Blowback 300 project.

[MIKEONE]

Hey guys!

Ive been lurking around for a while here, im in the process of building a blowback operated 300 on the AR platform with a 9mm bolt and some other
little goodies.

My question is, have any of you attempted anything like this on any platform?

This will be a dedicated subsonic gun as to keep the pressures in the 25,000 psi range.

I have a feeling this puppy will be pretty cool once the EJECTOR is figured out.

Mike:smile:

[Colohunter]

Sounds pretty interesting, I'm not aware of it being done. Maybe it hasn't been done because of the higher pressure loads available for the platform.

[cav_scout_tj]

Use 9mm ejector parts, they should work.

Blow back is louder than DI. This just doesn't pass the "cost / benefit analysis" test.

[MIKEONE]

The main reason for the blowback is that a super short barrel can be run.
Im toying with ideas on how to delay the action a bit for more dwell time in the chamber.

I need a pic of the 9mm barrel extension and or how the barrel is machined where it fits in a 556 extension.

Mike

[cav_scout_tj]

All sorts of 9mm info:
http://thegunwiki.com/Gunwiki/AnalysisAR9mmRRA

A lot of uppers use an ejector on the mag block
http://www.rainierarms.com//img/shop/product/5591c98d3dacaae9506898fee0151a1f.jpg
or fixed in the lower.
http://www.ninthstage.com/wp-content/uploads/2006/03/9mm%20Lower%20S.jpg

Olympic uses one fixed in the upper:
http://www.ar15barrels.com/tech/oly_ejector_holes.jpg
http://www.olyarms.com/components/com_virtuemart/shop_image/category/arpr_pistol-cal-bca-parts.jpg
The parts in the box lower right are what get mounted in the upper. The finger on that spring ride in the left channel on the bottom of the carrier.
http://www.olyarms.com/index.php?option=com_virtuemart&page=shop.browse&category_id=53&Itemid=37

Note that the oly system and the colt system are very different.
9mm FAQ (http://www.ar15.com/forums/topic.html?b=3&f=15&t=108261)

[MIKEONE]

Fantastic info thanks!

Id like to know more about this set up fixed on the lower

http://www.ninthstage.com/wp-content/uploads/2006/03/9mm%20Lower%20S.jpg

But the OLY set up looks interesting also.

Its funny how I seem to have completely disregarded the 9mm AR set up until

now, Thanks for the info!

Mike

[cav_scout_tj]

Actually, thats sort of a misnomer, since yes the ejector can be fixed, but its part of a mag block like in the first pic. Those come in several flavors, some pop in from the bottom of the magwell, some from the top, and some of which are fixed in the lower, wedged with screws.

Since whispers use normal magazines not mag blocks, the Oly setup looks much better for your project since its part of the upper only.

I had a 40S&W Oly upper at one point, ran fine. Sold later.

[MIKEONE]

Im still working on it:rolleyes:

[tp555]

The 9mm barrel doesn't have an extension.Its all one piece.The oly system has a real weird extension with a taper.

[Fritzcat]

I have the Oly 9mm with a 10 1/2" barrel. The Oly system uses the mag agenced the front of the magwell and the upper can be pinned directly onto a 5.56 lower without any mods.

[MIKEONE]

Can you post a pic of the inside ejector side of the receiver so I can see how the thing sits in there?

Thanks Mike

[Garrett]

Just thinking in a different direction - could you use a .223 bolt & carrier and modify the bolt so it does not rotate and lock into the barrel extension? You would then have the ejector issue taken care of.

You could use the guide / key from a 9mm bolt.

Of course, it may end up being more work than using a modified 9mm bolt.

[redtazdog]

I'm waiting to hear someone say that it wont work well because the bottle neck brass wont get enough support in a blowback system without the brass getting blown straight wall looking like a 30 carbine or worse.
To keep the brass looking like 300 whisper the delay of the bolt moving will
need to be long i would think.

[Hoser]

It *could* work. Both the 9x19 and Whisper run around the same pressures.

[cav_scout_tj]

Thats silly, the 357 sig and 400 Corbon are bottle neck pistol rounds. They run fine blow back.

The AR57 uppers are blowback, and the 5.7 round is a higher pressure bottleneck round.

[redtazdog]

The lugs on the bolt hold the bolt closed longer with the delay of the gas
then the hi preasure is reduced to safer low levels before the bolt comes back.
on a blowback the bolt comes back with very little delay, in fact its
almost instant.
Yes a pistol is blowback but they are made to have delay before
the slide comes back on some calibers like a 357 sig.
If you think a blowback 300 whisper has not been tried,
call Olympic Arms or JD Jones and ask how there AR blowback
prototypes in 300 whisper and 357 sig worked. :wink:
They will tell you they did work but the brass was destroyed after
the first round.
Oly tried a AR 357 sig prototype using a 40 sw bolt and a 9mm barrel rechambered for 357 sig
and it blew up the brass.
There is a artical about a blowback AR 357 sig that Kurts Custom firearms built and it destroyed the brass but was fun.

[redtazdog]

It *could* work. Both the 9x19 and Whisper run around the same pressures.
9x19 is not a bottle neck like 300 whisper.

[redtazdog]

Thats silly, the 357 sig and 400 Corbon are bottle neck pistol rounds. They run fine blow back.

The AR57 uppers are blowback, and the 5.7 round is a higher pressure bottleneck round.
357 sig and 400 corbon pistols have barrels that cam back and cause a delay
to reduce the pressure before the slide comes back.
If the barrel came straight back without delay the hi pressures would
make a straight wall case out of a 357 sig or a 400 corbon.
.
Figure out how to delay the bolt from coming back until the hi pressure is dropped and a bottle neck
will work in a AR.

[MIKEONE]

LOL, Well we will just have to see.

[Hoser]

9x19 is not a bottle neck like 300 whisper.

And your point is?

I was just saying it could work. Not that it should/would work.

I dont have enough experience with blowback rifles, other than a SBR 9x19 and a SBR PS-90, to know if the case design would be too much of a player.

[cav_scout_tj]

357 sig and 400 corbon pistols have barrels that cam back and cause a delay
to reduce the pressure before the slide comes back.
If the barrel came straight back without delay the hi pressures would
make a straight wall case out of a 357 sig or a 400 corbon.
.
Figure out how to delay the bolt from coming back until the hi pressure is dropped and a bottle neck
will work in a AR.
When forming 300W brass from .223 brass, the shoulder needs to be fireformed, and blown out a little. Shooting subsonics does not generate enough pressure to do this, have to use more powder. As such there is no way there is enough pressure straighten out the bottle neck shooting subsonics. Supersonics is another matter....

If you found a way to delay the bolt, then this would not be a blow-back project anymore. It would be direct impingment, or delayed roller lock or short stroke piston or whichever...

[redtazdog]

I know how to fireform brass as I have been for over 20 years,
around 15 years just with the 300 whisper.
I made a 357 sig in a blowback AR a couple years ago just to use sub
ammo and found that the brass would blow out to easy when i used a can so I removed the barrel and made it a 40sw.
If the 300 whisper works as a blowback then great!
but I'm thinking there may be problems so I'm waiting for this build to
get done and tested. :wink: