Tuesday, May 01, 2007

The A-Max experiment

Last week I talked about the experiment I was going to try with my new heavy barreled AR; and Hornadys 75gr A-Max bullets.

These bullets have a ridiculously high ballistic coefficient for a 75gr because they have a long ballistic tip. They're also a hell of a lot cheaper than other bullets with a comparable BC; like the Berger VLDs that retail for about $0.30 each, as opposed to $0.15 per for the Hornadys.

The problem is, with their very low drag design and thus very long overall length, they are generally considered unsuitable for magazine feeding through an AR platform rifle.

They've been on sale everywhere lately for under $15 a box; so I decided to scoop some up.

I was also looking to pick up some Varget and CCI small rifle primers to load them with at the same time; but they, and every place else within a 40 mile drive of my house, were out of both. In the mean time, I picked up the only powder they had in stock that I thought would be suitable for some test loading; a pound of AA2230; and their last box of Winchester Small Rifle primers.

Given the powder and primers, I'm not expecting world beating accuracy from these loads; what I'm trying to establish is whether I can make these VLD A-Maxes function and mag feed properly.

Anyway, I loaded my first batch of 20 over 23.5gr of 2230, and I seated them to the shortest dimension that didn't put the case mouth too far over the ogive; which turned out to be 2.260". Importantly, I also backed the seating die out to give no crimp at all; and gave them the very slightest bit of a taper crimp using a lee taper crimp die.

By very slightest bit, I mean all it did was fold the chamfer over and slightly increase neck tension.

My second batch I went up to 24.5gr; and I loaded to the longest length I could get to feed from the magazine without issues, 2.280"; also giving it the slight crimp.

Now, both 23.5gr and 24.5gr are relatively hot loads compared to factory .223; but neither are high pressure. The pressure ceiling on the round is 55k psi, and both loads listed should be below 50k. I plan on loading up to 26gr in .5gr increments, which should put them right on 55k; checking for overpressure signs with each increment, and then backing down one when I hit it.

Then, this weekend I went out to function test the new ammo, and new upper (using JohnOCs lower; which is substantially the same as mine, but has a standard weight buffer and spring; and a currently a standard DPMS mil-spec trigger - though he's putting the national match trigger in soon, he just hadn't installed it yet).

First news, the upper is great. Using factory federal 62gr, there was basically no felt recoil, and no muzzle rise or jump at all. Offhand, from the bench, even handheld like a pistol, the muzzle stayed on target; and I was able to watch the bullet strike through the sights (strictly irons today)

I also did a featherfire test; and I was able to light up a 10" circle with 30 rounds at 50 yards in under 5 seconds, and had no problem whatsoever.

I love those heavy barrels... so long as I don't have to hump one all day every day (I miss my bushy superlight).

We only had a 50 yard range this time out, so I didn't even bother trying to shoot for accuracy; besides which the gun isn't ready yet; I just wanted to test function; and with the factory stuff it functioned fine.

Then I switched over to the first batch of my A-Max loads, and fired off a couple. With them, there was the slightest bit of noticeable recoil; but there weren't any problems cycling at all, even when I rapid fired about 10 off at once.

Importantly, none of them pushed back, misfed, or failed to eject. They also, very crudely measured, held a 2" or so circle at 50 yards for 5 rounds. Again, I wasn't looking for accuracy here, I jsut wante to make sure everything was working OK.

The second batch were loaded a bit longer, and I expected trouble; but I didn't have any feed problems whatsoever.

Unfortunately, about the tenth round in had a failure; the bolt locked up about 3/4 of the way through the chambering stroke; and the forward assist didn't clear it.

Now, I said above I didn't have and failures to feed, and I didn't; the problem wasn't the fresh round, it was the previous one. I had blown out a primer, which had jammed the bolt.


Now I hadn't brought out my chrony so I couldn't check the velocities in question; but the blown out primer wasn't pierced, nor were any of the previously ejected rounds; and there's no way 24.5gr of 2230 should have blown out a primer.

So I chalked it up to a fluke, or maybe soft primers (and they are ridiculously soft; which is why I wanted CCI in the first place. In years past Winchester primers were harder, but if you look at the firing pin impressions on a WSR primer shot through an AR these days, you'll see what I mean).

Well, five round later, again the bolt locks. I clear it and whadya know, another primer blowout.

At this point I stopped the test, because obviously we had an overpressure problem.

...which was a good thing because a T-storm was coming in fast; and we finished packing up the truck just in time to avoid the gullywasher.

We did manage to recover all the brass though; and I just inspected it prior to cleaning.

What I found was very slightly flattened primers on both sets of my handloads; and ALSO on the factory federal loads; from both Johns gun AND mine.

Ok good, that means it wasn't my loading or my gun, but...

Now theres no way the Federals should have been overpressure and flattening those primers, so what did I neglect to take into account?

Well, this is Arizona...

... and it was 100 degrees on Saturday, and very bright and sunny before the storm came in...

...and we had left the black aluminum rifles, and dark grey aluminum magazines out in the hot sun...

There is a reason why the British used to have "tropical" or "African" loads, and standard loads for their rifles. Load pressure standards are developed at 60 degrees farenheit; and temperatures over 80 degrees can make pressures rise greatly.

Temps well over 100 (remember, black rifle black magazine, hot barrel) can take even relatively lightly loaded factory ammo, and make it flatten primers.

Combined with the hotter load, the neck crimp, and the hot barrel (remember the blow outs were only happening at the end of a rapid fire string); in retrospect blowing the primers out wasn't exactly a shocking occurrence...

... or it wouldn't have been had I remembered any of that at the time. As it was, I was worried I'd done something stupid like mis-calibrating my charge scale or somesuch; or worse, that it was my gun malfunctioning causing the problem.

My concern was that the gun was unlocking early, withdrawing the bolt from the case head while obturation was still holding the case in the chamber; allowing the pressure to push the primer out in the pocket a bit, and then either blowing the primer out; or slamming the case head back into the withdrawing bolt face as the pressure eased, flattening the primers.

This is in fact what was happening; but it wasn't because of a malfunction in the gun as I had feared, it was overpressure.

In a normal pressure situation, this wouldn't have happened; but with too much gas pressure at too high a velocity, the bolt was unlocking too fast, and the brass was sticking too hard to the chamber walls; allowing what I describe above to occur.

It's a rare day when you can say you are happy about an overpressure condition.

My thoughts on the matter were confirmed when I resized the cases after cleaning; and the small base sizing die stroke took more effort on a well lubed .223; as reforming a thick walled magnum case. That's a clear sign of overpressure (an indicator of excessive obturation).

I'm probably going to have to cull this brass out, and only use it for one or two more generations, because of the overworking.

So, anyway, the good news.

The rounds functioned jsut fine through my AR, magazine feadding from a 30rd mag with no problem at either the shorter 2.260" OAL, or the longer 2.280" OAL. They also shot very well offhand, at short range.

Next step, finish the gun; and load up some more (with even less crimp, some CCI BR primers, and a slightly lighter load for the heat of summer) so I can shoot the loads for accuracy.