Ok that makes some sense.

The low velocity is due to the shorter barrel. I have to look it up but I believe the SAAMI test barrel is 12 or 14in. So if the date you are using was worked up in a SAAMI barrel the velocity will come in low.

1680 is a slow burning magnum pistol powder,it is so popular in 300blk subs because it produces a lot of gas needed to operate the AR-15 gas system. The beauty of 1680 is you cannot put enough powder in the case to over pressure the round. For a subsonics the targeted velocity is the limit not the pressure.

This is probably one of the few cases where adding more powder will help solve a few things.

1) you’ll get the velocity you want. ~1050fps
2) it’ll raise pressure enough to be closer to the pressure it needs to ignite (low pressure can cause partial ignition and “blow sand” out the end of the barrel)
3) It’ll help clean up the mess as it burns more completely.

Subs are very finicky and only a few powders have the right combination of burn rate and gas production. If you’re finding IMR 4227 is working better for you, then I’d stick with it. 1680 is a US made powder and is typically cheapest which is why it is found in so many factory rounds.


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