.450 Bushmaster SBR project

[MCMXI]

I think it would be interesting to start a thread covering my trials and tribulations with the .450 Bushmaster that I have. I always learn more when I screw up or when something doesn't work as opposed to when things work "perfectly" so I'm sure to learn a lot with this project. I wanted a "big bore" AR for quite a few years and in January of 2014 I finally received a 20" .450 BM upper to put on a BM lower that I've had for years. I ordered the 20" version but if I were doing it again with no plan to build an SBR I would order the 16" version. Right off the bat their were some oddities such as having two gas blocks installed, one functional and one decorative but overall I've been very pleased with the performance. I've had no magazine, feeding or cycling issues and I've shot a few hundred rounds of Hornady and Remington ammunition and the rifle has proven to be 100% reliable with no issues at all. I think that the magazines benefit from a little careful filing/sanding of the interface between the feedlips and cartridge case to remove the sharp edge but other than that they seem to work well.

I decided to go with the .450 BM because I have a .45-70 Govt rifle (Marlin SBL) that I use for hunting so another .458 cal rifle didn't interest me that much, and .45 cal is one of my favorite calibers. I have a bunch of pistols, revolvers and rifles chambered in .45 ACP, .45 Colt and .454 Casull and I have a lot of .450 BM brass and suitable bullets for reloading so it made sense to me.

Here's how the rifle started out with a factory 20" upper and BM lower with a Geiselle SSA trigger, VLTOR Emod stock, TUBB flat wire buffer spring, KNS anti-walk pins and a Magpul pistol grip



A couple of years ago I added a Daniel Defense Modular Float Rail, removed the fake gas block and figured out the the muzzle was threaded 11/16-24 UNEF.



Recently I had the barrel cut down to 12" and the muzzle threaded 5/8"-24. The rifle is shown with an AAC Ti-RANT .45 attached and the barrel and suppressor have an overall length that's less than the factory 20" barrel and brake. This rifle is nice and light and balances well.



I have 500 of the Hornady 300gr XTP Mag bullets on order for subsonic load development and will use Remington AccuTip bonded 260gr bullets for supersonic loads. This is a fun gun for the most part and won't see any serious use other than the possibility of bear protection if out hiking/camping in NW Montana. My objective for this rifle is to eventually shoot 100% suppressed, both subsonic and supersonic loads, so I'll most likely replace the Ti-RANT with a Hybrid from SilencerCo. My other objective is to have the rifle cycle both subsonic and supersonic ammunition which might be a fool's errand. I put together 10 rounds last night using a 300gr lead bullet from Oregon Trail and 12.8gr of Trailboss and was on the Mach 1 bubble at 1,120 fps to 1,130 fps and that load wouldn't cycle the bolt. Yes, I know that lead bullets can be problematic with gas ports but these are hard cast moving at relatively slow speed and I only shot 10 rounds so I don't think this is an issue. I have a 10.3" barreled DD that will cycle perfectly with 187gr subsonic ammunition so I was a little surprised and disappointed with the first test. Maybe a 300gr subsonic bullet/load doesn't generate enough gas to cycle the bolt. Perhaps the buffer is too heavy. Maybe the gas port isn't large enough or there's not enough dwell time. These are all questions that I plan on answering. The first thing I'll do this weekend is shoot some supersonic ammunition to make sure that the rifle will cycle with that ammunition at least and go from there. These projects are always interesting and I look forward to learning a lot more as it progresses. The worst that can happen is that I end up with a suppressed SBR that is a single shot with subsonic ammunition. We'll see.

 

[horsemen61]

Awesome keep us posted on what happens!

 

[MCMXI]

Thanks ... and I will. Shooting quasi subsonic 300gr loads with the suppressor attached is really fun with minimal recoil, minimal noise and a satisfying clang on steel at 75 yards. I don't think I'll be putting a scope back on this rifle. I like the simplicity and effectiveness of the iron sights.

 

[pintler]

I may be all wet, and know nothing about the 450, but my understanding that the powder can matter for 300BO subsonics, i.e. you may get better function with powder A than powder B even if both produce the same muzzle velocity. I have a publication from AAC when the 300 was new that advised using AA 1680 for subsonic loads. It reads "AA1680 is recommended because its bulk density matches the case capacity and it will generate enough gas pressure to cycle the weapon."

I dunno if that helps for the 450, but trying other powders might be something to try.

 

[MCMXI]

I may be all wet, and know nothing about the 450, but my understanding that the powder can matter for 300BO subsonics, i.e. you may get better function with powder A than powder B even if both produce the same muzzle velocity. I have a publication from AAC when the 300 was new that advised using AA 1680 for subsonic loads. It reads "AA1680 is recommended because its bulk density matches the case capacity and it will generate enough gas pressure to cycle the weapon."

I dunno if that helps for the 450, but trying other powders might be something to try.

You make a good point re the 300 Blackout and powder selection. I tried Trailboss for the subsonic .450 BM loads because it will fill up about 80% of the case when pushing a 300gr bullet to around 1,050 fps. If I used H110 it would fill up about 45% of the case which makes me nervous since the location of the powder in the case can really affect how the powder burns. Bad things happen when there's lots of free space available in the case! The suppressor increases back pressure and typically will increase the cyclical rate which should help with subsonic loads, but if there isn't enough gas or enough gas velocity generated, or it's not generated fast enough then it's not going to work. I have QuickLOAD which is useful in comparing powders.