Why direct impingement?

[Fudgie Ghost]

Why did Stoner pick direct impingement as the operating system in the AR series?

What are it's advantages?

What other successful rifles utilize this system?

Enquiring minds want to know.

 

[Badger Arms]

Let's clarify... direct impingement has certain connotations that simply aren't true. The Stoner system takes hot gasses from the bore and cools them down over about 12 inches. The gasses are then directed into a cylinder formed by the bolt and bolt carrier. This makes the carrier a moveable cylinder where inertia is imparted by the high-pressure gasses. Once the carrier has moved enough and gained enough energy, excess gas is vented through two holes in the side of the carrier and out the ejection port.

Why did did he choose this. His patents speak for themselves. The primary patent that covers this is 2,951,424. The main reasons he states are lightweight, simpler mechanism by elimination of a separate gas system and using the bolt and carrier instead, and having in-line recoiling of all recoiling parts. Fouling is the major problem with this system. Stoner was an engineer and felt that the good outweighed the bad initially. Even he realized the error of his ways and ditched the system on every design subsequent to the AR-15.

 

[Detritus]

Swedish AG42 Ljungman, Hakim (egytian license built Ljungman, in 8mm mauser), and Rasheed aka Hakim redesign in 7.62x39 with Ljungman gas system, and SKS based features (such as the addition of a REAL bolt handle, and a folding Bayonet).

the Ljungman is a WW2 era rifle and is held in High esteem by many. i believe it was the G&A writer that produced the first article i ever saw on the Ljungman made the comment "shoots as good or better than any garand i've ever had". Gas operated, 6.5 swede, detatchable Magazine, it accuracy, coupled with it's semi-auto action are considered by some to have been a deterrent to a german invasion of sweden. one thing of note though is that the Ljungman has an action that is open at the top, so after the inital "impulse" of gas to work the bolt the gas and vaporized crud had a larger escape route than in the AR15/M16 design.

so yes at least a few rather sucessful rifle designs used direct impingment. the Ljungmann, and Hakim were/are still heavy due to the fact that they needed to be because of thier chamberings (6.5swede and 8mm Mauser) lighter guns would have beat the hell out of the shooter. going to a lighter round allows a direct impingment system rifle to be lightened to a great degree and not lose a great deal of accuracy.

stoner was trying to design the most compact, lightest rifle he could around the perceived "needs" of a soldier or shooter. and after what now 30years? the longest a single rifle design has been in first line service with the US forces, and with a tremendous number of NATO countries using the rifle on at least some level... i think that the system works and stoner did a great job. i advise any critics of the AR that say "oh it was a piece of carp when it came out, stoner should have known better" to go back adn LOOK at who made the "fatal decisions" that screwed up the M-16 those first few years, Ball powder (and other ammo deficiencies), no chrome lining, and lack of cleaning kits had NOTHING to do with Stoner, those can be laid at the feet of, army ordnance, Robert S. McNammara, and Colt Firearms respectively.

 

[Archie]

Detritus

The M-16 design would not have the problems it does IF the gases were not vented through the bolt. It's a self-dirtying action.
I've heard the idea was the high pressure gasses would "self-clean" the bolt everytime it was fired. Oh, well.

Had Stoner put a pushrod and vent in the gas tube and left out the piston ring setup in the bolt/bolt carrier group, the rifle would work just the same.

One of you mechanics, fool with that idea.

 

[Badger Arms]

so yes at least a few rather sucessful rifle designs used direct impingment.

Detritus: You are under the false impression that the Stoner system is similar to the Ljungman system. In fact, they are worlds different. The Ljungman uses direct gas impingement. Hot gasses are vented down a STIFF gas tube that acts as a cylinder. The front of the bolt carrier is a piston. The Stoner design allows a lightweight tube to be used as it doesn't take as much stress, it is merely a conduit for the gasses. It always gets under my skin whey they compare the two systems. This mistake is also made when people say that the AR-18 uses a FAL operating system. The differences in these systems is significant.

The AR-15 does NOT use direct impingement, it uses a standard piston like most other systems do today. The difference is that the piston and cylinder are the bolt and bolt carrier respectively. The Ljungman utilizes the gas tube for a piston and a recess in the bolt carrier as a cylinder.

 

[cheygriz]

Why direct impingement?

BECAUSE IT WORKS!

 

[Fudgie Ghost]

So let me see if I got this straight--- The AR series utilizes an action system (or one that is similar) that was heretofore used on three (rather obscure) military rifles of countrires not known for their small arms design, or may I say, even fighting prowess.

And the main reason for this was to save weight?

How much does the operating rod in an AR-18 weigh anyway?

Is there any real disagreement that the system Stoner put into the AR rifle does create more crud in the chamber/reciever over a set amount of rounds than an operating rod (a la AR-18, AK, FAL, etc)?

If not, I am still not clear on why he chose, what seems to me, to be an obscure, problematic system over one more tried and true.

Thanks for the information in any case. I know I an always count on the members of this board to inform and educate.

 

[Handy]

I think it's a mistake to talk about "tried and true" when talking about the decisions of arms designers in the 1950s. There weren't many designs to go by, and the Garand system has not become a fixture in any modern design. The three major designs of the era, FAL, Cetme and AR-10 are all still with us in various .223 forms. All were advances (or huge leaps) over previous designs.

Stoner was likely aiming at not just a light, simple system; but one that did allow the crud a place to flow to, rather than letting it condense on the piston and gas tube walls.

Before one spends any time critiquing the idea, ask yourself how much development it's actually had. Two rifles: AR 10 and 15, and that's it (which are essentially the same design). If ANYONE else has worked on such a system we would have more perspective on it. But it is a one rifle system and judging the system's pros on a single example is foolish.

One other note, the AR's accuracy is owed in large part to its gas system. The non-rigid gas tube makes the AR barrel mostly free floating as the tube can expand without putting pressure on the gas block.

 

[Gewehr98]

I'll agree with Detritus.

The Ljungmann does tend to operate much more cleanly than the AR-15, even though it's a direct impingement design. Then again, the AG-42B Ljungmann isn't routing the combustion products through the bolt carrier and bolt, it simply uses the gas pressure to push the bolt carrier straight back.

One would never guess, in this day of free-floated barrels and minimum moving masses, that such an obsolete design would be so darned accurate and easy to shoot, especially with a 55 millimeter long case, instead of an assault cartridge. But shoot they do!




Anybody seen the SR-25/AR-10 quasi-conversions that McCann's offers for the gas-impingement French MAS49/56? Weird!