Sounds like the model you want is the Bushmaster Patrolman's Carbine. It has a 16" threaded barrel with a flash suppressor. Comes out to like 17-1/2 inches with the suppressor, if I recall correctly. With the flash suppressor removed, you still have a 16" barrel to remain legal. If you want 16" overall length with a threaded barrel and removable suppressor, that would be a 14.5" barrel, and therefore, a Short Barreled Rifle, needing a $200 stamp and sign off by CLEO in your area. There are other ways to get into NFA items, but that's another topic. This model has a carbine length gas system though, not a mid-length.
Or maybe I have mistaken your post and you want a 16" barrel with nothing attached on the end.
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Or maybe I have mistaken your post and you want a 16" barrel with nothing attached on the end.
?