Question regarding breech plug threads for smooth bore pistol barrels.

[Rolf]

Blackley sells smooth bore pistol barrelblanks caliber 0.625 . He threads these for plugs 3/4" coarse Whitworth threads. How many complete threads do you get on a 1/2" long plug?

What is the closest UNC thread size?

Strengthwise, is Whitworth equal to UNC?

Are American threaded plugs UNC or UNF?

Best regards

ROlf

[FL-Flintlock]

10tpi ideally you should get 5 threads on a 1/2" hole but there's losses on both ends of the hole.  BSW (Whitworth) have a 55 degree angle, USS/UNC have a 60 degree angle.  In any equal grade fastener, I don't think there's any strength difference.  Remember the first thread takes more of the load than any other - I'll email you the thread load chart later.  Most US barrels are UNF.

[Acer Saccharum]

Coarse threads have more metal mass interlocked than fine, so NC is stronger than NF.