Failing to cycle with supersonic

[Deansstern]

Recently I built a suppressed 300 AR-15 with a 9inch Faxon barrel, a .104inch pistol length gas port, adjustable gas with an Omega45k can. It cycles 1080 FPS 208, 220 grains subsonics flawlessly at most any settings. But with 1900fps 150 grain or 2400fps 110 grain supers I’m getting feeding issues, short strokes and failures to lock open on empty at most settings. The brass is consistently 6 feet and slightly forward on super, and 4 feet slightly back on sub.
The failures Seem backwards to me, can anyone help explain what’s going on?
Is it the reduced dwell time perhaps? Can overgassing cause it to unlock too early?

[dellet]

With those kind of numbers, depending on the powder, you’re not only over gassed but potentially dangerously over pressure.

[BobinNC]

I concur.

110 grain velocities in a 9" barrel should be in 2150 FPS range + or - 25 FPS.

With a 150 grain bullet your velocities in a 9" barrel should be in the 1700 FPS range + or - 25 FPS.

Your velocities are way too high.

So yes, over gassed and over pressure.

[Deansstern]

If indeed overgassed Can you explain Why that would cause the problems listed above? The primers also look normal, brass has no signs of difficulty in extraction, or over expansion. Ejected Brass, as I said has a very consistent close placement.

[dellet]

There is so much pressure, creating so much bolt speed that the magazine can not move fast enough to move the bolt latch into place to lock the bolt.

I don’t even use a port that big in 10” carbine gassed rifle.

Load data would likely confirm pressure issues.

Look at my avatar, I know exactly what it takes to get the kind of speeds your talking about.

[BJK]

Dean, if you have a micrometer that reads to the .0001" (that would be the 1/10,000") you can easily check for over pressure loads. A caliper won't work. While some observations may or may not be accurate for over pressure I find the following method to be a bit more certain. The link helps me from writing a book.

http://www.hodgdonreloading.com/reloadi ... le-reloads

[Deansstern]

Ok, thanks for the advice. I’ll drop my powder loads for the supers,(Accurate-1680) and choke off my adjustable gas block fully and try to dial it in again. If only I had infinite primers for this exercise!