safearm
US Veteran
I had been looking for a semi-auto rifle to keep feral animals, up to the size of coyotes, and other vermin away from my horses. I have a number of rifles in 5.56 mm/.223 Remington, both bolt action and semi-auto, but my wife complains they are too loud (I live in a fairly rural area of south central Mississippi and no one complains about the sound of gunfire). My current “go-to” rifle is a M85NP with a CNC Warrior folding stock and faux suppressor/barrel extension, and it’s admittedly loud with significant muzzle flash/blast. I had read about the virtues of the .300 AAC Blackout in a number of magazine and on-line articles, as well as Patrick Sweeney’s “The AR-15 Volume 4” from Gun Digest. I was particularly intrigued about the use of heavy, sub-sonic rounds which reduces the overall noise signature but still delivers potent energy out to 100 yards (about the limit of my off-hand ability).
I decided on a CBC upper in .300 AAC Blackout (on Amazon.com since I had a gift card to redeem!). The upper did not come with either a bolt/carrier group or a charging handle, so I ordered an Anderson BCG and a DPMS charging handle from Brownell’s (on sale no less) and everything arrived in the mail the same day.
The upper is an A3 style with a 16 inch medium weight barrel and an aluminum four rail free-float handguard. The upper has a carbine length gas tube. Twelve plastic rail covers are included with the upper, and the rail slots are numbered sequentially from the charging handle forward. The rail height on the upper and the free float handguard match perfectly.
I wasn’t sure at the time if I wanted another rifle, or just another upper (one of the real benefits of the .300 BLK is the ability to swap uppers, keeping the number of registered guns the same) so I installed it on my Colt M4 lower and pulled the Burris AR-223 1-4x scope from the Colt upper to install on the CBC upper.
Everything fit together without any issue and I was off to my back pasture range. I had a couple of boxes of ammunition to test: Remington 220 grain sub-sonic and Remington 120 grain Open Tip Flat Base (OTFB) supersonic (this round is no longer cataloged by Remington, but is readily available even at Wal-Mart). I also had three different brands of magazines: Thermold 30 round, PMag 30 round (polymer magazines), and D&H 20 round (aluminum magazine).
I really wanted to test the sub-sonic ammunition so I loaded up a Thermold magazine with 10 rounds of the 220 grain ammunition, locked the magazine into the well, and released the bolt. As a side note, in my haste to get outside, I did not clean or lube the upper, except to make sure the bore was clear. So when I released the bolt, the round did not chamber fully, but a push on the forward bolt assist did the job.
The first shot was on the paper, low and to the right of the aim point (the scope was sighted in with 5.56 mm on my M4). The recoil and muzzle flash/blast were very noticeably less than 5.56 mm. Two problems became immediately evident: The round failed to eject (partial stovepipe) and a new round did not chamber. A quick pull on the charging handle cleared the stovepipe and chambered another round. Another round goes downrange, point of impact was practically on top of the first round, and while there was no failure to eject this time, a new round failed to feed. Again, a rack of the charging handle chambered a fresh round and I was ready for another shot. After three shots, I adjusted the scope and continued. To cut to the chase, the gun was very accurate, 1 MOA or less, but essentially I had a bolt action semi-auto as I had to work the charging handle every shot to chamber a new round.
After the first 10 rounds of sub-sonic ammo, I decided to test the supersonic ammo that was loaded into a PMag. I locked the magazine into place, released the bolt, and had no problems through the next 10 rounds; no FTF or FTE. Point of impact was about 2 ½ inches higher and slightly to the right of the sub-sonic point of impact. I readjusted the scope to the POI of the 120 grain round (as I had more of it than any other ammo). At this point, I decided to take a break to clean and lube to gun, as well as to get some different magazines.
Back at the range with another Thermold magazine and a D&H magazine; I loaded the D&H with the sub-sonic roads and the Thermold with the supersonic rounds. Again, with the sub-sonic rounds, I had no problem with ejection, but new rounds would not chamber. There was one failure to feed with the Thermold; it appears that the bolt carrier is dragging on the feed lips as there were some scratches I didn’t notice before; luckily I have plenty of PMags and D&H magazines to dedicate to the .300 BLK.
I located a couple of boxes of Hornady 208 grain sub-sonic at the Bass Pro Shops and decided to give them a try, since all of the literature stated that it was the best sub-sonic .300 BLK. Unfortunately, I got the same results: Very effective ejection (about 90 degrees for 8 feet) but the new round would not chamber. Accuracy with the Hornady round was spectacular with multiple cloverleafs.
Sweeney does a good job discussing gas pressure issues with the .300 BLK. “You may or may not be able to get sub-sonic loads to work with the carbine set-up, as gas pressure has dropped markedly by the time the bullet gets to the gas port in a carbine design. At least, .300 gas pressure. Conversely, if you opt for the pistol length gas system, your supersonic loads may work the system excessively hard, what with the extra gas pressure provided by the supersonic loadings.”
Other than the one magazine issue, all of my problems have centered around the sub-sonic loads and I believe it’s as Sweeney states, that my carbine length tube does not generate enough gas to cycle the bolt carrier back far enough to eject the empty case and chamber a new round. At this point, I thought I only had three options: stick with supersonic rounds; accept that I have a single-shot semi-auto when using sub-sonic rounds; or adjust the weight (trim the length) of the recoil/buffer spring. Since I don’t like the first two options, and the cost of buffer springs is negligible, I decided that with a little experimentation, I could find the right spring weight/length to pair with a specific sub-sonic load. Because of this, I also decided to buy a lower receiver to be dedicated to the .300 BLK upper. Once I got all of the details worked out, I’d color code the spring(s) to keep everything matched.
I stopped by my local dealer to order a complete lower and told him of my situation. Luckily for me, he had already experienced the same problem and knew of a solution. His solution was to send the gun to Dixie Precision Rifles and have them expand the gas port. His gun did the exact same thing as mine and after expanding the gas port a few thousandths of an inch, it now feeds and fires everything, sub or supersonic, with no loss of accuracy. So it’s a trip to Dixie Precision to drop off my upper. Hopefully, it’ll be done by the time my new lower arrives next week.
Overall, I’m favorably impressed with the cartridge and the upper. Both are accurate; recoil and muzzle blast are subjectively less than the 5.56 mm, even with the supersonic rounds; and the new gun/cartridge gives me something to tinker with. The next part of this journey may be to buy some reloading dies (and a chronograph) to see what kind of loads I can develop to achieve the perfect, at least for me, mix of energy, projectile, and reduced noise signature.
I decided on a CBC upper in .300 AAC Blackout (on Amazon.com since I had a gift card to redeem!). The upper did not come with either a bolt/carrier group or a charging handle, so I ordered an Anderson BCG and a DPMS charging handle from Brownell’s (on sale no less) and everything arrived in the mail the same day.
The upper is an A3 style with a 16 inch medium weight barrel and an aluminum four rail free-float handguard. The upper has a carbine length gas tube. Twelve plastic rail covers are included with the upper, and the rail slots are numbered sequentially from the charging handle forward. The rail height on the upper and the free float handguard match perfectly.
I wasn’t sure at the time if I wanted another rifle, or just another upper (one of the real benefits of the .300 BLK is the ability to swap uppers, keeping the number of registered guns the same) so I installed it on my Colt M4 lower and pulled the Burris AR-223 1-4x scope from the Colt upper to install on the CBC upper.
Everything fit together without any issue and I was off to my back pasture range. I had a couple of boxes of ammunition to test: Remington 220 grain sub-sonic and Remington 120 grain Open Tip Flat Base (OTFB) supersonic (this round is no longer cataloged by Remington, but is readily available even at Wal-Mart). I also had three different brands of magazines: Thermold 30 round, PMag 30 round (polymer magazines), and D&H 20 round (aluminum magazine).
I really wanted to test the sub-sonic ammunition so I loaded up a Thermold magazine with 10 rounds of the 220 grain ammunition, locked the magazine into the well, and released the bolt. As a side note, in my haste to get outside, I did not clean or lube the upper, except to make sure the bore was clear. So when I released the bolt, the round did not chamber fully, but a push on the forward bolt assist did the job.
The first shot was on the paper, low and to the right of the aim point (the scope was sighted in with 5.56 mm on my M4). The recoil and muzzle flash/blast were very noticeably less than 5.56 mm. Two problems became immediately evident: The round failed to eject (partial stovepipe) and a new round did not chamber. A quick pull on the charging handle cleared the stovepipe and chambered another round. Another round goes downrange, point of impact was practically on top of the first round, and while there was no failure to eject this time, a new round failed to feed. Again, a rack of the charging handle chambered a fresh round and I was ready for another shot. After three shots, I adjusted the scope and continued. To cut to the chase, the gun was very accurate, 1 MOA or less, but essentially I had a bolt action semi-auto as I had to work the charging handle every shot to chamber a new round.
After the first 10 rounds of sub-sonic ammo, I decided to test the supersonic ammo that was loaded into a PMag. I locked the magazine into place, released the bolt, and had no problems through the next 10 rounds; no FTF or FTE. Point of impact was about 2 ½ inches higher and slightly to the right of the sub-sonic point of impact. I readjusted the scope to the POI of the 120 grain round (as I had more of it than any other ammo). At this point, I decided to take a break to clean and lube to gun, as well as to get some different magazines.
Back at the range with another Thermold magazine and a D&H magazine; I loaded the D&H with the sub-sonic roads and the Thermold with the supersonic rounds. Again, with the sub-sonic rounds, I had no problem with ejection, but new rounds would not chamber. There was one failure to feed with the Thermold; it appears that the bolt carrier is dragging on the feed lips as there were some scratches I didn’t notice before; luckily I have plenty of PMags and D&H magazines to dedicate to the .300 BLK.
I located a couple of boxes of Hornady 208 grain sub-sonic at the Bass Pro Shops and decided to give them a try, since all of the literature stated that it was the best sub-sonic .300 BLK. Unfortunately, I got the same results: Very effective ejection (about 90 degrees for 8 feet) but the new round would not chamber. Accuracy with the Hornady round was spectacular with multiple cloverleafs.
Sweeney does a good job discussing gas pressure issues with the .300 BLK. “You may or may not be able to get sub-sonic loads to work with the carbine set-up, as gas pressure has dropped markedly by the time the bullet gets to the gas port in a carbine design. At least, .300 gas pressure. Conversely, if you opt for the pistol length gas system, your supersonic loads may work the system excessively hard, what with the extra gas pressure provided by the supersonic loadings.”
Other than the one magazine issue, all of my problems have centered around the sub-sonic loads and I believe it’s as Sweeney states, that my carbine length tube does not generate enough gas to cycle the bolt carrier back far enough to eject the empty case and chamber a new round. At this point, I thought I only had three options: stick with supersonic rounds; accept that I have a single-shot semi-auto when using sub-sonic rounds; or adjust the weight (trim the length) of the recoil/buffer spring. Since I don’t like the first two options, and the cost of buffer springs is negligible, I decided that with a little experimentation, I could find the right spring weight/length to pair with a specific sub-sonic load. Because of this, I also decided to buy a lower receiver to be dedicated to the .300 BLK upper. Once I got all of the details worked out, I’d color code the spring(s) to keep everything matched.
I stopped by my local dealer to order a complete lower and told him of my situation. Luckily for me, he had already experienced the same problem and knew of a solution. His solution was to send the gun to Dixie Precision Rifles and have them expand the gas port. His gun did the exact same thing as mine and after expanding the gas port a few thousandths of an inch, it now feeds and fires everything, sub or supersonic, with no loss of accuracy. So it’s a trip to Dixie Precision to drop off my upper. Hopefully, it’ll be done by the time my new lower arrives next week.
Overall, I’m favorably impressed with the cartridge and the upper. Both are accurate; recoil and muzzle blast are subjectively less than the 5.56 mm, even with the supersonic rounds; and the new gun/cartridge gives me something to tinker with. The next part of this journey may be to buy some reloading dies (and a chronograph) to see what kind of loads I can develop to achieve the perfect, at least for me, mix of energy, projectile, and reduced noise signature.
