Blowback 300W AR?

[Fastshooter03]

I did a search and saw a few things about it but no real results.
Is anybody running a blowback AR for subsonic loads?
Anybody know what would be needed to make it a safe prospect(other than not forgetting to only run sub loads through it)?
I know the 9mm AR's are blowback. Do they have a higher rate spring and heavier mass?
I wonder what would happen if you blocked the camming action of a normal AR bolt or have it only partially cam? I know the back of the carrier would be into the buffer tube when the action is closed but I don't think that is a problem other than having to pull both pins for disassembly.
Seems like a cool idea-if it's safe and doesn't make a huge mess-not having a gas system to mess with. It would also save you from having to drill a hole in the barrel.
Let's here some thoughts


Nick

[natec]

I think that subsonic loads with light-for-caliber whisper bullets (150s at 1000fps) would be close enough to subsonic loads in 9mm (147 at 1000 fps) that you could likely get it to work. Keep in mind that 9mm in an AR sometimes relies a bit of tuning to get to work reliably. Some of the AR guys can chime in there.

The hiccup would be that supersonic loads are generally held to the same pressure standards of a 223 remington (at least they were in Contender circles when I last loaded the 300W) which would absolutely wreck a blowback action - 125gr at > 2000 fps.

So I do believe it could be done. I do not believe that you will ever see one commercially available.

(oh yeah, and this is my first post. I've been lurking for a bit - great forum.)

Nate

[JFettig]

With H110 up to 10 grains behind a 220, it should only be at 24000 psi. According to my calcs and neglecting case friction, with a 16" barrel and a 1.5lb bolt, the bolt will travel about .4" when the bullet exits. It should be less than with a 9mm.

The main difference here is the pressure curve. .300 whisper uses slower burning powder than 9mm. It takes longer to burn and increases the time that the bolt is loaded. A 9mm with 33000psi burns the powder faster and isn't under pressure as long, it only opens about .2-.3".

They make bolts for simunition that have the lugs fixed back in the unlocked position, you'd need to add a ton of weight and open up the bolt face to accept a .300 whisper case head(they make them smaller).

Jon

[cav_scout_tj]

You could probably get it to work using light loads, fast powders and 9mm AR parts: 9mm bolt carrier group and 9mm buffer.

BUT, why would you want to?

Blowback would be far louder shooting subsonics. Super-sonics would be a no go, too much pressure.

The AR guys are trying to make a Direct Impingment 9mm work. DI is much smoother, and easier on the parts. Blow back is for 380s and 22LR. Anything bigger, you want a locked breech.

See:
http://www.ar15.com/forums/topic.html?b=3&f=15&t=366336

We have a perfectly good DI system, why mess with it? Get the gas port at the pistol positon, sized about .08", and rock and roll. I must be missing something.

[Fastshooter03]

So what's the reason a blowback would be louder?
On a 16" wouldn't a pistol length gas port open the bolt too soon?
I like trying to come up with different ideas of how things work. Most of the time it goes nowhere.
When I make my next 16" 300w I want it to work without too much fuss with 220 subsonics.
Do 9mm AR barrels have a special barrel extension?
I also thought of blocking off one of the gas exhaust holes in the bolt carrier or running the LBC "slickside" carrier which isn't relieved in the exhaust port area to help make use of the gas better in the ring area.
I'd be interested to find out if any of the piston uppers would work at all with 220Subs.


Nick

[Artful]

Originally Posted by Fastshooter03

So what's the reason a blowback would be louder?

It doesn't have too but generally blowback don't seal the chamber until the pressure in the barrel drops to null as it's using the gas to push the case and bolt back. You can change spring rates and bolt/buffer weights to balance this to optimize but it seems to work best with a fire upon closing blowback (open bolt SMG like an Sterling, UZI or STEn)

On a 16" wouldn't a pistol length gas port open the bolt too soon?

Again if gas operated you can time it buy changing spring and buffer weights as well as changing port pressure curve by chaning powder burn rates.

I like trying to come up with different ideas of how things work. Most of the time it goes nowhere.

That's normal, if progress was a straight line without branches we'd be much further along.

When I make my next 16" 300w I want it to work without too much fuss with 220 subsonics. Do 9mm AR barrels have a special barrel extension?

No, but bolt is designed for blow back operation and it has a heavier buffer.

I also thought of blocking off one of the gas exhaust holes in the bolt carrier or running the LBC "slickside" carrier which isn't relieved in the exhaust port area to help make use of the gas better in the ring area.

I'd be interested to find out if any of the piston uppers would work at all with 220Subs.

They can but you'll have to tune the amount of gas it taps to move the piston.

If you decide to go with blowback operation keep in mind you will have to adjust the components for a specific load.

Artful

[cav_scout_tj]

I think the barrel extension is the same for 9mm, many other parts change. Getting reliable ejection/extraction may be the hard part. Here is a 9mm bolt diagram from Olyarms:


I think blow back with a bottle necked rifle case might have too much pressure in the case neck, and you might get stuck cases or have the neck broke off in the chamber.

What was the previous whisper setup which required so much fussing with? If you follow the best practices in 300W, they are very reliable. Mine has been 100% with subsonics from day one. Supersonics required more tinkering with the loads.

[Fastshooter03]

I think you're right about the cases sticking.
Sounds like I should just go with a pistol length gas system and about a .080" gas port?
What is the specified port location dimension?
My fussy upper was a 16" with carbine system. Even with a port of over.100" it needed a slower powder like RL7 to cycle with 220s. It was shoot, wait a few seconds, then it would cycle. I think it didn't completely burn either and just made a mess.
Now I just need to get some blanks on order.


Nick