Hybrid Gas Tube and Tappet System?

Has any manufacturer experimented with a hybrid gas system with an M16 style gas tube feeding a gas cylinder mounted on the front of the receiver containing an M14 style gas cutoff and expansion tappet that would push the bolt carrier to the rear.

An AR upper using such a system would retain the gas tube from the front sight base. It just wouldn't extend into the upper receiver. Instead, I would mount a gas cylinder on the front of the upper receiver at 12 o'clock (above the barrel). The gas cylinder would have a hole at 6 o'clock to admit gas from the gas tube. The cylinder would contain a gas cutoff and expansion type tappet (a la the M14) with a matching 6 o'clock gas hole. Expanding gas would enter the tappet and push it to the rear. At the same time, the rearward motion of the tappet would minimize the amount of additional combusion gasses entering the gas cylinder. The tappet would push the bolt carrier to the rear through a hole in the upper receiver (a la the FAL, SKS and other designe).

I'm not an engineer, but intuitively, it seems like such a system would minimize barrel bending moment, weight and combustion gasses entering the receiver. It would also eliminate springs above the hot barrel (the bolt carrier would push the tappet back into place just as the operating rod on an M14 does). You'd just have to implement the system in a way that would permit easy cleaning of the gas cylinder, piston and tube.

I'd be interested to hear what solidpoint and other participants in the ideal AR thread think.

part of the problem is the gas tube's propensity to clog. I'm not sure how common that is, since we are using cleaner burning powders now, but if that puppy clogs, then there is no way to clear it and you just made yourself a bolt action. An alternate idea is to have a free floating piston system just above your barrel and have it fed by a "Gas tube" thereby minimizing the attachment issues. IN fact it doesn't even have to be "free floating" since the barrel is the important part. I'm currently working on something like that in my "Ideal AR" development process (as shown in the Ideal AR thread).

BTW, good thinking though. I was just thinking about the same thing the other day.

Your free floating piston system idea had occurred to me as well. Specifically, I was wondering if you could get enough gas for a cutoff and expansion type system by aligning a hole in the top of a free floated barrel with an identically sized (or larger) hole on the bottom of a free floating cylinder above the barrel with enough of a gap between them to keep the barrel free floated. You would get gas leakage, but as long as the cutoff and expansion system got enough gas under all circumstances, that would be fine.

I'm sure the gas jet would impart some downward barrel bending moment on the barrel, but you could address that at least two ways. First, you could locate the gas port to minimize such barrel bending moment (balancing higher gas pressures closer to the breach against more leverage closer to the muzzle). You could also use a stiffer barrel (heavy, fluted or composite (e.g. ABS's carbon wrapped barrels).

Maybe the solution is not so much to have a gas piston tube, but a reciver that incorporates a rail for the gas piston assembly and allows the "gas jet" to impinge on it without it being inside the bolt track housing (like the AR-10/15) or having it touch the barrel. Most piston operated designs allow it the piston to contact the gas block on the barrel. A tappet with a standoff distance might do the trick. the distance between the tappet and the end of the op rod would be something like 1/16 of an inch, while the travel of the tappet could be as much as a quarter of an inch. the tappet punches the "rod" which rides on rails. When the rod returns to postion, the rails stop it before it contacts the tappet and gas block. The rails could be part of the reciver and to keep weight down, be steel inserts in a polymer frame.

I posted this idea on another board and someone pointed out that the old HAC-7 had a hybrig gas tube and tappet system.

The possibility of a clogged tube would be a drawback, but it woud be a good way to supply gas to a piston system isolated in the receiver while allowing the barrel to float. That way, the gas would push against the piston and cylinder (part of the receiver) and minimize or eliminate the forces exerted on the barrel.

I like Coolhand's idea, too. Instead of a tube, maybe you could get enough gas to "jump" accross a small gap to power a gas cutoff and expansion system like the ones on the M14 and M60, but some folks have told me they think port erosion would become an issue.

I stumbled accross the same concept in Collector's Grade Publications' SPIW The Deadliest Weapon That Never Was. At one point in SPIW development, they ditched the primer actuated operation in favor of a floating piston fed by a gas tube. IIRC the piston was even mounted on the receiver.

Found out that the ZM rifles use an extended gas key. It extends outside the reciever, so any of the excess gas would be bled off into the handguards instead of into the reciever. Still uses some of the gas bled off through the bolt carrier to "soften" the unlocking of the bolt from the barrel extension, so its still kinda dirty, but it should cut down on the overheating and the amount of carbon fouling.

I read that, too. It sounds like ZM might have been as interested in getting the action spring out of the buttstock (to permit the use of a folding stock) as in changing the gas system. Having excess gas go out of the vent holes in the side of the carrier instead of into the receiver may have just been gravy. I'll try to learn more about it.

Eugene Stoner's original direct gas impingement design fed gas through a hole in the side of the upper receiver into a hole in the side of the carrier. When the gas inside the carrier pushed it back, the hole in the carrier moved away from the hole in the receiver, cutting off (or at least restricting) the flow of gas. In that sense it was sort of like the White gas cutoff and expansion system on the M14, in which gas goes through a hole in the side of the cylinder and into a hole in the piston where it expands and pushes the holes out of alignment, cutting off the gas flow). Another Armalite designer moved the gas tube position to the top to make manufacturing more economical.

There are actually 2 excellent choices currently made:

Patriot Ordnance Factory - long-stroke adjustable (not self-regulating) gas-piston system. All gas components are chrome-plated or silicon/nickel coated.

Leitner-Wise Rifle Co. - short-stroke self-regulating gas-piston system.

POF plans to make a Grendel later this year, as soon as Wolsf ammo is available.

Leitner-Wise will build one now if you send them a barrel and 40 rounds of ammo.

Has any manufacturer experimented with a hybrid gas system with an M16 style gas tube feeding a gas cylinder mounted on the front of the receiver containing an M14 style gas cutoff and expansion tappet that would push the bolt carrier to the rear?

I'm aware of the traditional piston-and-operating-rod systems for the AR. I was asking about a system that would isolate the barrel from the mechanical forces of a traditional piston-and-operating-rod system while keeping gun gas (or at least most of it) out of the receiver. In a hybrid system, a gas tube would feed a cylinder mounted on the receiver. The gun gas would push against the receiver and a piston tappet - not atainst a gas manifold mounted on the barrel. It would also reduce reciprocating mass.

IMO a piston-and-operating-rod or hybrid system isn't necessary. IMO there are pros to direct gas impingement (e.g., the forward gas pressure on the bolt offsets some of the rearward pressure exerted by the casing on the bolt, reducing the stresses associated with turning the bolt). The effects of the heating of the bolt carrier group by gun gasses can be mitigated by using chrome silicon springs. Cleaning can be made easier by using modern coatings.

Many very informed people believe that the reliability (and accuracy) of the AR relative to the AK, for example, has more to do with operating tolerances than the direct-gas-impingement system. The fact that the M16 was found more reliable than roller-locked and piston-and-operating-rod systems including during the Marine and NATO small arms trials in the '70s and '80s and UK SAS trials in the '80s or '80s suggests reliability isn't a major issue.

The number one drawback to DI is not that it brings fouling back into the receiver. As you have stated, that can be cured with modern coatings etc.

The huge problem is that DI brings excessive amounts of HEAT into the receiver, and THERE IS NO WAY to eliminate heat from a DI system, that is why the military is buying a tappet system (FN SCAR).

To illustrate how much heat is present in the receiver:

If you were to run 200-300 rounds full-auto through a standard AR-15 you will melt the gas tube?!?! :eek:

I hear what you're saying, but IIRC the basic combat load is 210 rounds. (I know infantrymen carry all they can and understand that is is possible to need to dump seven to ten magazines in a row in combat, but how often does it happen?)

Also, didn't Colt start using a heavier gas tube when they started making LMG's out of M16's beccause it was required in order to fill the LMG role? I assume that would be included on M16s intended for sustained fire. I've seen photos of M4 barrels that have failed before the gas tube. IIRC That's why the SOCOM M4s are now heavier under the handguards.

As to the rest of the system, chrome silicon springs can handle heat better and better materials can minimize heat embrittlement. That said, IIRC, the bolt carrier group components already last longer than the military specification. For example, Colt and others have repeatedly tried to sell the military more durable bolts, but the military doesn't want to pay for them/introduce additional SKUs to the supply chain.

Let me be clear - I am not an M16 Kool Aid drinker. I just want to see some very compelling reasons before we spend god knows how many taxpayer dollars to replace a system that has served us well for 40 years. The replacement had better be demonstrably and significantly better than the existing system and/or more economical upgrades to the existing system.

If you were to run 200-300 rounds full-auto through a standard AR-15 you will melt the gas tube?!?! :eek:

On several occassions I've run *at least* 200 rounds through one of my ARs with a RDIAS as fast as I could change mags... Gas tube didn't melt, didn't even warp.

I've also handled the bolt carrier after one of these full-auto orgies and it wasn't particularly hot... (I was swapping uppers to let that one cool down). While I had the bolt-carrier in my hand, the guy who's silencer I was torture testing sprayed some Rem-Oil onto the silencer and the Rem-Oil ignited on contact in a big flash.

Has any manufacturer experimented with a hybrid gas system with an M16 style gas tube feeding a gas cylinder mounted on the front of the receiver containing an M14 style gas cutoff and expansion tappet that would push the bolt carrier to the rear.

An AR upper using such a system would retain the gas tube from the front sight base. It just wouldn't extend into the upper receiver. Instead, I would mount a gas cylinder on the front of the upper receiver at 12 o'clock (above the barrel). The gas cylinder would have a hole at 6 o'clock to admit gas from the gas tube. The cylinder would contain a gas cutoff and expansion type tappet (a la the M14) with a matching 6 o'clock gas hole. Expanding gas would enter the tappet and push it to the rear. At the same time, the rearward motion of the tappet would minimize the amount of additional combusion gasses entering the gas cylinder. The tappet would push the bolt carrier to the rear through a hole in the upper receiver (a la the FAL, SKS and other designe).

I'm not an engineer, but intuitively, it seems like such a system would minimize barrel bending moment, weight and combustion gasses entering the receiver. It would also eliminate springs above the hot barrel (the bolt carrier would push the tappet back into place just as the operating rod on an M14 does). You'd just have to implement the system in a way that would permit easy cleaning of the gas cylinder, piston and tube.

I'd be interested to hear what solidpoint and other participants in the ideal AR thread think.



Check out the gas piston conversion at Ares Defense.

It is setup for carbines, but...

Thanks gewing, but that's not what I mean. Again, I'm well aware of the many piston-and-op-rod systems for the AR.

What I am talking/asking about is a HYBRID/combination of the M14 (White gas cuttoff and expansion/tappet) and M16 (gas tube/direct gas impingement). As noted above, a gas tube has been used with a piston attached to the operating rod in the Hac-7 and with a free-floating piston of the SPIW prototypes. AFAIK, no one has tried it with a White-style tappet like the M14s. If the tappet was housed in a front receiver extension, it would keep most of the gun gasses and heat out of the receiver while isolating the barrel from mechanical forces - the best of both worlds (although gas tube failure would still be possible).

I like your idea Essayons. I was actually thinking of something similar. My current efforts have been scaled back to using 90% AR internals with a new arangement and magazine/Mag well to get the effect on design length and balance I want. I might wind up looking at the hybrid system after I figure out how to make a DI system work with the design I have in mind.