Rifle Design Question #2

[TheToyMaker]

Thanks to Bullseye 57 for the answer to question 1. Since the AR's method of barrel attachment will not work for a variety of reasons, I find myself forced to design my own barrels.

At any rate, here's question 2.

In a blowback design with no mechanical lock, is there significant rearward force on the barrel or is it all applied directly to the bolt face?

Example:

A Ruger 10/22 cycles via free blowback. That being said, the force used to cycle the bolt is applied directly to the bolt face. But is there any other force that might attempt to force the barrel rearwards, as was theorized in the long-dead Savage automatic pistols?

 

[Hoppy590]

hot gases exiting the barrel will great a pressure change that would want to force the barrel ( or anything else that stands between hi and low pressure) rearward. i barely passed HS conceptual physics so i cant say how much it would want to move, but i imagine it WOULD want to move

 

[TheToyMaker]

Since the G3 receiver is unstressed and I believe that the barrel is fixed directly to it, I can't imagine the forces being all that significant. Thanks for the help.

 

[Bohica_arms]

Hi -

The G-3 Reciever uses a heat treated steel trunion that the barrel is pressed & pinned into. The rollers pop out into a detent and are held in place by the firing pin efectively locking the bolt in place until the pressures have dropped significantly.

IMHO you are asking for injury or possible death and probably lawsuits if you build a unlocked blowback rifle caliber weapon. Pistol calibers work because the gas expansion force is equal in all directions, there is more case area on the sidewalls than at the base of the cartidge. While this is true for rifle caliber cartiges as well the chamber pressure spike is much longer and will send the bolt backward with VERY high velocity. Probably destroying the weapon at the same time.

There is a good review of what pressures can do on an aluminum reciever at www.dsarms.com. If memory serves a LOCKED action FAL lower in aluminum was destroyed by the pressures after a very low number of rounds.

The reason the AR-15 works is because the bolt is locked to the barrel extension. You could fire a AR-15 barrel with a bolt and no rifle arround it. (not recomended)

I encourage you to seek some professional design help in your project (no offence intended) - I really hate to see some one injure themselves because of something they missed in a design.

Good luck in your endeavour

Chris Rosequist
Bohica Arms Corp

 

[GunTech]

The aluminum receiver on the FAL was destroyed after shooting proof rounds, followed by commercial 308 (commercial 308 can be loaded to higher pressures than 7.62x51 NATO). There were many examples of many thousands of rounds fired through the aluminum FAL prior to DSA. For those of us who followed the whole debacle of the aluminum receiver of the FAL files, the concern didn't pop up until DSA blew one up. There is no denying the aluminum FAL is much less strong than the steel one, but DSA's interest in warning customers away from it had more to do with competition than concern for safety.

An aluminum receiver can be used if designed properly and the right alloys are used. Typically, you will want to install a steel or similar locking block, or use a steel barrel extension like the AR. Locking lugs should not rest on aluminum.

 

[Bohica_arms]

GunTech,

That's my whole point about the locking block. I was using the DSA receiver to make a point that that was a LOCKED bolt action and still failed catastrophicaly. (sp)

I would suggest that Toymaker get a Cetme or G3 kit from someone like Centerfire or GB and take a careful look at how the delayed blowback system on HK rifles works. Another good example is the Benelli M2 shotgun (recoil operated)

JMO

Chris Rosequist
Bohica Arms Corp

 

[Monkeybear]

Just a quick question, do you also post as Nolo ?

 

[Bohica_arms]

Who? - Not me!

 

[GunTech]

Benelli M1 and M2 are recoil unlocked blowback. It relies on a fairly decent recoil and a high bolt to gun mass ratio. It's not going to work for any rifle with a high pressure round and moderate recoil.

Delayed blowback requires very good enginnering. The difference between safe and dangerous unlocking is pretty small.

 

[corncob]

Yes, there is a rearward force on the barrel as the rifle fires. The (enormous) pressure on the inside of the case forces the case against the chamber, causing a proportionately large frictional force. This force is why all the delayed blowback rifles have fluted chambers. Otherwise, the case bodies actually "stick" to the chamber walls while the case heads blow off--very bad. There is a reason no one builds them anymore.

Also, don't forget that there is also a large forward force on the barrel as the rifle fires due to the the projected forward area of the bottlenecked rifle case.