Last month, the JP Engineering Team conducted a post mortem on a pair of catastrophic failures involving our Ultra Low Mass aluminum carriers. They did a great job reading the signs of both the carriers and the cases. If you didn't get a chance to read it, make sure to catch up before continuing.
Our JP engineers do a bang up job on product design and development, but they don't have the same reloading experience I do. I've been reloading my own bullets for over 40 years. So, I could see there was still more to this little episode than just a bad recipe.
The reloads in question used a 69-grain Sierra Match King with 23.4 grains of H335 powder loaded to an OAL of 2.250 inches. I've burned many pounds of this propellant in different loads:
Although it works great for me, it performs optimally with the lighter-weight projectiles in applications like these. It is not a good choice for the heavy match bullets; its burn rate is a bit too fast for that application. Ramshot TAC, Hodgdon CFE223, Varget or IMR-8208XBR would be a better fit.
As a general rule, you should select a propellant that nearly fills the case capacity with the recommended max. charge. Too much air space indicates that the propellant is not well suited for the application. In the .223/5.56, depending on the particular powder's density, a charge of ~24-26 grains is more optimal.
As an example, I've used copious amounts of TAC with both the 69 and 77 SMK projectiles. These loads take a charge of 24.5 grains with Federal 205M primers and an OAL of 2.250 inches. I could add at least a grain more with the 69, but I chose the 24.5 since that's optimized for the heavier 77-grain bullet.
Doing this allows me to switch between the 69 and 77 for either 3-Gun or .223 long-range precision rifle without adjusting my powder measure. This charge yields about 2725 ft/s with the 77 and 2850ft/s on the 69—right in the accuracy nodes for those bullets.
The CFE requires a bit more at 26.8 grains on the 69 and 26.3 grains with the 77. In any case, these are pretty close to case capacity loads with these propellants. They have perfect internal ballistics for a gas gun with good port pressure. Their chamber pressure is in a range that allows for multiple reloads on a case.
In other words, case life is a good indicator of a safe load.
I have the luxury of testing my loads right in my barn, performing my load development in my makeshift ballistics lab. I will use one case over and over to test and chronograph a load until I settle in on a charge weight. Absent an actual pressure tester, I take notice of the primer pocket retention as a good indicator of chamber pressure.
As soon as you notice a primer retention reduction when seating the next primer, you've gone too far and need to back off. A good load should allow you to reuse a case about 8-12 times before the primer retention becomes inadequate.
What is "inadequate?" You'll know it when you feel it in contrast to a good, once-fired case. You may actually feel almost no tension upon seating the primer.
Getting to know the "feel" of your reloads is a skill that takes time and repetition to develop.
This brings us to a related subject: loading on progressive presses vs. single-stage or multi-station manual presses.
I never load centerfire rifle on a fully progressive press. Performing multiple operations at once eliminates your ability to feel the neck tension or primer seating force during the process. You simply cannot track these all important yardsticks of quality control at once.
That brings us to neck tension. If you've seen our tongue-in-cheek instruction video GasGunBasics, you've already heard this sermon. In the case of these destroyed carriers, I think the real culprit was inadequate neck tension in the hand loads.
With inadequate neck tension, the projectile can be pushed deep into the case upon seating. It will actually go beyond the case neck/bearing surface range into the case itself. Now, it is sitting on top of the powder charge, no longer precisely aligned with the bore.
At this point, the bullet is no longer a projectile waiting to be launched into the lands and grooves of the barrel. It's now an obstruction with gases escaping around the bullet upon ignition. This leaves the pressure too low to eject the bullet, yet still too high for the case to hold together.
The result is a catastrophic failure of the rifle as the rear of the case fails, taking the bolt carrier with it. In my expert opinion, that's just what happened with these carriers. The results of the post mortem are irrefutable since, indeed, the bullets were still inside the cases.
So, if you are committed to loading on a progressive machine, you'll need to take extra precautions.
Box your ammo upside down. Press hard with your thumb on each case head to see if the bullet will collapse into the case. This will uncover your rejects and save you from destroying a perfectly good rifle.
As stated in last month's write-up, the aluminum carriers failed exactly right under the circumstances—a textbook destruction where no one was hurt. This is the beauty of the AR-15 design.
I've seen many catastrophic failures of rifles from many manufacturers over the years. Yet, whatever the reason for failure, I've never seen anyone seriously injured. The failure mode of the design is engineered to prevent this from happening, and thank God for that.
Stay tuned for a more in-depth discussion of the reloading process. Writing about this, I had to hold myself back from going off on too many related subjects. I have plenty more advice and quality control checks that will keep you safe and on-target and your rifle in one piece.
We hope you found these Post Mortem articles useful and enlightening. As the engineering guys said in Part 1, we take parts failures very seriously at JP. We want to learn everything we can from them. If we can pass some of that knowledge on to you, so much the better.