COTEP.org - Loading subsonic 300 Blackout

I built a 10.5" Carbine length upper for one of my SBR lowers to use with my Silencerco 762 can.

Shame on me, I did not really do a whole lot of research into the gas system, other than making the mental note that 300 BLK tends to be a fairly dirty round, so a carbine length system would be a touch cleaner than a pistol length one.

Needless to say, it works great as a single shot rifle, but not so well as a semi-automatic.

Doing some searching on the web, I had found some published information stating that using between 10.1 and 11.3 grains Reloader 7 powder with a 200 grain bullet cycled the action of a 300 BLK.

So I loaded some at 11 grains and gave them a shot, and got a 50% success rate.

Then I went back and checked the data again, and realized that the published data was for a 16" barrel, not a 10.5, so the velocities given for the powder load were incorrect for me.

Time to do math.

A year or so back, I wrote a small program that calculated the velocity of a projectile for a given barrel length.

Example: If the bullet velocity is 850 fps out of a 16 inch barrel, what will the velocity be out of a 10.5" barrel?

So I plugged the known values into my program.

Minimum load data - 10.1 grains of Reloader 7 with a 200 grain bullet 16" barrel = 850 fps
(see attachment 1)

Solves to 747.68 fps out of a 10.5 inch barrel.

Maximum load data - 11.3 grains of Reloader 7 with a 200 grain bullet 16" barrel = 1050 fps

(See attachment 2)

Solves to 949.35 fps out of a 10.5 inch barrel.

Since the speed of sound is approximately 1116.1 fps @ sea level, I could increase the load by just shy of 167 fps and still remain subsonic. The additional powder should also provide more gas volume to cycle the action more efficiently. Since these are subsonic loads, pressures should not exceed 35000 PSI, over-charged loads should not be a concern. (SAAMI Pressure range for 300 AAC BLK is 35000 - 60000 PSI)

So now I needed to determine what charge of powder would get me as close to the speed of sound without breaking it.

Since I know that using 10.1 grains will give me 748 fps, and 11.3 grains will give me 950 fps, I could use math to determine the velocities for the velocities between 10.1 and 11.3 grains, and then use that data to estimate the velocity for powder charges greater than 11.3

So I added a tab to the velocity calculator and made a screen that would perform the calculations and return them in a window incremented by 0.1 grain of weight.

To check the math for values between the mix/max values, I ran it against the original values, and got a velocity within 1.6 fps of the published data.

Since I needed data that is above the maximum given weight, I added the utility to add 1 grain of weight to the maximum charge and/or subtract 1 grain of weight from the minimum charge.

The results

(See attachment 3)

Linear interpolation indicates that a charge of 12.2 grains should produce a velocity of 1110.82 fps with a pressure of just over 31000, which is still below both the speed of sound and the SAAMI minimum pressure values.

Loaded some today, I will test them tomorrow.

If the gun blows up in my face, I will assume my math is incorrect..:D