Gas tube/piston over barrel... or under barrel

[Frohickey]

I noticed that the M1Garand and the M1A, the gas piston/op rod mechanism is underneath the barrel. That tends to place the sights much closer to the bore axis, and I would think that this would be a good idea to have.

On the FAL, and on the AR15/M16 and SU-16 and AK variants, the same mechanism is over the barrel instead.

Is there a particular reason this was done? Would putting it above instead of below the barrel make for a much simpler and reliable design? Or is it about heat from the gas operation?

 

[PO2Hammer]

I think the under barrel design is old school thinking.
The over barrel designs have one advantage, less muzzle rise. It is also directly in line with the bolt, which simplifies design. Reliability could go either way.
I don't see any advantage of having the sights closer to the bore line.
If they are higher, you get a little longer 'point blank' range, as the bullet rises up to the line of sight at a greater distance.
I don't think heat is an issue, the barrel is probably hotter than the piston/cylinder or gas tube.

 

[Badger Arms]

The main reason for having the gas system below the barrel was to make the gun acceptable to the 'old school' as was said earlier. It was specified that the new autoloader must follow, as close as possible, the size, shape, weight, and look of the old rifle. I guess they had settled on the best proportions and the designer be darned if he had a better idea.

 

[shoobe01]

With the piston over the barrel, and the mag in its normal place down below, the modern design rifle can have a straight rod of metal. Lighter, less movement, and much cheaper to make. The dog-leg on the Garand style op rod is somewhat squirrley, and just works because good people spent a long time designing and making them.

Note that most current MGs are flipped over. Since the feed is up top, they have an inline piston underneath the barrel. So, same theory.

There is significant evidence that the closer (lower) you are to the boreline when shooting in battle, the more likely you are to survive. Less head exposed and all that. This is hard to accomplish with modern inline or bullpup designs. Also, offset (the distance between the sights and bore) becomes significant at close ranges. For 3 yard shots (which happen all the time in cities and such) you will hit a couple inches lower than the aimpoint says. That could matter a lot.

 

[Badger Arms]

The FG-42 had a piston under the barrel made possible by a magazine which fed from the side of the receiver. The Johnson 1942 completely did away with a gas system in favor of a Johnson multi-lug bolt and recoil operation. (yes, that year was a good one for small arms development FG-42, Johnson 1942, MG-42) The Original patent for the AR-15 Gas System shows the gas tube going to the side of the barrel! Not many side-operated gas systems around, eh?

There is significant evidence that the closer (lower) you are to the boreline when shooting in battle, the more likely you are to survive.

In the book, "Shots Fired in Anger," The author relates how the BAR gunners had an advantage over the Jap NAMBU gunners because you could see their magazine columns sticking up and bouncing rhythmically above the grasstops. The BAR gunner would simply aim the burst a little below that point and the enemy MG would stop rattling. That said, the Garand is the ultimate low profile weapon. The Johnson LMG had an advantage over the BAR in that respect also. Mag changes could be done in position and from a lower profile generally.

 

[Jim K]

The main reason for putting the gas piston/tube over the barrel is to keep the barrel closer to the center of mass line of the gun and reduce upward movement in full auto fire. It is the same reason as for using the pistol grip.

A rifle recoils around its own center of gravity and only secondarily around the center of gravity of the gun-shooter system. So anything that keeps the barrel in line with the gun's center of gravity and also in line with the shooter's shoulder reduces muzzle climb.

Jim

 

[Frohickey]

So, if you could actually design a cylindrical action, that wraps itself around the barrel, and also an ammunition magazine that is cylindrical in configuration that wraps around the barrel/action, that would be the least recoiling/muzzle climb firearm?

Hmm... I could see how it could be made to work, but it sure won't look like your traditional rifles or firearms.

Maybe some Japanese anime franchise has already made drawings of such a beast.

 

[Jim K]

That is about it. If you could get the center of mass of the gun on the center line of the barrel, the gun would recoil straight back with no climb at all. But of course, the gun is only part of the gun-shooter system. When firing any gun with significant recoil from an upright position, the tendency is for the top body mass of the shooter to move back with recoil, leaving the feet in a fixed position. When that happens, the shooter pulls the gun upward (try it!), regardless of what the gun itself does.

So recoil reduction is still important, which is why designers of full auto rifles went to light cartridges. While there were other reasons, the main reason for adopting the M16 and the 5.56mm was that the gun is controllable in full auto fire, while the M14 was not.

Jim

 

[shoobe01]

but it sure won't look like your traditional rifles or firearms.

Sure it could. Think about the layout/look of the recoiling action rifles. The Barretts, Auto-5 shotgun, Johnson rifle (and I am sure others) have a moving /barrel/ and look like normal guns.

Think about the action tubes wrapped around the Benelli or some other shotgun's feed tubes (11-87?). That's what a piston around the barrel would be like. Seems to integrate too many systems together, so the barrel cannot change over time to suit design/mission changes.

Anyway, the position of the piston seems to be of no actual significance to actual recoil forces. When the AUG came out, the gas tap being at the 8 o'clock position freaked out a bunch of people, who decided the gun would twist, or recoil sideways or something. Actual army tests proved them wrong, and I have fired one. The bullet is throwing all the recoil, and past that just the bolt mass.

Turning off the gas on my FAL, for example, seems to have no notable effect on the recoil of the whole weapon, incidentally.