AR-15: piston or DI?

Having owned a number of AR-15 rifles I have a few comments about operating systems that may be helpful. First, the Direct Gas Impingement (DI) system can be perfectly effective. Second, all piston guns are not created equal and in some cases may perform worse that their DI counterparts.

I had an Olympic Arms tactical carbine that I purchased new and put somewhere between 15-20k rounds through before replacing it. After initial cleaning that gun NEVER had a failure to feed, failure to fire or failure to eject. I did have the bolt break in half at about 10k rounds (Oly had me return the entire rifle so they could check it and it returned fully repaired) and the barrel was pretty beat by the end but as far as the gas system, I never had a single problem. DI is a brilliant idea in terms of the weight savings and lack of moving parts. It also puts force behind the bolt allowing the energy to push forward on the bolt and backward on the carrier. This lets the carrier go directly backwards. Very clever and if it weren’t for the fouling would be perfect.

For decades everyone has understood that using a DI gun requires proper cleaning and lubrication. If this is done the gun will work long and hard without a problem. Like most things in life, once you get used to the process the cleaning and lubing is done without much thought or effort. However, objectively speaking it does take a fair amount of time, effort and cleaning solvent to really clean up the gun after several hundred rounds have been fired. The area of most concern is the portion of the bolt aft of the gas rings. This is the area that gets most of the carbon fouling which can build up very quickly. I have found that dropping the entire bolt into a container of Hoppe’s No. 9, tail first, can help tremendously in softening all the carbon. It can then be scraped and scrubbed off. You should of course remove the extractor first before doing this since the rubber bushing under the extractor spring doesn’t like solvents.

So, our DI guns can and do work very well but they do require some effort to keep in good working order. So, why even think about pistons? They add weight and cost to an already-expensive gun and in some cases it appears that they may not be as reliable as the DI platform.

Much has been said and written about the dreaded “carrier tilt”. This is a condition in which the piston’s force is transferred via the push rod to the bolt carrier key in such a way that the force driving the carrier rearward tends to force the rear of the carrier downward. This makes sense given that the original AR design was never meant to accommodate these off-axis forces. Most major manufacturers of piston AR systems have addressed this issue by creating bearing surfaces either on the carrier, inside the upper or both. This keeps the carrier in position as it travels rearward and saves the wear inside the bottom of the buffer tube. From what I am seeing, H&K, Ruger and POF all have addressed this issue adequately.

A word must be said at this point about conversion kits. I can’t say that I’m a fan because in most cases the kit is not sufficient to address the carrier tilt problem. Perhaps some manufacturer will produce a kit that will be very specifically tailored to a particular gun that will lend itself to CT mitigation but I just haven’t seen or heard of such a system yet. I would say that if a piston system is desired then start from the ground up with a fully-engineered solution.

So, if the piston gun has accounted for TC and is otherwise well-made, what advantages does it provide over a DI gun? The biggest is of course that the gun runs much cooler and very much cleaner. The nickel-silicone of the H&K and the chrome of the Ruger are amazingly slick, require little lubrication and clean with a dry cloth. The carriers can be removed immediately after a mag dump for inspection or replacement in the event this is necessary.

Another advantage of most piston systems is that they provide for pressure adjustment. This means that the operating pressure can be increased or reduced to accommodate weather conditions, ammunition variations and the condition of the gun. Some users of the Ruger SR-556 have noted that after break-in they can reliably run the gun at the “1” position on the gas regulator. In the Ruger system “0” is no pressure (suppressor use), “1” is low pressure, “2” is normal and “3” is high. If the gun can run with less pressure then it will save wear and tear on the entire system. Most users will likely leave the setting at 2 and never notice any difference but it is a nice feature.

So, if pressure adjustment isn’t valuable to you and if you don’t run the gun hard, is there really any reason to consider a piston system? The answer, like almost everything, is “it depends”. The real benefit for me is that the gun is so much cleaner that I 1) don’t have to clean nearly as often and 2) the time to clean is cut by %50 or more. What would take at least 20 minutes with a DI gun takes less than 10 minutes with my piston gun. To many people this might not matter but to me it is significant.

So, there you have it. I have in fact gotten rid of all my DI ARs at this point. Will I have a DI gun in the future? Almost certainly I will but I must say that I am spoiled with piston systems. It’s surprising now to see just how dirty a DI gun is after using primarily piston guns for a while.

What should you use? that will depend on what factors weigh more heavily for your situation. Hopefully my experiences will be of assistance.

William