I'm running a Remington 870 with an 18 1/2" barrel in 3 gun. After my last match, it was suggested that I get a choke to tighten up my groups for some of the more challenging steel. I don't believe my barrel accepts chokes, I think it's a fixed. 1) Does Remington make a factory replacement that I can drop in that will accept chokes? This one looks like it would work. 2) What choke should I use? I usually shoot the cheapest birdshot the gun store has on the shelves in 2 3/4. Thanks for all the help!