Originally Posted by mcseal2
I got my barrel purchased today for the re-barrel on my 243, a 1 in 9 #4 Kreiger that will finish out at 24". I plan to shoot 85-90gr bullets through it for coyotes and a low recoil option for long range practice.
Now for my next question, several people on other forums have said I should chamber it to 6mmAI to gain extra powder capacity. I did not know if it would be worth going to the AI in a 24" barrel or if I would gain much with that length. I would like to get 3300fps from the 90gr bullets, can I do that with the 6mm or should I go 6mmAI?
I shot a 6mm Ackly out of a 24" barrel for a couple years. I got 3450fps with 85 grain bullets, without doing anything out of the ordinary. I also had access to a very similar rifle, but with a 27" barrel (both were Pacnors and were identical). Chambers were cut with the same reamer and the same lathe and gauge used. Headspace was within .001" between them. The 27" barrel might have been 75 fps faster, but the accuracey loads proved different between the two. The 24" barrel was faster in the accuracey loads (and in the end that's what counts)! Also the shorter barrel seemed to have a better selection of rounds that grouped well (both barrels were cut from 28" #7 blanks). But looking back at the situation again, I think that a 27" barrel with a 1" strait contour (both were savage 112 single shot actions) might have helped that gun out a lot. Also Doug and I swapped barrels thinking there might have been a difference in the actions. We got the same results. Also I've found that for some odd reason the 6AI is somewhat more selective about the actual powder you are using.
Being as the standard 6mm Remington is really about the max powder capacity for a 6mm bore; you don't really gain as much as some folks say. Maybe 150fps max. There are a couple of tricks out there that will add another 50 to 75 fps to this, but now are very remote. I think that a case that's about 54mm long, but with the Remington neck is a better solution. You just run a 6mm Ackley reamer in short. You'll use less powder, and doubt you'll see 50 fps difference. Yet shoot better. For a die; you simply cut off a 6mm Ackley die, and run the reamer in a .243 Ackley seater sleeve from Forster.
Now adays I shoot a 6mm/250AI out of a 26" barrel. It's a fast twist barrel, and is throated for 105 grain Amax bullets. It's good for mid twos on a perfect day, and will always shoot mid to low threes. I can max load a 105 grain bullet for right at 3100 fps, so an 85 grain bullet ought to do about 3250 fps at least. Going to do another barrel for it in 1:10 twist, that will be about 24" long in a strait 1" contour. I'll throat it for 70 thru 80 grain bullets