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It looks to be 12 and a tiny bit....assuming your calipers are accurate.
Ring gunworks and ask them.
I'm going with x24 as above but a mismatch of diameter of can and barrel threads.....that is...

It could quite possibly be 9/16x24 but your can and barrel threads are different sizes, It is quite common to turn a thread until the part threading on just "fits" and threads on nice without...

It could be whatever depending on what gear the guy on the lathe decided to put it in, I have come across all manner of non standard threads.

Im thinking 9/16 x? need the tips of the caliper on the tippy top of the threads to get the pitch, set caliper to 1/2" and see if it lines up neat on ten threads.... then it would be 9/16x20

Line the points of the caliper up with the tippy tops of the threads 10 threads apart and measure the diameter with the thick part of the caliper so you can hold it square without the caliper tips...

Yep that if you have calipers, measure od and measure the tops of the threads over as many as you can but say 5 or 10 to keep the maths simple.