After taking a look at the parts and guts on the Colt 6940 Piston carbine last time, it is now time to show the results of testing the carbine for accuracy and reliability.
For my accuracy testing of the carbine, I used the Leupold 18x target scope on a Larue SPR mount and my usual bags and test as I am wont to do. I fired all groups shown at 100 yards and 200 yards using a variety of match factory ammo as well as my own match handloads. I also fired the gun at 1,000 yards and 500 yards in my typical test to push it as far as possible. Once again for the long range resting, the 18x target scope was used,
To make the job a easier , I did use a SSA trigger int he carbine this time. The reason for using the SSA trigger instead of the milspec trigger this time, was because there is a reputation of piston guns having a little less accuracy than DI guns. My thinking was to try my best to eliminate anything I could that may give results that I , or anyone, may be biased to attribute to the piston system. So I used the match SSA trigger and a very secure front rest and sand bag set up from a bench. I wanted to get every bit of accuracy I could from the carbine.
Above are the 5 rounds groups fired at 100 and 200 yards. Due to limited amounts of some of the test ammo, I was only able to use 5 round groups after zeroing the gun and settling in. While all groups are what I considered great, I did notice small changes in the group size with certain match ammo from the DI guns to the piston. When using the DI carbines some of those brands shoot better in about every DI carbine/rifle I have used and other bands are not as tight while it seemed to be the opposite with the piston. Now, this is a small amount and not worth even talking about in a practical matter, I only noticed because of firing the ammo through so many guns that I was able to notice the change, Practically speaking , and from the outlook of field use, It is irrelevant. You can notice the SSA and the TAP strings vertically at 200 yards and beyond, I shot these at a later time with a cold clean bore and with a cold dirty bore and hot dirty bore. Those brands of ammo string vertically in the gun after you get to 200 yards. Again, practically speaking, it is not enough to matter or worry about in a carbine with a milspec barrel meant for fighting. It may be just this one gun, or may be those brands are sensitive to a piston operated carbine. I have no idea. But I present the info to you regardless.
Above is the target with the boxes of some of the brands tested. Below is a closer picture of the groups for closer inspection.
After seeing the results of the groups and being pleased with the accuracy , I determined it was worthwhile for long range testing. With the guns potential in mind, I and my friend loaded up and went to the mountain top strip job for the long range testing 3 weeks ago. Weather was mid with slight winds. Being on top of the mountain, it is hard to catch a windless day. The wind without fail travels right to left and can be seen on target as can be seen in almost all long range test targets from me.
I used a cardboard target with two orange panels to make target ID easy and to give me a better aiming point. Readers will notice I have used as variety of different target types and styles for long range testing, This is an ongoing project of mine to determine the best target and color combination to make long range testing as easy as possible to center the target in the optic for precise aiming, This system worked well on a sunny day, but the color or the paper was not much help late on when the sun was not shinning on it directly.
The shots fired at 500 yards , I circled with a sharpie. The 1,000 yard shots I drew a square around them. The one hole with a star like squiggle drawn around it, is a hit that I am not sure is a 500 round or 1,000 yard shot. I thought it was a 1K shot but later I thought maybe I intended to mark it when I fired the 500 yard group. So I marked it as a 500 shot to not give myself the benefit of the doubt and make a note of it. I feel it is more honest in this case to just call it as a 500 yard hit. On top of that, the 1,000 yard string obviously shifted to the bottom left corner and I feel it was unlikely that one of the 1K shots hit that far right and high.
The first fired 10 rounds at 500 yards using the Black Hills 77 grain MK 262 MOD 1 ammo. Five hundred yards is not a serious challenge for a quality carbine. Especially off of a bench rest and bags with an 18x optic. As per my usual method, I fired 10 rounds on a steel target gong to confirm my zero. I think fine tuned on a few skeet I lay around the target to make sure it is refined, then fire my “record group” of 10 rounds. As you can see I missed the target completely on one shot and of course the specially marked hit that may or may not be a shot at 500 yards. So NOT giving myself the benefit. 8 out of 10 rounds on target at 500 yrds. But, this is a very good group. The wind showed me some mercy while I fired the 10 shots and it shows. Once again, you can see the vertical stringing sneaking into the group.
Last I fired 20 rounds at 1,000 yards with 6 hits and then the hit in question that may have been a seventh round hit, Once again, not giving my self the benefit, I toss this shot out since it is in doubt, I give a count of 6 hits. The wind at that distance carried the shots further to left and I used several minutes to get me on the target this much. For the 1,000 yard group,I switched to my personal hand loads, It is a pet load that out performs factory ammo and is hot enough I do not share the load data. Now, whether it shoots better at this range or I just have more confidence in it, I have no idea really. But confidence is a huge factor, so I stick with it since it has always performed well for me. It takes extreme effort to get a 16 inch barreled carbine on target at 1K. Using a 20 or 18 inch barrel or better yet, a 24 inch barrel 556 gun is like heaven compared to the gymnastic it takes to get a carbine on but it can be done. Once again, I show it, just to show what a person can do with an M4.
The 6940Piston has some benefits in the long range testing in the fact that it comes with the SOCOM profile heavy barrel that is a big help. If the piston does disrupt the barrel from its extra movement and vibrations, then the heavy SOCOM barrel meant for harsher full auto firing schedule, helps cut this down possibly.
Last we come to the reason that the piston M4 carbine is supposed to exist. More reliability and especially in hard use with little cleaning, And of course in a military context, full auto fire with little cleaning and lube.
Last week friend of Looseournds.com and my neighbor , Tug Valley Armaments brought his full auto guns out for us to do some hard testing of the Piston Colt. Since getting the gun in the mail from Colt. I have rnot cleaned or lubed the gun. After 784 rounds of no cleaning and no lube, It was time. We put the upper on the full auto lower and fired up a few 40 round Pmags to get it so hot, it took glove to even hold it by the KAC vertical fore grip. I stuck a full surefire 60 round mag in the bone dry, very dirty gun with zero lube on it and held the trigger down until empty.
The gun went through the magazine without issue. Let me tell you it was hot before I fired the mag, and it was smoking after., We got the carbine dangerous hot.
You can see the barrel of the carbine smoking from the heat of the 60 round mag dump after not taking a break after also firing through five Magpul 40 round Pmags and various USGI 30 rounders. There was no problem form the gun. It ran wonderfully. I cannot make any dubious claims of the BCG being cooler because it was a piston though since by the time I stopped shooting even the receiver extension was hot to touch.
One observation we did not expect is that the gun on full auto would not run with the full auto lowers carbine buffer. We slapped the upper on the Class III lower and left the buffer it had in it in place. I went to auto and it was semi auto only. After thinking about it a second, we put the H2 buffer that comes standard in the 6940Piston, in the NFA lower and the gun ran perfectly. Just more reason why I have always appreciated Colt giving at least the H buffer in their carbines and heavier buffers based on what the gun was intended to do.
The piston 6940 is a superb piston AR15 carbine. If you are the type who thinks he has to have a piston to kill the commie invasion, I can not see you being let down by this gun or find any complaints. If you just want a great gun and you like this one and do not have any strong thoughts on the piston vs DI, you are gonna love this gun. If you are a DI die hard guy like me? You are still going to really like this gun. I won’t be switching to piston nor do I feel the need to, but I am impressed by this gun. I think the DI does edge it out in accuracy with match ammo, but in practical field use it is not really a factor. Since I used match ammo for the testing in the part, I will be using milspec issue ammo testing in the next part to see how it does and possibly a direct shoot off between the 6940 Piston and the standard 6940 DI gun. So, if you are interested check back for that info.