Hi 2-piper, my term recut in my post was an unfortunate generalisation term. As you say, of course you would not be able to make 1/4" BSF into 1/4" UNF by tapping it out. But, I have seen an example where a breech plug nipple hole thread was so badly worn, the remains of the thread had to be bored out, a plug screwed/welded back in, and the plug drilled and tapped to the desired thread. Once cleaned up with an end mill, you would never have known it had been done (other than being very clean).
Hi SKB, very interesting that you have seen many English Guns with 1/4" 28 tpi threaded nipple holes with presumably a UNF type thread form. We all live and learn!
Also interesting to note that BSW and UNC threads have the same tpi, but different thread form.
