Effects of Low Charge when Fireforming the 6mm/22-250 Ackley Improved Cartridge

Introduction

While working up loads for my 6mm-250 Ackley Improved, I tried IMR4895 in fireformed cases with good results. I then decided that this medium-fast powder might make a good choice for a fireforming load. Since I wanted to shoot these loads on my upcoming prairie dog hunting trip, I chose H4895 over the IMR variety due to it’s better temperature tolerance. What subsequently happened was a real eye opener for me, and shows how non-linear internal ballistics can be in small arms reloading.

Conditions

I use Norma 22-250 cases expanded to 6mm, then neck sized with a Lee collet die. I decided to standardize on the Federal Gold Match Large Rifle Magnum (215M) primers, as they proved to be the best choice for the AI loads, and for the proven H4350 fireforming load. The bullet used was the 107 grain Sierra MatchKing, seated at an OAL of 2.700”. This rifle was chambered to match the Norma cases, with a positive 0.004” crush on the case. In addition, the bullets are seated such that they are into the lands by about 0.020”, further guaranteeing that the case head is flat on the bolt face upon firing. I have never observed any issues with this rifle with either non-fired or fireformed cases until this event.

The rifle is instrumented with a Southwest Products PressureTrace strain gage and data acquisition module, and an Oehler 35P chronograph was used to measure the muzzle velocity. Since the proven AI load of 35 gr IMR4895 was well within maximum pressure limits, I decided to start with 31 grains of H4895 and work up from there. This charge takes up about 83% of the unfired case volume.

Observations

The first shot showed an expected low velocity of about 2580 FPS, and the PressureTrace plot showed the peak chamber pressure to be right at 40KPSI. The bolt lift was normal, but the condition of the extracted case was anything but normal. Thinking that this was somehow related to a bad case, I tried another shot. The measured pressure and velocity data was the same, as was the case condition. Figure 1 shows the primers of the two cases.

Figure 1 - Flattened Primers from Low Charge Condition

The primers are embossed with the tooling marks from the bolt face, and the ejector impression can be seen over the second number 2 in “22-250” on the head stamp of the case on the right. Just below the web, bright lines can just be seen in this photo showing incipient case head separation. Figure 2 and Figure 3 clearly show the near complete separation of the head from the rest of the case.

Figure 2 - Bright Lines from Near Case Head Separation

Figure 3 – Case wall almost separated

With a sense of morbid curiosity I decided to see if increasing the charge from 31 to 33 grains would improve the condition. I know what you all are thinking, I must be nuts. Possibly, but curiosity got the best of me.

The 33 grain test load case was similar after firing to the 31 grain cases, but definitely less flattening of the primer, and no sign of case head separation. The pressure was around 50KPSI, and the velocity up to about 2700 FPS, still way below the normal 3050 FPS for the normal fireforming round of 39.5 grains H4350.

Conclusions

Since there was no possibility of excessive headspace due to the chambering and the care with which the ammunition was made, and the case indications were improved when more powder was added, I have come to the conclusion that this was a simple  mechanical process that resulted in the failed cases.

With a light powder charge, the pressure was not enough to stick the case wall against the chamber during the initial expansion, so the increasing pressure caused the case to flow forward to fill the empty shoulder volume, pulling the case head away from the bolt (the 22-250 case has no rim). As the pressure continued to rise, the case walls then firmly gripped the chamber, and the primer started to back out a bit and rest against the bolt. As the pressure further increased to maximum, the head was pushed back, flattening the primers, and causing the separations.

This explains why the case condition was so bad with only a light charge and at such low pressures. It also explains why the higher load seemed to be much better, as the pressures rose fast enough that the case walls were immediately stuck to the chamber, keeping the head on the bolt face, and causing the forward part of the case to stretch and fill the "improved" volume in the chamber.

Live and learn. I learned something new from this one. That is always a good thing.