This is my first time changing barrels. I've got a 10p 308 that I'm putting a 26 Shilen on. My factory barrel spun right off and I put it back on with my go nogo guages just to make sure I was doing things correct then when I tried to put the shilen on it wouldn't go more than two turns and I didn't want to mess up the threads by forcing it on. I got the calipers out and the factory was 1.05 the shilen threads at 1.053 I don't know if that makes a difference.