If you are getting 3 1/4" chamber length reading, I think you are not measuring accurately. I have a 20 gauge Flues that has had the chamber lengthened to 2 3/4". I would try the dowel rod or credit card method to see what reading you get with either of those. My Flues is choked 1 and 4 if I remember correctly. Also, I have a Beretta 409 that has 25" barrels that I am told were cut off as Beretta didn't offer a 25" barrel. It has the bead on the end and it is hard to tell it has been cut.