It has been a bumpy road for me trying to get my AR-15 in 300-221 Fireball functioning properly. The gas system functions great but I have had issues with my reloads, the AR magazines, and (I think) with the accuracy of my chronograph.
I started out not being able to get a round to feed from my AR magazines. I solved that problem by installing Magpul anti-tilt followers in the magazines. Even though I was firing one round at the time and cleaning the barrel for suggested break-in, I was attempting to load each round from the magazine to check for proper functioning and then release the magazine so that the AR would fit in my sight vise.
Then I was experiencing slam-fires about every third time I would push the bolt catch and let it slam into the cartridge. Luckily I had the gun pointed safely at the ground the first time it happened
. I was told the shoulder was probably not set back far enough for my particular chamber on the 223 brass. All I know is that when I used my Redding 2-die set to form cases out of 221 Fireball brass, I have not had another slam-fire. Another indication that the 223 brass was not fitting my chamber properly was that I would have to pull really hard on the charging handle to eject an unfired cartridge from the chamber.
I then started trying to come about an accurate subsonic load using Hodgdon 110 and Sierra 220 grain BTHP MatchKings. I was only loading and shooting 3 cartidges at at time trying to get close to the 1050-1060 fps range:
Grains, Average Velocity, Extreme Spread
9.2, 924, 12
9.5, 925, 77
10.0, 998, 65
10.2, 999, 16
10.5, 1061, 57
10.7, 1070, 41
The first time I heard the ballistic crack was with all three shots of the 10.7 grain load. As shown above, that three shot average displayed 1070 fps, but I had calculated the speed of sound today at 80 degrees Farenheit to be 1139 fps. I really question the accuracy of my Pro Chrono Digital. I was surprised when the chronograph showed me to be so far below 1050 fps with the 9.2 grain loads I had seen published several places on the internet. The chronograph displaying about 100 fps too slow and the speed of sound being on the high side at 1139 fps today with the 80 degree temperatures would seem to explain this discrepancy.
I sometimes solve problems in my sleep. Maybe I will wake up tomorrow morning with ways to work out all the details...
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