DON’T TRY THIS: Pushing 6GT to the MAX (Alpha Munitions, Bat Machine)

We previously pushed the limits of Alpha Munitions .308 brass and BAT Machine’s Ignitor action. Do NOT try this at home, but our curiosity has led Bruce Thom and me to explore the limits of 6 GT with Alpha Munitions brass and the BAT TR action.

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Experiment Planning

Bruce Thom and I started with research and data calculations in QuickLOAD Ballistics Software. We agreed we wanted to take the 6mm GT at least to 6mm Creedmoor velocities.

The closest data we had in QuickLOAD was for 6mm Dasher. We used this and Hodgdon data (24” barrel) for both 6mm GT and 6mm Creedmoor to set our parameters. (We are also using a 28” barrel for our testing.)

Alpha Munitions once again recommended Varget for this project and we set our starting load at 30.6 grains. Our data is for 107 grain bullets, but we used Berger 105 grain VLD bullets.

We used the same reloading press for this project as we did for our .308 high pressure project: the Area 419 ZERO press, as well as the RCBS Chargemaster Link, Primal Rights Competition Primer Seater, and Area 419 loading block.

Each round was composed of brand-new Alpha Munitions 6mm GT small rifle primer brass, Berger 6mm 105 grain VLD bullets, Federal 205 primers, and Varget powder.

Bruce and I loaded a variety of cartridge weights starting at 30.6 grains, and headed up to the mid-mountain range.

Testing and Results

This time, we have equipment to get actual pressures, not just estimated ones.

Our pressure data acquisition system consists of a specially-designed pressure ring, Caldwell Velociradar chronograph, PCB 119C12 Charge Output 100k PSI Pressure Sensor, and DEWESoft Sirius data acquisition module with DEWESoftX software.

We mounted the barreled action in the Arrow Products Rock Vise via Picatinny rail. This way neither of us would have to be close to the action or barrel during testing.

Bruce and I designed and built a pressure ring that we slid over the barrel shank and drilled into the barrel. The PCB sensor will measure up to 100,000 psi, our end goal.

A call to PCB revealed we couldn’t just attach the sensor to the ring—we had to pack it with grease to avoid heat issues. This fix also results in more uniform pressure curves.

We obtained baseline calibration data with the minimum load before jumping to the maximum load: 34.0 grains of Varget.

We had loaded increasing amounts of powder in 0.5 grain increments up to 37.0 grains, recording the velocity and pressure associated with each charge weight.

The new setup and sensors made data collection much faster and more efficient, but we still had to do the math to determine our correction factor from the published data.

By 35.0 grains of Varget, we exceeded 6mm GT maximum published velocity, though we have to keep in mind we have a longer barrel. With our initial goal met, we continued, gathering data and watching for pressure signs. Muzzle blast intensified with the hotter loads.

High pressure signs emerged in the primers at 36.5 grains. There was slight cratering and one of the cases had a faint ejector swipe. 36.5 grains of Varget was our last “usable” load under CONTROLLED circumstances. (Again, do NOT try this at home!) Alpha Munitions brass is what made this experiment even possible—regular brass would fail long before this.

We finally stopped the experiment at 37.0 grains when a puff of smoke escaped the pressure ring along with an odor of burnt Cerakote. Losing gas is not just a bad sign, but also indicative that any further data won’t be accurate.

Zoomed out View of Triggered Event in DewesoftX

There was also a significant pressure spike between 36.5 and 37.0 grains. Each of the three pieces of brass we tested at 37.0 grains had ejector swipes. Our greatest observed actual pressure was 86,632.7 psi, far over SAAMI maximum psi of 62,000.

6 GT Pressure Test Summary

6 GT maximum published velocity is 2,909 fps. 6mm Creedmoor maximum published velocity is 3,009 fps, exactly 100 fps more. We observed an average velocity of 3,215 fps at 37.0 grains, 6.85% over maximum 6mm Creedmoor velocities.

Another interesting data point is the case capacity of the two cartridges. 6 GT’s case capacity is 88.01% of 6mm Creedmoor’s capacity and we were still able to get a higher velocity.

Conclusion

While this experiment is incredibly interesting, there really is no need for the pressures we attained in these cartridges. (Again, do NOT do this at home.)

Besides confirming the quality of Alpha Munitions brass, perhaps the most interesting part of this experiment was seeing just how close our numbers were to published data.