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300 Blackout (300 Whisper) Cycling Failure - Fix and Reloads

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300 Blackout (300 Whisper) Cycling Failure - Fix and Reloads

Postby flintlok » Sun May 02, 2021 7:20 pm

Some of this discussion concerns the AR 15 chambered for the 300 Blackout or 300 Whisper. In the UK AR 15 semiautos are not generally available however the caliber 300 Blackout cartridge is available in bolt action rifles.

I bought a 300 Blackout upper receiver complete assembly from Aero Precision last winter. I was unable to get to the range until recently. I had previously bought a complete 300 Whisper upper receiver assembly from SSK Industries around 1990. I dealt with JD Jones when I bought the Whisper. The Whisper had an adjustable gas group the was located on the rifle's gas tube. I could adjust the gas flow to any load. I made thousands of rounds for the Whisper both subsonic and supersonic. I sold the Whisper in an auction. It had a 1 inch thick 16 inch bull barrel. I wanted something lighter and I still had about about 900 loaded cartridges of mixed subsonic and supersonic loads. As an aside when I bought the Whisper I had to form my cases from Remington 221 Fireball brass first expanding the neck size to 6 mm then to 308. The cartridge was a wildcat then and properly marked casings were generally unavailable. All of my Blackout cases are marked 221 Fireball which confuses some folks when they inspect my reloads.

The Aero Precision had no adjustable gas group. It does have a pistol length gas system with a 16 inch barrel. I placed the upper on an Aero Precision lower assembly provided with a LaRue trigger group and a standard AR 15 carbine buffer and buffer spring. I installed a nickel boron Tool Craft bolt carrier group with a properly head spaced Bushmaster bolt assembly. I took the rifle to the range with 240 grain Sierra Match King subsonic loads with an 8.5 grain charge of 2400 powder (use that load at your won risk as I'm not a reloading manual). The loads wouldn't cycle. The bolt opened but only partially retracted to about 50% of total travel. I also had some moderate velocity supersonic loads. They wouldn't cycle either. I pulled all of the weights from the recoil buffer and still no cycling at all with either load. Now the scramble started. I did a literature search on the internet for posts from people with similar issues. There's all kind of perturbations for fixes from drilling out the gas port on the barrel to 0.125 inches, using adjustable gas blocks or gas tube, to using lighten skeletonize bolt carry group, and lighten recoil springs. Note that the Aero Precision barrel gas port diameter is 0.075 inches. I just simply sent the upper receiver assembly along with the bolt carrier group to Aero Precision for an warranty evaluation. The techs at Aero Precision found as I had that the rifle would not cycle. They then installed an Aero Precision bolt carrier group in the upper with their standard lower and everything worked fine with subs and supersonic ammo.

They sent the upper back to me with their report. I swapped out the bolt carrier group with a Colt AR 15 bolt carrier group (not as the M16 bolt carrier). Everything worked fine with all of my loads with the bolt locking open on the final round. The problem was simply gas blow by from the carrier key. Either the gas key port was oversized or key's base was improperly sealed. The fix appears simply to be the carrier key replacement. I had just learned that all carrier keys are not the same. All of the other stuff is just appears to be expensive crap. I posted this to make life easier for folks with similar cycling issues and the 300 Blackout (300 Whisper). I did installed a lighter recoil spring for reliability as I intend to shoot mostly subsonic ammo but I'm not sure that it was really necessary to do so.

Now, what I really like about the 300 Blackout (300 Whisper) is it's 1 and 7 rifling twist rate and it's ability to shoot heavy i.e. 250 grain bullets either jacketed or relatively soft lead cast bullets. The heavy bullets have a trajectory similar a 22 long rifle over the first 100 yards but unlike the 22 long rifle it sheds very little energy over velocity over the 100 yard and if sighted at 75 yards it's near point blank shooting over 100 yards. At supersonic velocities with light bullets the rifle's performance approaches the 7.62x39mm performance if not better. Subsonic loads tend not to expand and terminal's performance relies on bullet placement as similar to a 22 rim fire as found by other researchers (
https://digitalcommons.unl.edu/cgi/view ... dmconfproc ) . Above all that I really like is the rifle's accuracy. It's a tack driver even with my lead loads. I recently found that some folks have stated to powder coats lead bullet without the use of any lube. The powder coat is the lube. The heavy jacketed bullets are expensive. It seems that powder coating cast bullets is the way to go with the subsonic loads as well as perhaps the supersonic loads. In either case loading the cartridges uses relatively small amounts of powder. As I approach retirement it seems like a nice way to shoot 30 caliber inexpensively.
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Re: 300 Blackout (300 Whisper) Cycling Failure - Fix and Rel

Postby libensvolenspotens » Mon May 03, 2021 5:00 am

Thank you for taking the time to relay the information which is of interest even to those of us denied the use of semi-auto.
I was recently actin as Range Officer on our range with a group of armed response Police, who are allowed and use semi-auto. (They had quite a variety to test)
They mentioned that they were hoping to change to 300 blackout which made your input illuminating.
Thanks,
Dave
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