying to see how much lathe work is needed on original barrel, and if juice is worth the squeeze.  The gas block is fake on the 522 so moving it or getting another fake block at the right length might be tricky and have to add material around the barrel.

552 and 553 are the same.
551 and 556 are the same.
556 Patrol and P556 are the same and unlike any of the above.

Let me clarify -

556 patrol/P556  versus Sig 552.

A P522 is similar to a P556/Patrol, As its a fake gas block, thinking of moving it backwards to 552 length, cutting barrel rethreading, also would need a sleeve for the outside of the barrel as the faux gas block sits on a wide part of the P522 barrel and closer to the receiver it is thin and would need some diameter added to accommodate the faux gas block being moved back (of course the 552 needs a different handguard front cover.

I forget the exact length difference but it’s about an inch or two.  Depending on what handguard you want to use, you might need a handguard retainer ring if you choose a real 552/553