different threads

[drgbike]

I have several receivers with different threads. I was unaware they used different threads besides the 12 flat type. Can anyone school me on these?

[Mainspring]

Add some models and calibers and any other info that might help people answer your query.

[drgbike]

I have several no1 receivers. The one which is different has a barrel in Spanish bottle neck. Its threads are much finer. I also have a #5, with a larger shank diameter. I also have a Numerick barrel which has fine threads and does not screw into any of my receivers.