Many older Rem 870/1100 barrels are NOT suitable to choke. So many were made out of round and not concentric (Thicker and thinner rather than consistant in diameter).
Many moons ago, Remington admitted the problem and did a recall with a coupon option to get a new barrel....all well past and gone away.
Rem Chokes are not suited to most older barrels. Colonial Chokes, with a full line of screw ins, are available via Brownells. Check with local gunsmiths or on line to find somebody with the tooling to install the chokes for you.
The first step is to measure your individual barrel to see IF it is suitable for screw in chokes.
For safety, reliability and pattern consistancy, I would watch for a Cabela's sale after the season , on replacement Remington barrels.
Hope that helps !