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View Full Version : Piston uppers and grip length


MrTokarev
09-19-2012, 8:04 PM
I was looking at the AR piston uppers midway has on sale right now and they have a couple different grip lengths (all the ones on sale are 16"). Now it seems to me that the grip length pushes forward tho position of the gas block and a longer grip would require a longer op rod. Would making this part longer affect the reliability of the weapon? It seems like the longer op rod would be more prone to stress damage. Is this the case? Are shorter piston systems more rugged?

starsnuffer
09-19-2012, 8:32 PM
I dunno. HK has the same gas block location and op rod for all of their barrel lengths for the 416 and G36, doesn't matter if it's the 10" barrel or the 20". There's no reason to move it on a piston system.

-W

MrTokarev
09-19-2012, 9:40 PM
These are the particular piston uppers I'm talking about:

Adams Arms mid length (http://www.midwayusa.com/product/674968/adams-arms-ar-15-a3-base-carbine-gas-piston-upper-assembly-556x45mm-nato-1-in-7-twist-16-barrel-melonite-finish-with-m4-handguard-flash-hider)

Adams Arms carbine length (http://www.midwayusa.com/product/665965/adams-arms-ar-15-a3-base-mid-length-gas-piston-upper-assembly-556x45mm-nato-1-in-7-twist-16-barrel-melonite-finish-with-m4-handguard-flash-hider)

It looks like the gas block on the mid length model is further down the barrel.

FeuerFrei
09-20-2012, 8:59 AM
Midlength is longer than carbine.
I am still testing a midlength AA kit I installed (first ML assembly) and the OP rod is supported in the receiver by a bushing. All AA kits have this feature. Not likely to bend or deform unless it is faulty steel from the git go.
Properly made op rod and installed correctly will run forever when using normal pressure ammo.
All of mine run ammo from Wolf to mil surp (193 & 855).

MrTokarev
09-21-2012, 12:07 AM
Midlength is longer than carbine.
I am still testing a midlength AA kit I installed (first ML assembly) and the OP rod is supported in the receiver by a bushing. All AA kits have this feature. Not likely to bend or deform unless it is faulty steel from the git go.
Properly made op rod and installed correctly will run forever when using normal pressure ammo.
All of mine run ammo from Wolf to mil surp (193 & 855).

Neat. Thanks for the response.

Son of BAR7
10-09-2012, 5:00 PM
For anyone looking at Piston uppers, or conversion kits, I found an informative Master's Thesis on the subject.

Very interesting reading.

http://www.ewp.rpi.edu/hartford/~ernesto/SPR/LeBlanc-FinalReport.pdf

After reading that thesis, the Armalite paper on DI, and the piece from AR15barrels.com about the function of gas systems (pistol, car, mid, & rifle, Linked below), I have come to the conclusion that since I am not dragging my rifles through mud bogs and/or swamps, and I typically clean them every 100 rounds or so, piston systems do not seem to offer a significant advantage, and seems to be disadvantageous in a couple ways, namely the oft mentioned wear due to tilt caused by the asymmetric loading of the gas piston on the bolt carrier, but also (and this barely mentioned in any literature I've seen, so may be insignificant) an increase in wear and strain on the lugs of the bolt and barrel extension not present in DI actions. In DI actions, when the BCG cam is rotating the bolt, the bolt is essentially free floating due to gas expanding behind it; the bolt and carrier are forced apart, the bolt towards the barrel, and the carrier towards the buffer, engaging the cam and rotating the bolt. With a piston system, the bolt is forced back with the carrier, the bolt lugs impact the barrel extension lugs, then the cam engages forcing the bolt to rotate, presumable while the lugs are still butted against each other under high pressure, a condition that could conceivably cause deformation of the lugs over time.
If anyone out there has any thoughts on this, I’d like to hear another take on the issue.

In any case, I’m linking to these resources as it finally helped me make the decision WRT AR piston systems, and may help others.

http://www.armalite.com/images/Tech%20Notes%5CTECH%20NOTE%20104%20DESIGN%20AND%20 RELIABILITY%20OF%20ARs%20120222%20Rev%200%20Final. pdf

http://www.ar15barrels.com/prod/operation.shtml

starsnuffer
10-09-2012, 7:03 PM
Thanks for raising a necro thread to have yet another debate on piston vs. DI.

Let me put it to you this way:

Arguing about piston vs. DI on an internet firearms forum is like running in the special Olympics. Even if you win, you're still retarded.

-W

Son of BAR7
10-10-2012, 10:08 AM
^clever and original

I just wish I'd come across those 3 papers two months ago when I started looking for piston retrofit kits, would have saved me a lot of time. Thought it might be helpful to others

whlgun
10-10-2012, 10:14 AM
Havent had any problems with my midlength AA upper. No tilt, No op rod problems. Runs great and runs clean.

I prefer midlength because of the longer rail.

Son of BAR7
10-10-2012, 4:55 PM
Havent had any problems with my midlength AA upper. No tilt, No op rod problems. Runs great and runs clean.

I prefer midlength because of the longer rail.

Have you noticed any abnormal wear on the bolt lugs?