AR possibilities

[tombogan03884]

That's why HK uses a fluted chamber. It's the only way they can get their basically blowback rifles to extract.

[Big Frank]

I never had anything with a fluted chamber. I think I know how they work but I could be wrong. I think a fluted chamber would make what les snyder was talking about even worse. It seems to me that when the case extracts, the neck starts expanding as it's crossing the shoulder of the chamber and keeps expanding up to body diameter. Wouldn't a fluted chamber leaking gas between the chamber and case offer less resistance, speeding up the extraction that's already too fast? I know blowback firearms need a heavier bolt and/or stiffer springs to handle more powerful cartridges, like in a a grease gun vs. a 10/22. And you need the right balance of bolt weight and spring tension/compression. I think that's about all there is to it. I can't think of any blowback operated guns that come from the factory chambered for bottleneck cases, just straight-wall pistol cases and and WSL rifle calibers. Maybe fear of the cases splitting open when the neck expands is why.

I haven't seen the inside of mine in years but IIRC the Mech-Tech CCU has 2 springs that stretch, rather than compress under recoil like most blowback guns. Like the screen door springs our WWII-era barracks had on North Fort Polk in the early '80s.

[tombogan03884]

The fluted chamber creates a cushion of pressure out side the casing to prevent it from sticking .
I suspect it also bleeds off some of the excess pressure . It's a work around to make a poor design function.
You could make a pure blow back 30 - 06 but you would need a 27 Lb bolt.
The reason you don't see blow back in bottle neck cartridges is because the available ones now are to high powered for safe blow back operation.
7.62 X 25 was a SMG round and .357 Sig was both to potent and never popular enough to be worth the investment.

[TAB]

The fluted chamber creates a cushion of pressure out side the casing to prevent it from sticking .
I suspect it also bleeds off some of the excess pressure . It's a work around to make a poor design function.
You could make a pure blow back 30 - 06 but you would need a 27 Lb bolt.
The reason you don't see blow back in bottle neck cartridges is because the available ones now are to high powered for safe blow back operation.
7.62 X 25 was a SMG round and .357 Sig was both to potent and never popular enough to be worth the investment.

I have an ar in 9x25 dillion that is just blow back.  I cant telk you the number of springs i destroyed getting it to run.   Its not 100% and i doubt i would ever get it to be so, but its just a tinker toy.

Just before i built it i did a google search to see if any one else had done it.... what did i find?   My posts on here asking if any one ever had about 8 years prior.     Now somethibg like a roller delay might work, but now you are talking a different gun altogether.   I tried drill out some spots on the bolt carrier and adding some tungsten, but it just did not work. 

[tombogan03884]

I think Les Snyder will have some suggestions here .
From what you said about trashing springs I'm ASS U Me ing  (    ) That the bolt is slamming back to hard .
Try ADDING weight to the bolt so getting it moving takes more energy.
Another thing is sometimes they use heavier mag springs so the feed keeps up with the bolt travel speed.
I don't know about the chambering, but that CMMG radial blowback upper may help.

[Sponsor]

[TAB]

No, the prob was i was using way to heavy of a spring and would cut coils til i found one that worked.  Between that and adding/ subtracting bolt wieghs i think i went thru about 50 springs.   I even tried doubling them for a while.  It was one of those things if i had to do over again i would have a custom bolt made and tried using the gas system. 

[les snyder]

my first dive into the PCC pool was with a MechTech Glock format...the extension spring (think screen door) was immediately identified as a problem, as I was looking to produce the lightest recoiling carbine possible... I looked hard at a Granger catalog but just couldn't find what I was looking for, in addition to one of the pistol lowers had a tolerance problem, and would go full auto... I didn't even get a chance to run some cartridges across the chronograph...historical note... I did shoot PCC division for a little over a year with a blow back 9mm... using an NFA lower and NFA bolt with a 6.2oz rifle buffer and rifle length AR buffer tube and spring... I then switched to the CMMG radially delayed bolt..


a half dozen years ago I purchased a 16" pencil barrel, can't remember the manufacture... it did not produce the accuracy that I was looking for... with the better pencil barrels now available, and as I have a couple of spare uppers, I may look at a light weight carbine for a project... re:  AR pistols, Tom has seen what I use for a buffer tube for my AR pistols... I think they are the quickest handling of any of my ARs, and would be a tough call not to grab one for any civil unrest ...  if I lived where auto loading rifles were not legal, a 16" model 92 (not 94) in .357 mag would be my pick

[Rastus]

<snip>
a half dozen years ago I purchased a 16" pencil barrel, can't remember the manufacture... it did not produce the accuracy that I was looking for... with the better pencil barrels now available, and as I have a couple of spare uppers, I may look at a light weight carbine for a project... re:  AR pistols, Tom has seen what I use for a buffer tube for my AR pistols... I think they are the quickest handling of any of my ARs, and would be a tough call not to grab one for any civil unrest ...  if I lived where auto loading rifles were not legal, a 16" model 92 (not 94) in .357 mag would be my pick

For a lightweight build I made about the same time (14-1/2" bbl) I picked up the lightest I could find which was at Spike's Tactical.  It is a forged barrel and is really accurate...I don't know what another 1-1/2" would do to the accuracy but I suspect not much.  Come to find out...it was produced by FN.  Just my 2 pennies....

[Big Frank]

my first dive into the PCC pool was with a MechTech Glock format...the extension spring (think screen door) was immediately identified as a problem, as I was looking to produce the lightest recoiling carbine possible... I looked hard at a Granger catalog but just couldn't find what I was looking for, in addition to one of the pistol lowers had a tolerance problem, and would go full auto... I didn't even get a chance to run some cartridges across the chronograph...historical note... I did shoot PCC division for a little over a year with a blow back 9mm... using an NFA lower and NFA bolt with a 6.2oz rifle buffer and rifle length AR buffer tube and spring... I then switched to the CMMG radially delayed bolt..


a half dozen years ago I purchased a 16" pencil barrel, can't remember the manufacture... it did not produce the accuracy that I was looking for... with the better pencil barrels now available, and as I have a couple of spare uppers, I may look at a light weight carbine for a project... re:  AR pistols, Tom has seen what I use for a buffer tube for my AR pistols... I think they are the quickest handling of any of my ARs, and would be a tough call not to grab one for any civil unrest ...  if I lived where auto loading rifles were not legal, a 16" model 92 (not 94) in .357 mag would be my pick

I couldn't remember if they had a pair of extension springs or just a single spring, but yeah, they're just like those screen door springs. They pull instead of push which is a little unusual for firearms. I've seen a whole lot of guns with compression springs and the occasional helical torsion spring, but not many extension springs. There are some in the trigger groups of a few guns. I think my Remington 12 gauge has one for the shell lifter but I may be thinking of a different gun. That one's overdue for a cleaning if I ever get my front Fire Sight on. I think I have everything else done and have to detail clean it.

The AR-15 ejection port cover spring was one of the first torsion springs that came to mind but it seems like almost all gun springs are compression springs. Not much of anything else that I know of in commercial firearms, but I don't know much about the innards of most of them. Back in the army someone brought an M240 in that was messed up. The buffer spring is a stack of Belleville washers that are supposed to be stacked like ()()()() and someone took the buffer assembly apart and put them all back in like )))))))). They took up less than half the space inside the buffer tube and it didn't work. So it battered the hell out of the bottom of the hole when the piston drove the bolt and operating rod assembly into it where there should have been a cap sticking out with a stiff spring (the washers) behind it. I thought they were called bell washers but looked them up before and they're Belleville washers. They look just like heavy duty flat washers except they aren't flat.

https://en.wikipedia.org/wiki/Belleville_washer

[Big Frank]

I downloaded a picture of a MechTech CCU recoil spring from their site. It's just like a screen door spring like I said. They sell replacement springs but I don't know how to tell when my spring needs to be replaced. That's the only reason I didn't buy one with my other stuff. If it was a compression spring I could measure it in a relaxed state and see how long it is. Now maybe I can measure it with a certain amount of weight hanging on it to see how much it stretches. Or just wait and see if the bolt beats up the rubber buffer until it causes some damage. I'll have to ask about it.