Sanity check on a .450 bushmaster build

[WindstormSCR]

So I’ve been playing a lot with .460 S&W mag recently, and yesterday was looking at my reloading components and my dead air primal and thinking I really need to build a .450 BM upper for suppressed use

The sanity check comes in here: Would there be any reason a rifle length gas system on a 16” barrel would not work in this application?

plan is to use suppressed exclusively (if it becomes a bolt gun unsuppressed idgaf)

Parts for full picture:

• X-caliber custom barrel, wide gas port, 16”, rifle gas
• Superlative Arms bleed-off adjustable gas block
• JMac 45cal Keymount muzzle device (adding to the backpressure)
• Keymount mounted Dead Air Primal (high backpressure .458cal)

Loads will be a combination of 395gr Sub-X subsonic loads and supersonic 250gr FTX

Did some reading in other places, but haven’t run across a good answer for this particular question.

 

[steve101610]

I wouldnt go any longer than mid length gas on a 16” 450. Even with mid length subsonics probably won’t cycle suppressed. Carbine with an adjustable gas block would be ideal if you’re shooting subs and supers. Your best bet would be to contact mos-tek and see what they recommend and have them make your barrel the way you want it.

 

[n2ishun]

Call the subsonic pros at SSK industries.
I would wager deep that there is no one else on the planet that knows subsonics better.
That is JD Jones's company, you know, from the 959JDJ and 300 Whisper fame ?

 

[tekmann2377]

If you want it to cycle with subsonic, then go shorter instead of longer to maximize dwell time. The ideal gas system is determined by the bore to case size/volume ratio. Rifle gas in what is basically a large pistol cartridge is a recipe for function problems.

The 300 BLK shares a similar bore to case volume ratio. What is the typical gas system length for a 16" barrel? It is carbine, or even pistol gas. In fact when I first started shooting 300 BLK, I had a 16" carbine gas barrel. It would not run any subsonics period without a can, and was very unreliable even with a can. I switched to a 16" Noveske with pistol gas which resolved all of the reliability issues. Now due to fixed gas block it is probably over gassed when shooting supers/suppressed, but practically speaking it doesn't beat up brass and there have been no reliability issues to speak of with any combination of loads/suppressor (subs and supers with and without suppressor).

By trying a rifle gas in 450 BM you are trying to reinvent the wheel.

 

[45-90]

The 450 Bushmaster headspaces off the case mouth.
I use a carbine length gas system on my 16" barrel.
And get a 1- 16 twist barrel instead of 1-22 or 1-24 twist.
That way one can shoot cast or rezized .458" bullets in the 500 gr range, slightly super or subsonic. I run a 502 gr bullet in mine, but a 600 gr could be run subsonic in the 16 twist Bushmaster.

 

[WindstormSCR]

I’m aware of other cartridges and projectiles, but my use case here is very narrow, since I’m making use of components I already have for my 460 S&W Mag

the reasoning behind wanting to use a rifle length gas system was to keep it highly light weight by using delta ring furniture (probably magpul, but I hadn’t decided) on a KP15 poly lower I have. I could use carbine furniture and gas, but I have gorilla arms so I was looking for a comfortable option that left the gas block in reach for adjustment.

My thought on dwell time is that the suppressor should act much like a booster does in AK-104 platforms, with the blast chamber area keeping the pressure high enough to get enough gas to actuate the system

 

[45-90]

I have used the rifle length gas system on a 16.5" 5.56 on purpose, in conjunction with a titanium bolt carrier. For a gaming rifle with very high cycling rates of 1085 rds per minute, without hammer follow through.
It works in a narrow window of normal bullet weights, where ammo is cheap, and the same powder is used with bulk bullets 50 gr to 60 gr.
Works great, once set, never change anything, and it never misses a beat...it's unsuppressed.
The gas port was opened up to .122".

 

[91Eunozs]

I have carbine length gas on my 10.5” bushy upper (with an 11” can! LoL)… wouldn’t go past mid-length for any length barrel though, primarily for the ability to run subs suppressed.

Get what works for you, but good to have options. I really like 450 BM for close-in work popping pigs in the scrub brush we call woodlands here in the Tejas Hill Country. Makes nice big holes!

 

[WindstormSCR]

Did some QL tinkering for pressure data vs barrel length, and I think you guys are right, probably not worth the hassle, carbine or midlength it is then.

Any good recommendations for lightweight free floats besides the PRI carbon fiber stuff?