My JAB 20 was what I got when I wanted a TAC 20 and the smith didn't have a TAC 20 reamer. It looks like one.
Its a 223 case necked down in a shorted 204 ruger fl die which pushes the shoulder back 30 thou and gives the 204 shoulder angle and a longer neck.
All the internet data I found said 10x would give the fastest velocity.
I didn't try any other powder as it gives 3800fps with the 39sbk. From memory its 23.9gn. if its relevant to your 20 prac I'll turn on the chargemaster and check for you
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