View Full Version : LMT Piston Review.

11 June 2010, 19:01
Please post comments here (http://www.weaponevolution.com/forum/showthread.php?2383-LMT-Piston-Review.&p=21404#post21404).

I got my LMT 5.56 Piston Upper on 1-2-09. I had been wanting to try out the Piston ARís but I was very skeptical about them. I wanted a 5.56 upper to supplement my 6.8 SPC ARís because shooting the 6.8 SPC was becoming expensive. I never had any problems with my DI ARís in the Military or LE. I did hate having to clean them, but other than that, they ran fine. So I finally decided to give the Piston ARís a try, but I was still skeptical so I decided to do some researching. I finally decided to go with the LMT Piston for a couple of reasons:
1. LMT has always out good products.
2. I really like the monolithic concept.
3. I liked the idea of different calibers.
4. If I didn'tít like the piston AR, I could just swap it out with a DI barrel.

As picture below, it weighs 11LB loaded. (10 without the magazine.).

1. Recoil:
I had heard several different opinions on the recoil differences with Piston ARís. Some owners said the recoil was sharper, some said the recoil was less, and some said it was ďdifferent.Ē I honestly donít think Piston ARís are different from DI. It all depends on how large the gas port is and what buffer you use. Iíve had DI uppers that were gassed properly and were smooth, and I had some that were over gassed and shot like crap. My LMT Piston Upper happens to be one of the softest shooting ARís Iíve ever shot. I run it with the FSC556, Superior Shooting Spring and regular carbine buffer.

2. Cleaning:
This is where the Piston AR shines. We all have shot our DI ARís for several hundred and even thousands of rounds and the DI ARís still ran with just some drops of oil. The difference is that I have shot over 2K through my LMT Piston Upper and it looked like my DI after a couple of hundred rounds. I used to go to the range and when I got home, I cleaned my DI ARís, even if it was a quick wipe down. I never liked to let the carbon sit because it was just harder to clean up once it got harder. With my LMT upper, I come home and put everything away and I donít even bother to clean it. Sure, eventually I have to clean it, but it is more of a wipe down. I donít even disassemble the carrier most of the time. Some carbon does build up in the front in the gas block and piston, but that takes just a few minutes to clean and I donít even check until I shoot over 1-2K rounds. So, yes, I do have to clean my LMT Piston AR, but no where near as often, nor does it take as much time. For some people, cleaning their ARís isnít a big deal, but for some of us, it is. This should be the big selling point for Piston AR Manufacturers, instead of the goofy ass gimmicky advertising such as the guys popping out of the water and shooting or pulling out the carrier after a mag dump to show how cool the carrier still is.

These are two pictures of my BCG and inside the upper. I honestly can't remember the last time I cleaned it, but it is between 500-1000 rounds. I'll start over and keep track of the round count so I can be a little more accurate. It may be har to tell from the pictures, but it is still very clean:

3. Reliability:
I am close to 10K rounds and reliability has been great. The only issues Iíve had were due to my error. Forgetting to put primers on a couple of my handloads and not installing the piston correctly. Iíve primarily used Pmags and some USGI mags with the magpul followers. So how does reliability compare to DI? IMHO, for most practical purposes, a quality DI AR is just as reliable as a quality Piston AR. Now, in harsh environmets, that may be a different story, but that is just a guess and I donít have anything to back it up.

4. Weight:
Another complaint about the piston uppers was the added weight of the gas block, piston and the rod. I never thought this was a big issue because at most, we are talking ounces, not lbs like some people make it sound. When I got my LMT Piston upper, I also had a 16″ 6.8 SPC MRP Barrel, so I decided to check the difference in weight. I have a digital scale, which reads every 1/2 lb and when I weighed the LMT Piston Upper and compared it to the LMT MRP DI, I didn'tít see a difference. They both weighed the same.

IMHO, the whole weight issue is over rated for most practical purposes. If I was humping my AR all day long out in Afghanistan or Iraq, I can understand the ďevery ounceĒ counts, but for LE/Civilian use, Iím more concerned with how an AR balances.

5. Accuracy:
I remember reading all of the theories on how the piston ARís were not as accurate and it kind of made some sense. So when I purchased my LMT, I had the mindset that this thing was going to be a 2-3 MOA shooter, which I was OK with since I was going to be using it for plinking/training and not for accuracy/benchrest shooting. I purchased one of the Hornady 55 GR FMJ/BT 6K pack and tried some different powders. I also purchased some Hornady 75 GR BTHP to test out. To say I was shocked with the results is an understatement. With the Hornady 55 GR FMJ/BT bullets and even the Hornady 75 GR BTHP bullets, I got 1-1.5 MOA with most of the powders I used. Sure, some of the loads were over 2 MOA, but thatís what load development is all about. Since Iím using this for training/plinking and donít shoot much over 100 yards (occasionally I shoot at 200 and 300 yards), I decided to stick with the much cheaper Hornady 55 GR FMJ/BT bullets with 25.5 GR of Benchmark. If I canít find Benchmark, TAC or X-Terminator are also very accurate and even cheaper. And from accuracy reports that Iíve read on the LMT and other Piston ARís, accuracy has been similar to what Iím getting.

Some of the negatives I heard about the Piston AR was that the Piston System just wansít designed for the AR. That may have been true in the beginning, but the Piston AR has evolved. Some of the reported issues were carrier tilt, bolt thrust, and excessive wear inside the upper due to the cam pin gouging the upper. Here are some simple fixes that you can do yourself. Some of the companies are already taking care of these issues.

1. Bolt Thrust:
The Adams (http://http://adamsarms.net/details.asp?sku=SP-BLT) Arms Bolt Spring takes care of the bolt thrust issue. They cost approximately $5 per spring and I installed one on my LMT Piston Upper. For those not familiar with what bolt thrust is, Iíll do my best to explain it.
In a DI system, when the gas travels down the gas tube, it goes inside the carrier and the gas pushes on the bolt and holds it open for a split second while the extraction begins. With the piston system, there is no gas to hold the bolt open for that split second so supposedly that has caused some bolts to break. That spring holds that bolt open similar to the way the gas holds the bolt open. It makes sense, but I never seen a bolt brake or heard of any bolts braking from the piston owners Iíve spoken with. Just an added security. I hope I explained it correctly and easy to understand. It is kind of hard to pit it in words.

2. Carrier Tilt:
I noticed some minor carrier tilt pretty much after the first 100 rounds, but it was more cosmetic and it leveled off. Even though LMT uses the pads on the carriers, it still wans'tít enough to prevent carrier tilt. Some people never get the carrier tilt, some do. I didn'tít like the idea of carrier tilt, so I decided to address the issue with the PWS (http://http://primaryweapons.com/store/pc/viewPrd.asp?idproduct=139&idcategory=) Enhanced Buffer/Extension Tube and the Anti-Carrier (http://http://firearmreadysolutions.com/) Tilt Buffer from FirearmReadySolutions. With the PWS Enhanced Buffer/Extension Tube, the carrier just sits right on top of the lip of the extension tube. Iíve shot close to 500 rounds with the PWS and donít see any carrier tilt. At $150, they are expensive. If I knew about the POF (http://http://www.pof-usa.com/parts/arparts.htm) extension tube, I would have most likely bought that one as it is much cheaper and it has the same lip on it. Other than the price, Iím very happy with the PWS version.

Another item that is available to take care of the carrier tilt issue is the Anti-Carrier Tilt Buffer from FirearmReadySolutions. I purchased one from Seth, but I havenít had a chance to test it out yet. The only negative with the anti-tilt carriers is that in order to get to your bolt, you have to pop out both the front and rear pins and completely separate the upper from he lower. This is because of the nipple on the anti-tilt carriers.

This is a picture of the original buffer tube:

This is a picture of the PWS after approximately 500 rounds through it. If there was going to be any carrier tilt marks, they would have been there right away:

3. POF Cam Pin.
The POF (http://http://www.pof-usa.com/parts/arparts.htm) Roller/Cam Pin reduces friction and drag of the bolt carrier. I donít have any drag inside the upper receiver, but I did order one anyway. Iíll update this as soon as I test it out.

You often hear guys posting that they have shot their friendís Piston AR and donít see what the big deal is. Shooting your friendís Piston AR at the range isnít the same as owning one. I canít explain the feeling of shooting 1K rounds and opening up my LMT and the thing is still clean. The majority of the people bashing the Piston ARís are the ones that that have never owned one. If you ask the majority of the Piston AR owners, they will tell you that if given a chance, they would not go back to DI.

Support Used: Harris Bi-pod and rear sandbag.
Optic: Swarovski Z6i 1◊6 scope set on 6x.
Trigger: WOA Tuned RRA with Superior Shooting Springs.
Range: 100 Yards.
Shot Groups: 2 10 round shot groups.

Hornady 55 Gr. FMJ.
Brass: Winchester.
Primer: Wolf Small Rifle Magnum.
Crimp: Lee FCD.
OAL: 2.220"
Temp: 30.

24.5 Gr. of X-Terminator: Avg: 2885 fps, Lo: 2861 fps, Hi: 2911 fps, ES: 50, SD: 21
ACCURACY: 1.823"
25 Gr. of X-Terminator: Avg: 2954 fps, Lo: 2898 fps, Hi: 2982 fps, ES: 84, SD: 33
ACCURACY: 1.945"
25.5 Gr. of X-Terminator: Avg: 3005 fps, Lo: 2982 fps, Hi: 3023 fps, ES: 41, SD: 19
ACCURACY: 1.903"

24.5 Gr. of Benchmark: Avg: 2958 fps, Lo: 2930 fps, Hi: 2982 fps, ES: 52, SD: 21
25.5 Gr. of Benchmark: Avg: 2965 fps, Lo: 2936 fps, Hi: 2989 fps, ES: 53, SD: 19
ACCURACY: 1.307"
26 Gr. of Benchmark: Avg: 3021 fps, Lo: 3005 fps, Hi: 3089 fps, ES: 41, SD: 20
ACCURACY: 1.335"

23 Gr. of H322: Avg: 2761 fps, Lo: 2726 fps, Hi: 2789 fps, ES: 63, SD: 23
ACCURACY: 1.095"
23.5 Gr. of H322: Avg: 2826 fps, Lo: 2772 fps, Hi: 2855 fps, ES: 83, SD: 32
ACCURACY: 1.922"
24 Gr. of H322: Avg: fps, Lo: fps, Hi: fps, ES: , SD:
24.5 Gr. of H322: Avg: fps, Lo: fps, Hi: fps, ES: , SD:
ACCURACY: 1.681"

25.5 Gr. of H4895: Avg: 2891 fps, Lo: 2849 fps, Hi: 2943 fps, ES: 94, SD: 34
ACCURACY: 2.531"
26 Gr. of H4895: Avg: 2985 fps, Lo: 2949 fps, Hi: 3023 fps, ES: 74, SD: 26
ACCURACY: 1.585"
26.5 Gr. of H4895: Avg: 3049 fps, Lo: 3016 fps, Hi: 3079 fps, ES: 63, SD: 24
ACCURACY: 1.978"

25.5 Gr. of Varget: Avg: 2784 fps, Lo: 2721 fps, Hi: 2855 fps, ES: 134, SD: 56
ACCURACY: 2.015"
26 Gr. of Varget: Avg: 2859 fps, Lo: 2842 fps, Hi: 2892 fps, ES: 50, SD: 20
ACCURACY: 1.413"
26.5 Gr. of Varget: Avg: 2928 fps, Lo: 2898 fps, Hi: 2969 fps, ES: 71, SD: 31
ACCURACY: 2.423"

Hornady 75 Gr. BTHP:
Brass: Winchester.
Primer: Wolf Small Rifle Magnum.
Crimp: Lee FCD.
OAL: 2.250"
Temp: 30.

22 Gr. of H322: Avg: 2558 fps, Lo: 2520 fps, Hi: 2583 fps, ES: 63, SD: 25
ACCURACY: 1.119"

22.5 Gr. of H4895: Avg: 2420 fps, Lo: 2406 fps, Hi: 2428 fps, ES: 22, SD: 8
ACCURACY: 1.122"
23 Gr. of H4895: Avg: 2450 fps, Lo: 2424 fps, Hi: 2487 fps, ES: 63, SD: 24
ACCURACY: 1.701"
23.5 Gr. of H4895: Avg: 2525 fps, Lo: 2501 fps, Hi: 2549 fps, ES: 48, SD: 22
ACCURACY: 1.517"

23 Gr. of Varget: Avg: 2395 fps, Lo: 2372 fps, Hi: 2420 fps, ES: 52, SD: 21
ACCURACY: 1.537"
23.5 Gr. of Varget: Avg: 2477 fps, Lo: 2460 fps, Hi: 2492 fps, ES: 32, SD: 11
ACCURACY: 1.306"
24 Gr. of Varget: Avg: 2524 fps, Lo: 2510 fps, Hi: 2544 fps, ES: 34, SD: 15
ACCURACY: 1.704"

Hornady 75 Gr. BTHP:
Brass: Remington.
Primer: Wolf Small Rifle .223.
Crimp: Lee FCD.
OAL: 2.245"
Temp: 30.

21.5 Gr. of H322: Avg: 2557 fps, Lo: 2544 fps, Hi: 2578 fps, ES: 34, SD: 13
ACCURACY: 1.805"
22 Gr. of H322: Avg: 2593 fps, Lo: 2574 fps, Hi: 2609 fps, ES: 35, SD: 13
ACCURACY: 2.047"
22.5 Gr. of H322: Avg: 2684 fps, Lo: 2666 fps, Hi: 2704 fps, ES: 38, SD: 15
ACCURACY: 2.273"

23.5 Gr. of H4895: Avg: 2579 fps, Lo: 2564 fps, Hi: 2604 fps, ES: 40, SD: 15
ACCURACY: 1.219"

23.5 Gr. of TAC: Avg: 2562 fps, Lo: 2539 fps, Hi: 2574 fps, ES: 35, SD: 13
ACCURACY: 2.298"
24 Gr. of TAC: Avg: 2605 fps, Lo: 2599 fps, Hi: 2614 fps, ES: 15, SD: 6
ACCURACY: 2.375"


Hornady 60 Gr. V-Max TAP-FPD:
Avg: 2777 fps, Lo: 2766 fps, Hi: 2789 fps, ES: 23, SD: 11
ACCURACY: 1.528"

Hornady 75 Gr. BTHP TAP-FPD:
Avg: 2554 fps, Lo: 2525 fps, Hi: 2574 fps, ES: 49, SD: 18
ACCURACY: 3.021"

HSM (Hunting Shack Munition) 60 Gr. V-Max:
Avg: 2678 fps, Lo: 2645 fps, Hi: 2704 fps, ES: 59, SD: 23
ACCURACY: 1.402"

HSM (Hunting Shack Munition) 69 Gr. SMK:
Avg: 2574 fps, Lo: 2559 fps, Hi: 2588 fps, ES: 29, SD: 11
ACCURACY: 1.697"