yea, they made them a bit big that way they at least wouldnt have to worry about them being too small in some bodies and causing a ton of blowback.
most people run them without o-rings, they fit tight enough that they dont need them. and the nylatron will wear down to where it fits just right...or you could sand it a little to speed up the process.
funny thing about those slik kits with the bolt, back block and pin is they are supposed to be lighter than a lot of halfblock bolt & sleds.
