I picked up a used barrel cut to 18" from 28" by it's former owner. Patterned not to my liking with buckshot, as it had no choke after being cut back. I sent it off and had it reamed and threaded for Rem-Chokes. Now it's my favorite barrel.
Absolutely outstanding buckshot patterns, does nicely also on the clays. I'll try it for the first time on dove here in October.
Edited to ADD:
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Using Dial Calipers, measure the outside diameter (od) of your barrel:
Minimum od measurements for:
0.830 od = Tru Choke
0.845 od = Win Choke
0.860+od = Rem Choke
Helps if bore is concentric to OD, as if not, may neccesitate the use of 'thinner' choke tubes.
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I was fortunate, as my cut-down barrel measured over .880, and was fairly concentric. This allowed me to use Rem-Chokes, which are somewhat more available to me in my locale.
About 'Short' barrels:
Dave's on the money about the less than 18" barrel. Depending on your local/state government, You can most likely own one, with the proper registration forms, fees, background check, fingerprints, photographs and other requirements IN ADVANCE of the short barrel becoming yours. Having had that experience personally, there is very little value in my mind for the extra effort and expense of the shorter barrel.
My 14" barreled Mossberg is in the back of the safe, the 18" 870, much handier.
All the best,