The idea that the piston operated AR15/M4 would be an improvement that fixes all of the perceived short comings of the weapon has been something that has gained ground in certain corners since the dubious “dust tests” and H&K marketing from a bit over 10 years ago now. Miss-use by users in the GWOT and careful lobbying by certain companies has put the idea that the DI system is sub-par in the minds of some of the lesser educated. In fact ,if you did not know better you would think the piston operated AR15 did not exist until HK came out with the 416. Truth is Colt had already developed a piston operated AR15 since the 60s and had been playing around with it ever since. If you look close at the front sight, you will see some details that pop up a lot later.
Later Colt, in 2005-2006 colt started showing pictures of another piston gun they called the LE1020. It was a monolithic railed upper very close to the current 6940 uppers. It lacked the QD sockets,and some other small refinements but it was clear the idea was being refined. All this before others had started with their piston campaign. Yes, colt had been making and refining piston AR15s for a long time. Getting it the way they wanted it before deciding to offer it. We did not see the LE1020 hit the market back then because it was found the market and the Military was not that interested in a piston gun. It took ignorant gun writers and HK marketing to convince a lot of people that they could not like without a piston operated M4. Never mind some of those early piston ARs chewed up receiver extensions, suffered from carrier tilt, weighed a ton and were not very easy to modify.
If you are new to AR15s you may have missed the bright spike that was the peak of people wanting piston guns because so many believed a little dust caused a M4 to malfunction and History channel documentaries that were more or less HK 416 advertisements. That has craze has evened out now a days and while some SOF use piston M4s, the rest of the army found out the M4 with its DI worked just fine witht some oil and not trying to use the M4 as a SAW. But in that time, companies had some time to tweak the piston guns to get them to work right. Among those was Colt, who refined their piston model from all those years ago before any one else had even thought about making a piston AR15.
With that, we come to the present day. A few weeks ago, Colt once again was nice enough to send me a shiny new Colt 6940Piston for my grubby little hands to test and abuse for other peoples amusement. We will take a look at it in this first part of a longer review and test. just to get to know it a little. stick our nose in its nooks and crannies and put on the old rubber glove and tell it to bend over so we can get to know it a little deeper….
The Colt 6940P ( Piston) is essentially a regular 6940 from the outside. The lower is standard Colt milspec minus the full auto FCG of course. The buffer that comes with the P is the H2. This is standard issue with the gun as it comes with the the heavy SOCOM profile barrel we talk about in a moment. The SOCOM profile M4A1 barrel is always combined with the H2 buffer in Colt models. Piston guns with standard A2 flash hiders will have a bit more felt recoil than DI guns, and the H2 buffer can smooth that out, Though to be clear that it not why it is in the gun. As I said, with colt, the H2 buffer always is paired with the SOCOM barrel, but it is a nice side effect.
Looking at the picture of the buffer you will note there is no shaved metal from carrier tilt or eaten up lowers which was common on some other companies piston conversions.
As mentioned, the barrel is the SOCOM profile, which was made for the use on M4A1 full auto carbines. The cut flats a few inches from the front sight are for the M203 to mount around. The barrel is free floated in the monolithic upper. The free float 6040 uppers will give you every bit of accuracy the barrel is capable of. I have never seen a Colt monolithic upper that has given mediocre accuracy when using good ammo, but the piston parts may make a difference. We will see in part 2 with accuracy testing.
The upper rail is standard 6940 and the lower rail removes the exact same way. You can see just like the DI guns, this one has the QD sling points. The piston parts are hidden under the FF rail.
The piston comes out very slick and is retained neatly with a push pin much like those used for the lower. You simply push it to the side and slide the piston out.No muss no fuss.
The piston is Colt’s design with the articulating link. Not much to say about it since its a piston. Very robust.
Since we have a piston, we don’t need a gas carrier key. The early Colt P guns had a bolt on part, just like the gas key, this was changed to the current model. It is machined out solid on the carrier . No bolts or staking to worry over. Not that you ever really had to worry about a colt stake job in the first place. The Bolt carrier group fields strips for cleaning just like the standard non-piston BCG
The rear of the carrier has rails machined on it to make sure you get no carrier tilt. No tilt means your lower will not get chewed up like some of the early HK416 and conversion kits rushed out on the market. The truth is, the AR16 was not meant to be a piston gun, so careful changes had to be made for it to work out in the long term. With the rails to the rear of the carrier and a steel block added to the upper receiver, tilt is a non issue on the 6940P. In the picture below, you can see the part added to the upper. Buyers of even DI guns will notice this on newer 6940 DI guns and the 901 as there are plans to make piston 901 eventually and it simplifies production to make them all the same.
Above is the upper with lower rail hand guard removed with piston and bolt carrier.
From the outside, it looks like the regular 6940 until the educated eye looks at the front sight. The gun handles and balances no different, thought the SOCOM barrel adds a little more weight. To get ready for long term hard shooting, accuracy testing and full auto torture tests, I have added my favorite TD grip and Colt factory ambi safeties. For drills and general use it now has a CompM4 a B5 stock and a Knights 600 meter BUIS. Part 2 of the review will be the accuracy testing for group, long range to the weapons extreme limit and more. Full auto fire may be in part 2, or it may be moved to a part 3 for torture test and taking a look at cleaning the piston gun. Less fouling is often touted as one of a piston gun’s biggest advantages so it is possible I do a part devoted to that.