Q: I own a 20" bbl mod choke. Can this bbl be threaded for extended choke to shoot Tri-ball type ammo?

The Dixie Tri Ball is 3" or 3.5", so you'll have to at least have a the 870 magnum. I would just buy a new 870 barrel with Remchokes. But, yes, you could have a gunsmith thread your barrel for chokes as long as you have adequate wall thickness on the barrel in question.

Mike Orlen in Amhurst, Mass is the guy you need to call.

His choke tube installs are very reasonable and quick turnaround.

Doc

I have used Mike good bit and hes 1st rate

Thx all. The 870 is a mag so it will handle the TBs. I'll contact your 'smith with your endorsements.