No takers, I guess. Now that the lawn is mowed, I guess I can crunch some numbers.
The 10 ring of a Mid-range (600 yard) NRA target?
You guessed it, BTW. Actually the X ring.
Turns out my gunsmith and I are both right.
I'm having him build me something goofy, and I wanted to avoid the comments on the goofiness. I am having him put together a Garand for highpower. There are a bunch of negatives to that, but the one we are debating is .308 vs .30-06. His argument is that the air space in the case of a .30-06 will end up with a 100fps spread in velocities depending on powder position, which even with everything else being perfect will put me above or below the 10 ring at 600 yards. With .308 there is a full case, and less FPS spread, and that's what he suggests that I build. My argument is I never see that kind of spread with my loads, and I really want it in .30-06 because, well just because. He says he can make a sub-moa Garand in either chambering, but that the .30-06 will handicap me at longer range.
My question is, with my loads, how much actual vertical spread could I actually anticipate, because I'm not quite the hard holder enough to do the test at 600yds, and I would already need the rifle built to do the test anyway.
So based on his numbers, which I believe he gets from Clark's NRA article where the author carefully made sure the powder was all the way forward vs. all the way back, and there is a 100 fps difference in velocity between the fastest and slowest rounds.
What I get in my spreadsheet with a 2481 fps mean velocity (taken from my 600yd load) and one bullet going 50fps faster and the other 50 fps slower, the fastest bullet will hit 3.6" high, the slowest 4.2" lower than dead center. This is based on the time the bullet falls from it's highest point, the speed it falls, and the difference in arrival time at the target. This would put me outside of the 6" X ring with a perfect rifle, and with a 1 moa rifle, would throw shots outside the 12" 10 ring.
However, my 600yd load in .30-06 gives me a 42fps extreme spread last time I checked it, and these are straight out of the powder measure. This gives me a high shot of 1.6" and a low shot of 1.9" which are both in the X-ring, and with even a 1 moa rifle are 4.6" high and 4.9" low in the worst cases, which is still in the 10-ring. I think that I won't be too handicapped
Looking online, I find some testing that reveals Federal GMM with 168gr Match kings in .308 was found to have a 9 round extreme spread of 27fps. This calculates to 0.9" above and below the center for the fastest and slowest bullets. I could stand to shrink my group's vertical stringing by 1.5 inches at 600yds, or be .75" closer to the center by going with .308 versus .30-06.
The next step is that I have a box of loads with 10 shots each at my current load and +1.5 gr and +3.0 gr to test with hand-measured charges to see what my best possible spread is as I decrease the air space in the case. I expect that the spread will be a little bigger with 10 rounds, since my old data are from a 5 round set. If I get these close to the federal match ammo, I think I will cast my die with the .30-06. If the extreme spread opens way up, then .308 it is.
If anybody wants the spreadsheet, I can send it. It was actually easier to build than I though. It probably isn't perfect, but it should be within a few percent of reality.
Nerd hat off.
-J.
