AR Historical Development Question

[barnbwt]

Does anyone know at what point during the design process Stoner came up with the internally pressurized bolt? Before I examined the AR design with any care, I'd always assumed the gas-key was itself the piston, thrust away from the gas tube as pressure flooded in. I'm curious if the AR ever operated as such, and only subsequently became internally pressurized when it became apparent that not enough gas force could be applied to the top of the bolt body to cycle it without tipping or venting dangerously high volumes of gas through the ejection port.

TCB

 

[rcmodel]

No, I doubt it was ever even considered.

Eugene Stoner invented it, and the direct to the rear bolt carrier 'cylinder', using the bolt as the 'piston' to propel it, was a new concept based on earlier gas impingement systems that worked of something similar to bolt key impingement.
Stoner apparently understood all the drawbacks to the off-center bolt thrust, and designed something much better.

Center-line bolt carrier thrust.

His whole goal was straight to the rear recoil to make the gun more controlled in full-auto fire, dispute it's light weight.

From Wackipeedia:

In the Stoner system covered by U.S. Patent 2,951,424 Stoner states that the system is not a conventional impinging gas system, not that it is not an impinging gas system. ″This invention is a true expanding gas system instead of the conventional impinging gas system

rc

 

[Jenrick]

I'd agree with rcmodel. DGI had been around for a while in several weapons that had actually seen combat. Stoner would have been well aware of them and he was definitely a good enough engineer to see the advantages and drawbacks of what was out there.

Great question though.

-Jenrick

 

[Tirod]

The beauty of his design is that the gas expands in a cylinder, and as it pressurizes it, the bolt is pushed against the barrel to balance the pressure of the case head against the bolt face. That relieves the lugs from carrying all of the initial load, and lets them turn more easily to unlock.

A true impinging system would only add thrust to the carrier, and none to the bolt, which would have to take all the pressure of the brass against the bolt face onto the lugs.

This is why timing in the AR system is a lot more important that some give it credit - the gas pressure curve requires the action to conform at the right places. With carbine gas on a long barrel, gas pressure is much higher, and the bolt unlocks sooner with a lot more force. If the case is still under enough pressure and expanded against the chamber walls, the extractor slips or rips off the case head. If the case is partially extracted but pressure remains higher than the yield point of the brass, then explosive rupture can occur.

Most piston operated systems use guide ways or rods to control the bolt's travel to the rear, and a conversion to the Stoner bolt carrier results in it being levered on one side, tipping it in the other direction. Hence, "carrier tilt," which some vendors either ignored or could not mitigate. The cylindrical bolt of the AR requires thrust on it's axis to prevent carrier tilt. It's a system of compatible features, not a Chinese buffet of whatever you want.

What is curious has bee the focus in the DI system as it's main feature, when the barrel extension and controls are the most copied features in modern carbines over the last 40 years. We debate the merits of DI a lot, nobody questions the control layout is superior, or the simplicity of a barrel extension that relieves the receiver of all the major loads. It even allows the simplest method of headspacing without a press or gunsmith level training - assembly workers aren't machinist grade gunsmiths. It can even be done at home with just hand tools.

 

[briansmithwins]

Many other firearms used barrel extensions to take the load of firing before the Stoner rifle. The StG44, AK47, and MG42 come to mind.

The bolt isn't supported by gas pressure from the gas key while the loads are highest. The gas key doesn't see any gas until the bullet passes the gas port and by the time the bolt carrier is unlocking the bore gas pressure is dropping towards zero as the bullet has cleared the bore.

BSW

 

[Bartholomew Roberts]

I don't have anything to add; but as this is one of the few novel and interesting discussions of the AR gas system I've seen in several years, I'll give it a bump.

 

[sawdeanz]

Yes please explain more this is the first I've heard off the pressurized bolt and I'm sure others as well who get caught up in the piston ar hype

 

[MistWolf]

Eugene Stoner filed for the patent on the gas system used on the AR FOW in 1956

https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/US2951424.pdf

The beauty of his design is that the gas expands in a cylinder, and as it pressurizes it, the bolt is pushed against the barrel to balance the pressure of the case head against the bolt face. That relieves the lugs from carrying all of the initial load, and lets them turn more easily to unlock...

The pressure in the barrel s tens of thousands of psi. The pressure in the carrier might be as high as one or two hundred psi. There is no way the pressure in the carrier will balance out against the pressure in the chamber

 

[rcmodel]

Yes please explain more this is the first I've heard off the pressurized bolt

In simplest terms?
If you have a basic understanding of your car engine?

Gas/air mix is sucked into the cylinders as the engine rotates.
It is ignited by the spark plug and burns rapidly producing expanding hot combustion gas.
Which drives the piston down in the cylinder which rotates the crankshaft.

AR-15:
Bolt carrier = The cylinders in the engine block the pistons slide up & down in.
Bolt = the pistons inside each cylinder in your engine.
Gun powder = Produces the rapidly expanding hot combustion gas.

Upon firing, the bullet passes the gas port several inches down the barrel.
As the bullet base clears the gas port in the bore.
Low pressure gas comes out of the barrels gas port and enters the gas tube through the gas block, or front sight base.
It is piped back to the gas key mounted on the bolt carrier and directed inside the bolt carrier. (cylinder)

At that point, there is nowhere for it to escape, except to drive the bolt carrier backward, away from the bolt (piston).

As the carrier is driven backward off the bolt, a cam pin in the bolt follows a cam surface in the bolt carrier.
That rotates the bolt out of engagement with the locking lugs inside the barrel extension.

Momentum then drives the bolt carrier and bolt to the rear to eject the empty case, and load the next round in the magazine as the recoil spring drives it back foreword.

As the round is chambered, the bolt stops, but the bolt carrier continues driving forward over the base of the bolt again.

That causes the cam pin in the bolt to be rotated by the cam slot in the bolt carrier.
Which rotates & locks the bolt back into full lug engagement with the lugs in the barrel extension.

Just like a car engin cylinder & piston.
Except hopefully, the piston doesn't turn in your car engine and lock in place each air/fuel explosion in the cylinder.

rc