older Rem 870P Cylinder Bore

[SW CQB 45]

Some years ago, I purchased a trade in Rem 870P that appeared to have been hardly used.

I decided to bring it out to a recent police academy shotgun training/qual and much to my disappointment the pattern with low recoil buckshot was not as tight from 15 yards as a newer 870P.

I had one of the pellets just go outside the paper scoring area.

I discovered my 870P barrel is marked "CYL" where as the newer 870P barrel is marked with an Improved CYL.

I want to "choke" my muzzle to "IC".

suggestions on defensive use chokes and who to send my barrel to for this modification.

thanks in advance