Working through the details...

[too_many_hobbies]

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...

[RWBlue]

Originally Posted by too_many_hobbies

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.

IMHO, Something else is wrong.
An AR will not slam fire on malformed brass.
I would think that the firing pin channel, firing pin is bent, dirty....messed up
OR
The pimers are not set to the correct depth.

Please post picture of the slam fire brass. You should be able to tell when it fired.

[too_many_hobbies]

Originally Posted by RWBlue

Please post picture of the slam fire brass. You should be able to tell when it fired.

I am not sure which brass to take pictures of because I inspected the slam-fire brass at the time, but did not keep it separated from the other brass after the fact.

Since I posted above, I used the Hornady/Stoney Point headspace guage to determined that my 223 brass averaged the same size as my chamber. I used my dies to set the shoulder back .002" and have not experienced a slam-fire since with the 223 brass.

The primers appear to be seated properly (flush). The once fired 223 military brass I was using had the primer pocket crimp removed, but some would still not accept the primer. I set those to the side.

Another detail is that I apparently had some type of grit that was getting on the plunger that presses the primer into the case as I turned the shell plate on my RL550B press. I found small dimples with random placement on some of my unfired 223 brass. I was using Federal 205 primers and have been told that they have a relatively soft face. I was advised to try some Remington 7 1/2 primers.

So far most of my issues seem to be due to my ignorance, but I am learning. When your weapon goes off within inches of your foot, you gain an urgency to become a faster learner...

[RWBlue]

Things that go bang when you don’t anticipate them, really gets your attention. :-) Been there done that. It was one heck of a learning experiance.

Hmm,
If the brass doesn’t show unusual expansion, then it is not going off out of battery as I would expect if the firing pin was sticking out.
And if the primer is seated deep enough you shouldn’t be able to set it off when it is hit with a flat object like the bolt face, so the setback shouldn’t matter.

I have no idea what is going on. It doesn't make sence.

BTW, I noticed you may have the same problem with tight neck, but I was hoping a singe topic thread, may get us both a response. Have you measured the neck of your fired brass.

[too_many_hobbies]

Originally Posted by RWBlue

Have you measured the neck of your fired brass.

It is difficult to get a true measurement of the neck of my fired brass because is getting bent during extraction, as shown below:



I measured .005" expansion (+- .002" due to the out-of-round condition) relative to my unfired brass for both the 223 and 221 brass. From those measurements, I determined that I had adequate expansion space in my chamber and stopped trying to determine the actual amount to the .001".

[too_many_hobbies]

Originally Posted by too_many_hobbies

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 sent my chronograph to the manufacturer for calibration. I am going to wait till I get it back before I continue searching for an accurate subsonic load. I wish I had purchased an Oehler Model 35 (discontinued) with the 3 screens instead of the ProChrono Digital.