ivokalok
07-18-12, 19:28
I have recently done some research on the differences in operating systems, and wanted to share what I have found and allow others to correct any errors I have made. I didn’t see a concise summary of published data on the subject—just individual opinions. SECONDARILY, I will comment on these sources. I am relying on 4 sources that I found informative:
a) An article by the NRA on this issue http://www.americanrifleman.org/ArticlePage.aspx?id=2298&cid=1
What I found remarkable: The “sole” advantage of piston systems is that they are cleaner/cooler. External pistons are NOT more accurate nor are they more durable. External pistons have more felt recoil. Modified AR designs with pistons suffer carrier tilt. Newer “from the ground up” designs don’t suffer carrier tilt.
b) Results of an Army test on reliability http://www.defensereview.com/colt-m4-carbine-finishes-last-in-latest-us-army-small-arms-reliability-test/
What I found remarkable: The Colt M4 had more stoppages than the other 3 weapons combined. Some of the other weapons were ‘prepared’ for the test whereas the M4 wasn’t.
c) An interview with John Noevske http://www.defensereview.com/noveske-rifleworks-n4-light-recce-carbine-john-noveske-interview-part-one/
What I found remarkable: That if you are suppressed, back pressure will force hot gas and fouling back into the chamber, negating the primary issues separating operating designs.
d) Technical Note 54 from Armalite http://www.armalite.com/Categories.aspx?Category=35e8a0b0-7e71-4af3-b335-53f4a7dc8f08
What I found remarkable: Almost all of it—a very good read. Direct Gas Impingment (DGI) is not the correct term. Internal (gas) piston is. External piston designs are less accurate as evidenced by the lack of use in competition, but not because of moving parts when the bullet is in the barrel, but possibly because of barrel vibrations. While a gas system lets in more fouling, a piston system lets in more dirt—essentially negating the difference.
Above was my summary/recitation of published evidence---below will be my discussion/interpretation of it.
The heat issue has always bugged me as at best minimal in effect if not outright meaningless. Generally, metal is a great conductor of electricity…and heat. When you put an all metal pot on the stove, even the parts away from the flame like the lid and handles get hot really fast. I don’t doubt that directly blowing hot gasses on the bolt will get it hotter a smidge quicker than relying on conduction, but in a closed bolt design where there is metal to metal contact with the hot barrel…there’s going to be heat transfer and equilibrium eventually (and fairly quickly). Yes, an external piston has more metal parts to soak up the heat (at a cost of more weight), but this is a nominal difference, I expect. Fire X rounds out of each and wait 1 or 5 minutes…I’d predict the difference is nominal or non-existent on that bolt. I haven’t seen a formal test of such, and would be curious to see one that could resolve this once and for all. Even if the difference is 50 or 100 degrees…what effect does that difference have in the grand scheme of things? I just don’t accept that the thermal issue is a meaningful difference now.
One of the subtle but significant claims was that of a lack of external piston use in competition. I’m sure this is changing more recently with the SCAR and HK 416/MR556 becoming available. However, IF this is true—I find this significant. While on the subject of accuracy…Armalite cites an (older) Army test that seems to prove it is not a function of the inertia of the moving parts they suggest that it may be vibrations. Legion Firearms has a honeycomb barrel, and the claim by them is that it does reduce vibrations and increase accuracy. I didn’t see proof of that claim—but I don’t know it isn’t true either. The implication in my mind is that barrel vibrations may actually be an issue for accuracy more so in external pistons. This is not formally resolved in my mind.
I know that every Tom, Dick, and Harry will have an opinion on how dirty ‘their’ gun gets. As such, I don’t know that I have reliable data on a true (scientific) test of the fouling of the different types without suppressors—I don’t know of a vacuum test where they empty the trays/bags of different ones to show the difference. Noveske claims there is essentially no difference with a suppressor. I also openly wonder how much fouling gets blown back without a suppressor. I must confess that I’m a bit surprised at the Armalite citation of the Army study that more actual dirt gets in an external piston design, but I’m not in a position to refute it. Obviously other/newer external pistons than ones tested may not suffer the same issue to the same extent. As I intend to clean after use anyway, even if the difference isn’t nominal as I expect…it will be essentially nil after a cleaning.
The Army reliability study doesn’t resolve in my mind whether the internal piston is an unreliable design—or if the Colt M4 implementation of that design is sub-optimal. I don’t know if the failures reported were a function of tolerances or operating systems. Maybe someone else has dived deep into that one and can expound.
I didn’t really look hard at the SCAR, etc. for my next purchase. It is possible that they really have raised the ‘reliability’ bar by eliminating carrier tilt, and sealing a system better than those tested by the Army. For cost and other reasons, my review was focused on the AR market primarily.
So, in summary, the 2 big claimed advantages of external piston AR designs are both suspect if not negligible/non-existent, which leaves me looking at the ‘classic’ internal gas piston design of Stoner for my next purchase.
I cautiously invite others to comment—preferably with published data references as opposed to individual opinions. Hopefully this will serve as a decent summary/guide/starting point for others. I intended to share what I found more than incite a riot between the die-hards in each camp. We’ll see which effect is greater.
a) An article by the NRA on this issue http://www.americanrifleman.org/ArticlePage.aspx?id=2298&cid=1
What I found remarkable: The “sole” advantage of piston systems is that they are cleaner/cooler. External pistons are NOT more accurate nor are they more durable. External pistons have more felt recoil. Modified AR designs with pistons suffer carrier tilt. Newer “from the ground up” designs don’t suffer carrier tilt.
b) Results of an Army test on reliability http://www.defensereview.com/colt-m4-carbine-finishes-last-in-latest-us-army-small-arms-reliability-test/
What I found remarkable: The Colt M4 had more stoppages than the other 3 weapons combined. Some of the other weapons were ‘prepared’ for the test whereas the M4 wasn’t.
c) An interview with John Noevske http://www.defensereview.com/noveske-rifleworks-n4-light-recce-carbine-john-noveske-interview-part-one/
What I found remarkable: That if you are suppressed, back pressure will force hot gas and fouling back into the chamber, negating the primary issues separating operating designs.
d) Technical Note 54 from Armalite http://www.armalite.com/Categories.aspx?Category=35e8a0b0-7e71-4af3-b335-53f4a7dc8f08
What I found remarkable: Almost all of it—a very good read. Direct Gas Impingment (DGI) is not the correct term. Internal (gas) piston is. External piston designs are less accurate as evidenced by the lack of use in competition, but not because of moving parts when the bullet is in the barrel, but possibly because of barrel vibrations. While a gas system lets in more fouling, a piston system lets in more dirt—essentially negating the difference.
Above was my summary/recitation of published evidence---below will be my discussion/interpretation of it.
The heat issue has always bugged me as at best minimal in effect if not outright meaningless. Generally, metal is a great conductor of electricity…and heat. When you put an all metal pot on the stove, even the parts away from the flame like the lid and handles get hot really fast. I don’t doubt that directly blowing hot gasses on the bolt will get it hotter a smidge quicker than relying on conduction, but in a closed bolt design where there is metal to metal contact with the hot barrel…there’s going to be heat transfer and equilibrium eventually (and fairly quickly). Yes, an external piston has more metal parts to soak up the heat (at a cost of more weight), but this is a nominal difference, I expect. Fire X rounds out of each and wait 1 or 5 minutes…I’d predict the difference is nominal or non-existent on that bolt. I haven’t seen a formal test of such, and would be curious to see one that could resolve this once and for all. Even if the difference is 50 or 100 degrees…what effect does that difference have in the grand scheme of things? I just don’t accept that the thermal issue is a meaningful difference now.
One of the subtle but significant claims was that of a lack of external piston use in competition. I’m sure this is changing more recently with the SCAR and HK 416/MR556 becoming available. However, IF this is true—I find this significant. While on the subject of accuracy…Armalite cites an (older) Army test that seems to prove it is not a function of the inertia of the moving parts they suggest that it may be vibrations. Legion Firearms has a honeycomb barrel, and the claim by them is that it does reduce vibrations and increase accuracy. I didn’t see proof of that claim—but I don’t know it isn’t true either. The implication in my mind is that barrel vibrations may actually be an issue for accuracy more so in external pistons. This is not formally resolved in my mind.
I know that every Tom, Dick, and Harry will have an opinion on how dirty ‘their’ gun gets. As such, I don’t know that I have reliable data on a true (scientific) test of the fouling of the different types without suppressors—I don’t know of a vacuum test where they empty the trays/bags of different ones to show the difference. Noveske claims there is essentially no difference with a suppressor. I also openly wonder how much fouling gets blown back without a suppressor. I must confess that I’m a bit surprised at the Armalite citation of the Army study that more actual dirt gets in an external piston design, but I’m not in a position to refute it. Obviously other/newer external pistons than ones tested may not suffer the same issue to the same extent. As I intend to clean after use anyway, even if the difference isn’t nominal as I expect…it will be essentially nil after a cleaning.
The Army reliability study doesn’t resolve in my mind whether the internal piston is an unreliable design—or if the Colt M4 implementation of that design is sub-optimal. I don’t know if the failures reported were a function of tolerances or operating systems. Maybe someone else has dived deep into that one and can expound.
I didn’t really look hard at the SCAR, etc. for my next purchase. It is possible that they really have raised the ‘reliability’ bar by eliminating carrier tilt, and sealing a system better than those tested by the Army. For cost and other reasons, my review was focused on the AR market primarily.
So, in summary, the 2 big claimed advantages of external piston AR designs are both suspect if not negligible/non-existent, which leaves me looking at the ‘classic’ internal gas piston design of Stoner for my next purchase.
I cautiously invite others to comment—preferably with published data references as opposed to individual opinions. Hopefully this will serve as a decent summary/guide/starting point for others. I intended to share what I found more than incite a riot between the die-hards in each camp. We’ll see which effect is greater.