Long barrel on M1 Garand or Springfield M1A ?
- Thread starter Hammer1
- Start date
Jim Watson
New member
Never saw one.
But I have an AR with a 28" barrel by Krieger so I would call them first.
But I have an AR with a 28" barrel by Krieger so I would call them first.
An interesting proposal. Gas pressure will stay higher in the system and it will not drop off as readily as with the normal length barrel.
Just some loose thoughts. The helix of the op rod controls the rotation of the bolt. The additional gas will cause the bolt to slam back more violently. This in turn may impact the receiver. Cracked receiver? It might happen.
Just some loose thoughts. The helix of the op rod controls the rotation of the bolt. The additional gas will cause the bolt to slam back more violently. This in turn may impact the receiver. Cracked receiver? It might happen.
Dixie Gunsmithing
Moderator Emeritus
To do it correctly, the gas cylinder would have to be moved forward, too, which means increasing the length of the operating rod to match it. That increases the mass of the rod, so something might have to be done for it, I'm not sure. The timing is what needs to be kept, as the bullet needs to leave the barrel by the time the dwell portion of the cam stops, and the cam starts unlocking the bolt. That's about 3/8" of movement, and keeping it timed is a must.
Not sure about the M14 with its cutoff and expansion system, but with an M1, moving the gas cylinder out would put the gas port into a very low pressure area and I doubt the gun would function. Increasing barrel length past the gas port might have complications as well.
The problem with full/semi autos, more than with manually operated rifles, is that the whole package is designed to work as a unit with established parameters. That is why so many folks who try to "tweak" the AR-15 have problems; they don't understand interior ballistics well enough to know what effect, for example, a shorter/longer barrel or a bigger/smaller gas port will have on the rifle in the long term, even if it appears all right for a few rounds.
Jim
The problem with full/semi autos, more than with manually operated rifles, is that the whole package is designed to work as a unit with established parameters. That is why so many folks who try to "tweak" the AR-15 have problems; they don't understand interior ballistics well enough to know what effect, for example, a shorter/longer barrel or a bigger/smaller gas port will have on the rifle in the long term, even if it appears all right for a few rounds.
Jim
Dixie Gunsmithing
Moderator Emeritus
Jim,
My thought is, one would have to move the gas cylinder on out, and enlarge the gas port or the piston diameter for more area, or decrease the gas vent size. The extra weight of the operating rod would have to be countered, and this would most likely solve the timing, and the rod mass problem. The only other thing that could be done about the rod would be to adjust the spring pressure.
That is the problem with this type of gun, as everything is tuned to the other to time it.
My thought is, one would have to move the gas cylinder on out, and enlarge the gas port or the piston diameter for more area, or decrease the gas vent size. The extra weight of the operating rod would have to be countered, and this would most likely solve the timing, and the rod mass problem. The only other thing that could be done about the rod would be to adjust the spring pressure.
That is the problem with this type of gun, as everything is tuned to the other to time it.
Fully agree. That is why ideas like the OP's are good as ideas, but not feasible in reality. And I know of no long barrels being made for either rifle; a good gunsmith could probably make one from a blank, but it would take lots of time and money, probably more than any ordinary person would want to pay.
Jim
Jim
The main thing is the gas impulse. This is not well understood by most users. In the Garand, basically, it is a function of the size of the gas port hole, pressure in the bore when the bullet base is between the gas port and muzzle, the length of time the bullet base takes to pass from first exposing the gas port to that pressure until it has uncorked the muzzle and then until the pressure in the barrel has dropped, and then the rate at which gas bleeds back out of the gas port after pressure falls below the gas cylinder pressure. For the M14 action, the concern is only with how much gas gets through before the piston cuts off the gas port.
The Navy ran into the more complicated Garand gas system problems when it refitted Garands in 7.62 NATO in the 60's. The 7.62's smaller charge of powder produced lower pressure near the muzzle and the slightly lighter bullets in M-80 ball operating at higher peak pressure actually went faster in the Garand than the M2 ball it was compared to, providing shorter gas port exposure time. This combination caused the gas port to need to be opened up from 0.079" to 0.1065" to get near equal gas cylinder peak pressure from these two kinds of ammunition. That bigger hole, of course, let gas in faster, but at 0.1065" diameter, the hole has an area of almost 0.0090 in². The area of the clearance gap between the median wear tolerance op-rod piston (0.5255" diameter) and the media wear tolerance gas cylinder (0.5300" diameter) is only 0.0037 in². That means, once pressure in the bore drops toward atmospheric pressure, that large gas port hole also let gas bleed out of the cylinder faster than it bleeds out of the piston/cylinder clearance gap, where the original hole, at about 0.005 in² at its mid tolerance size, is closer to merely equal to the piston/cylinder gap leakage.
So it's a complicated relationship. One consequence was that rifle grenades could not be used with the .308 versions because that larger hole trumped the gas cylinder plug valve bleed-off rate, operating the op-rod at too high a velocity.
Anyway, I suspect extending the muzzle forward of the standard location is possible, but the gas port hole diameter will have to start small for the longer exposure time. A 24" barrel has the gas port 1.5 inches back. A 30" barrel would have it 7.5" back if you didn't move the gas system forward (inadvisable, since it would make the op-rod longer and more whippy and would require a custom, heavier wire recoil spring and parts modified to fit it). At first blush I would estimate the new gas port would wind up around 0.030" in diameter. Pretty tiny. It would probably clog with carbon fast. Seems like a lot of bother for an extra 120 ft/s or so. You could get half way there in a standard Garand by getting a Garand Gear gas port plug or a vented gas port plug and using IMR 4350, and not have to modify anything.
The Navy ran into the more complicated Garand gas system problems when it refitted Garands in 7.62 NATO in the 60's. The 7.62's smaller charge of powder produced lower pressure near the muzzle and the slightly lighter bullets in M-80 ball operating at higher peak pressure actually went faster in the Garand than the M2 ball it was compared to, providing shorter gas port exposure time. This combination caused the gas port to need to be opened up from 0.079" to 0.1065" to get near equal gas cylinder peak pressure from these two kinds of ammunition. That bigger hole, of course, let gas in faster, but at 0.1065" diameter, the hole has an area of almost 0.0090 in². The area of the clearance gap between the median wear tolerance op-rod piston (0.5255" diameter) and the media wear tolerance gas cylinder (0.5300" diameter) is only 0.0037 in². That means, once pressure in the bore drops toward atmospheric pressure, that large gas port hole also let gas bleed out of the cylinder faster than it bleeds out of the piston/cylinder clearance gap, where the original hole, at about 0.005 in² at its mid tolerance size, is closer to merely equal to the piston/cylinder gap leakage.
So it's a complicated relationship. One consequence was that rifle grenades could not be used with the .308 versions because that larger hole trumped the gas cylinder plug valve bleed-off rate, operating the op-rod at too high a velocity.
Anyway, I suspect extending the muzzle forward of the standard location is possible, but the gas port hole diameter will have to start small for the longer exposure time. A 24" barrel has the gas port 1.5 inches back. A 30" barrel would have it 7.5" back if you didn't move the gas system forward (inadvisable, since it would make the op-rod longer and more whippy and would require a custom, heavier wire recoil spring and parts modified to fit it). At first blush I would estimate the new gas port would wind up around 0.030" in diameter. Pretty tiny. It would probably clog with carbon fast. Seems like a lot of bother for an extra 120 ft/s or so. You could get half way there in a standard Garand by getting a Garand Gear gas port plug or a vented gas port plug and using IMR 4350, and not have to modify anything.
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Jim Watson
New member
Come to think of it, even my 28" AR has a gas tube 2" longer than standard to manage the "dwell time" after the bullet passes the port.
Dixie Gunsmithing
Moderator Emeritus
The dwell time of the cam of the Garand is different than what the extended cylinder of the AK47 does, though. Dwell, in that it is the time that the cam does nothing to the bolt, (moves the 3/8"~), or nothing happens, after firing, until the cam starts to unlock the bolt. I would say the extra cylinder length, on the AK47, is to allow gas pressure to act upon the piston a little longer before it is vented in order to give it the extra kick needed to work properly, or it was needed so the piston wouldn't run out of cylinder, as I don't think the AK47 has anything like the Garand's gas vent.
The 'double impulse' of the Garand's gas system may have to be completely changed over this, if using a longer barrel. In it, the gas pressure starts pushing against the face of the piston on the operating rod, and the face of the valve of the gas vent, closing the valve, thus applying all the pressure to the piston momentarily. Then, when the pressure drops enough, the valve opens under spring pressure, and the gas exhausts out the front, though the rod is still traveling to the rear.
Plug with valve shown open:
I was thinking the AK47 didn't have this type of gas system, which truthfully makes it much simpler. I also don't know how much, if any, dwell the AK47's bolt may have when the gas piston starts reward. On this, I would have to look, as it is a rotary locking bolt. Some of these rotary designs do have a dewll period set into the cam.
Edit: There is some dwell in the AK47 bolt, but how much, I can't tell from the video I watched.
The 'double impulse' of the Garand's gas system may have to be completely changed over this, if using a longer barrel. In it, the gas pressure starts pushing against the face of the piston on the operating rod, and the face of the valve of the gas vent, closing the valve, thus applying all the pressure to the piston momentarily. Then, when the pressure drops enough, the valve opens under spring pressure, and the gas exhausts out the front, though the rod is still traveling to the rear.
Plug with valve shown open:
I was thinking the AK47 didn't have this type of gas system, which truthfully makes it much simpler. I also don't know how much, if any, dwell the AK47's bolt may have when the gas piston starts reward. On this, I would have to look, as it is a rotary locking bolt. Some of these rotary designs do have a dewll period set into the cam.
Edit: There is some dwell in the AK47 bolt, but how much, I can't tell from the video I watched.
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Dixie Gunsmithing
Moderator Emeritus
I wanted to add this, about changing the gas system on the M-16 and the AR-15 rifles from the original arrangement to using an operation rod. To me, the time it takes the gas to travel through the tube, and into the bolt, where it expands the two pieces, is somewhat of a delay in itself, and if you use an operating rod, you will lose this delay, I would think. How much this delay would be is in question?
Dixie,
That valve was absent on the original Garand plugs. It was added as part of the rifle grenade launcher system. When the launcher is attached it opens that valve to prevent the extremely long dwell of the grenade (compared to barrel time of a bullet) from keeping up gas pressure long enough to slam the bolt hard open and wreck the op-rod. But in normal firing operation, like the original solid plug, the valve never opens. Instead, gas vents via an angled relief cut toward the rear of the gas cylinder that gives the gas a path around the piston head as it gets to its most rearward position.
That valve was absent on the original Garand plugs. It was added as part of the rifle grenade launcher system. When the launcher is attached it opens that valve to prevent the extremely long dwell of the grenade (compared to barrel time of a bullet) from keeping up gas pressure long enough to slam the bolt hard open and wreck the op-rod. But in normal firing operation, like the original solid plug, the valve never opens. Instead, gas vents via an angled relief cut toward the rear of the gas cylinder that gives the gas a path around the piston head as it gets to its most rearward position.
Dixie Gunsmithing
Moderator Emeritus
Ah, okay, as I've never owned a Garand, only read about them, but what I read, said the valve was a replacement and used on the later models, unless they had them all set up usable as grenade launchers, but I guess the later ones all could have been, or just some changed out. Every cut-away view I've seen, always depicted the valve open, as if it had a spring behind it, pushing it open. This was with and without the launcher in place. I think the only pictures showing this was the large wall chart the instructors used.
I knew the original didn't have this, due to the patent, and was just a cylinder, but I remember looking at the patent for the new plug, and it didn't show a cross section of its inners, only just how they changed the manufacture. It seems that there is not much info on this, especially in everything I have on it. The field manual just shows the plug taken out, but nothing about taking it apart further. I've never worked on one, and never had one to grace the shop, but it being what it is, I can see why. Anyhow, in reality, its just a straight blow back piston, with no metering.
If you get a chance to look at the wall charts, you'll notice it open in another view they have, besides the one showing the grenade launcher. They show an outline of the valve head sticking out into the recess where the piston stops, and the gas port is at. They also lead you to believe it is to blown closed, by a double-head arrow, pointing at both the valve and the piston, showing it as gas expansion, or what I always took it as. One is never too old to learn something new.
Edit: I thought I would add this, as its the best I can find, but you can see what I mean.
gv by matneyw, on Flickr
It looks to say, 'valve closes as shown when regular service ammunition is fired'.
Last, I did finally find it in the ordnance manual. They do say it is supposed to push in only. This is the 1947 manual, and they said to replace all solid plugs with the 'new' valved plug. Whomever did the drawing for the chart, it looks to me, got it backwards. The drawing even looks to have the spring compressed, with the valve closed, where it should be sprung open.
I knew the original didn't have this, due to the patent, and was just a cylinder, but I remember looking at the patent for the new plug, and it didn't show a cross section of its inners, only just how they changed the manufacture. It seems that there is not much info on this, especially in everything I have on it. The field manual just shows the plug taken out, but nothing about taking it apart further. I've never worked on one, and never had one to grace the shop, but it being what it is, I can see why. Anyhow, in reality, its just a straight blow back piston, with no metering.
If you get a chance to look at the wall charts, you'll notice it open in another view they have, besides the one showing the grenade launcher. They show an outline of the valve head sticking out into the recess where the piston stops, and the gas port is at. They also lead you to believe it is to blown closed, by a double-head arrow, pointing at both the valve and the piston, showing it as gas expansion, or what I always took it as. One is never too old to learn something new.
Edit: I thought I would add this, as its the best I can find, but you can see what I mean.
gv by matneyw, on Flickr
It looks to say, 'valve closes as shown when regular service ammunition is fired'.
Last, I did finally find it in the ordnance manual. They do say it is supposed to push in only. This is the 1947 manual, and they said to replace all solid plugs with the 'new' valved plug. Whomever did the drawing for the chart, it looks to me, got it backwards. The drawing even looks to have the spring compressed, with the valve closed, where it should be sprung open.
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Right. You can stick an Allen wrench or other object down and force the valve open, and you feel the spring pushing back on you when you do that. I suppose the artists showed the valve open just to show that it was a valve and that it would release gas when open. But the spring keeps it closed at rest, so it starts and stays that way for normal firing.
One design for using slower burning powders or to reduce op-rod wear takes one of those valved plugs and drills and threads the screwdriver cuts to accept a set screw you can use to crack the valve open in a partial and controlled way. This lets you bleed off some of the pressure to the op-rod runs more gently.
One interesting development has been a solid plug sold by Garandgear.com that is hollow inside. This increases the cylinder volume the gas flows into initially, and while that lowers the peak pressure it doesn't lower the total amount of gas so the op-rod still works, albeit slightly more slowly. The end result, he shows from pressure readings, is it allows any kind of .30-06 ammo, including ammo using slow powders like IMR4350 to be fired safely in the Garand without hurting the op-rod or having to adjust a gas bleeder valve to compensate for the higher gas port pressure.
One design for using slower burning powders or to reduce op-rod wear takes one of those valved plugs and drills and threads the screwdriver cuts to accept a set screw you can use to crack the valve open in a partial and controlled way. This lets you bleed off some of the pressure to the op-rod runs more gently.
One interesting development has been a solid plug sold by Garandgear.com that is hollow inside. This increases the cylinder volume the gas flows into initially, and while that lowers the peak pressure it doesn't lower the total amount of gas so the op-rod still works, albeit slightly more slowly. The end result, he shows from pressure readings, is it allows any kind of .30-06 ammo, including ammo using slow powders like IMR4350 to be fired safely in the Garand without hurting the op-rod or having to adjust a gas bleeder valve to compensate for the higher gas port pressure.