Gas Piston AR-15, how much weight does it add?

[LoneStarWings]

Hi, I've really got my eye on this gun, the S&W M&P15PSX:

I have some questions about piston system AR's.

S&W lists the weight of this rifle at 6.5 lbs, which is the same weight as the comparable models that use the DI system. How is this possible? Does the gas piston system simply re-locate parts that were originally inside of the reciever to a different location?

I realize that the piston is contraversial, and AR purists will say it's not necessary, but I like the prospect of added reliability, cooler temps, and easier cleaning that the piston offers. This gun uses a factory-installed adams piston system. Are there really any drawbacks to the piston system other than cost and parts commonality with other AR's? I have heard some say that it decreases accuracy, but that seems debateable.

 

[ArmedBear]

Why to get one...

Added reliability: Check, at least after X rounds have been shot without cleaning.

Cooler temps: I don't think this has ever been demonstrated, at least enough to matter.

Easier cleaning: there's more to clean. You have to clean the piston. The inside of the upper receiver stays cleaner, longer, but there's still fouling to clean in any semiauto receiver, and the inside is the same so the same nooks and crannies apply.

So I think that added reliability is the best, if not the only, reason. It's a mighty good reason, though.

Reasons not to:

Weight: Yeah, there's really no way it could weigh exactly the same as an otherwise-identical DI gun. It might not be that much heavier, though. I wouldn't sweat that.

Accuracy: Piston guns have stuff moving around on the barrel as the bullet leaves the muzzle. DI guns don't. Piston guns can't be quite as accurate. How much? I don't know.

 

[Dr. Tad Hussein Winslow]

It's a good question - in otherwise-identical guns, how much weight is added, exactly? 2 oz, 4 oz, 6 oz, what? I'd be curious to know. My guess is 2 or 3 ounces.

 

[LoneStarWings]

In fact, s&w lists the basic DI plain "M&P 15" model with the carry handle at 6.85 lbs. The basic DI model with the flat top upper w/ rail is also 6.5 lbs (M&P15OR).

 

[ArmedBear]

I've seen some pretty silly things in vendor weight listings.

Then again, I've weighed my S&W revolvers on a gram scale, and the claimed weights are dead-on. So maybe S&W isn't one of the many vendors whose claimed weights seem to be pulled out of the air.

 

[PMROY]

I converted my Bushmaster M4 to a Gas Piston rifle. The weight difference is. negligible. Although the actual piston weights a little more than the single gas tube, you have to remove the gas rings. I did not weigh the new bolt in the kit to compare to the stock one, but ir probably weighs a little less. I used the Bushmaster Gas Retrofit kit, which includes the new bolt, hand guards and piston system.
On the Bushmaster website , the M4 DI weighs 6.22 and the M4 Gas Piston weighs in at 6.5, that shows you that the difference is only .28lbs. In comparison an empty magazine, metal, 30 rounds, weighs .25lbs.
that will put it into perspective.
By the way, I Love my Gas Piston Bushmaster.

 

[Dr. Tad Hussein Winslow]

0.35 lbs on S&W
0.28 lbs on Bushy

Average is .315 lbs, or 5.04 ounces. Round it off to 5 oz, shall we?

 

[model of 1905]

IMO, the only reason to buy an AR with gas piston as a fighting rifle is it may require less attention to keep it running. The added weight on the front end is a major detractor for my way of thinking.

Keep your AR lubricated and clean and you will be just fine with DI.

 

[LoneStarWings]

Well, that's just it, according to S&W there isn't any added weight to the front end. They listed this DI one:

And this one with a gas piston:

As exactly the same weight: 6.5 lbs. I know the piston model has an upgraded one piece bolt with skis on the back to minimize bolt tilt, so maybe weight is saved there or somehwere else?

I'm sure I would probably be fine with DI, but if I could save myself some hassles when cleaning and get a more reliable rifle between cleanings, why wouldn't I?

Clearly they're cleaner:
http://www.therallypoint.org/forum/index.php?topic=1908.0

And clearly more reliable:
http://www.armytimes.com/news/2007/12/army_carbine_dusttest_071217/

 

[Bartholomew Roberts]

Clearly they're cleaner:
http://www.therallypoint.org/forum/i...p?topic=1908.0

Putting aside how clear that is from your link, even if we take your statement as fact, all that shows is that the LWRC system is cleaner. Not all piston ARs are equivalent (though the Adams system is by all accounts a good one)

And clearly more reliable:
http://www.armytimes.com/news/2007/1...sttest_071217/

Again, the HK416 isn't the same thing as the Adams system that the S&W uses. For one, it has a different sized upper receiver.

People come at the piston thing a lot of different ways, you can't simply assume that because one design does something that all piston systems will perform similarly.

 

[Dr. Tad Hussein Winslow]

Lonestarwings, for starters the gas block itself, as shown in that picture, is larger/heavier on the GP model. So even if everything else is equal, it'd still be heavier. I think their advertisement is suspect, unless they've shaved some contour off the barrel near the base on the GP model.

Well, the handguard do look different, too, but you just cannot save very much weight using a lightweight handguard - don't think you can save 4 or 5 oz there, can you?

 

[ny32182]

There are a lot of different AR15 piston designs out there as you can see: Adams, POF, LMT, LWRC, HK416, Ruger, etc, etc... some can be retrofit, some can't...

You definitely *cannot* assume that because one system performs a certain way, the other systems will do the same. There are a variety of totally different designs out there.

 

[ArmedBear]

"Cleaner" yes, in the sense that there will be less fouling inside the upper receiver body after 500 rounds.

"Easier to clean" means something entirely different. Every semiauto receiver ends up with fouling in it, and how easy it is to clean depends on the design of the gun. Since it's the same upper receiver, it won't be easier to clean. You have to get the fouling out of the same exact places, plus the piston, and disassembly is of necessity more complex, not less.

 

[MCMXI]

S&W lists the weight of this rifle at 6.5 lbs, which is the same weight as the comparable models that use the DI system. How is this possible? Does the gas piston system simply re-locate parts that were originally inside of the reciever to a different location?

It's not always easy to compare apples to apples when comparing the weight of two rifles. You mention "comparable models" but you have to consider the forend, sight or no sights, the stock etc. At the end of the day, there's no way that a GP model weighs the same as a DGI model if they have the same stock, sights, forends etc.

the only reason to buy an AR with gas piston as a fighting rifle is it may require less attention to keep it running. The added weight on the front end is a major detractor for my way of thinking.

My POF has a 16" fluted barrel which more than makes up for the minimal increase in overall weight of the rifle due to the heavier bolt carrier, gas plug, gas piston and op rod. Also, the reduction in weight is where it has the greatest benefit in terms of reducing forward weight ... the barrel.

For those of you that don't have gas piston AR rifles, I can tell you for a fact that they are MUCH easier to clean. It takes all of 5 seconds to remove the gas plug, piston and op rod and another 15 seconds to wipe them off and reinstall. Since there's no crap in the action, for the most part that's all the cleaning required. Cleaning my DGI AR is more involved since I remove the carrier, the cam pin, the bolt and firing pin and even after as few as 20 rounds there's a bunch of carbon in there. I still like my DGI AR ... it's VERY reliable and very accurate. I happen to like my two GP ARs more though.

 

[LoneStarWings]

Well thanks for all the replies, hope I didn't come off too know-it-all-ish, becaue I really don't know much about the platform.

One on hand gettig a DI model woudl be nice since that's what everyone else has and it's universally understood.

On the other hand if a good Piston design truly solves the problems the AR is notorious for, then it seems like a great opion. I'm pretty confident that S&W would take the time to do it right. I understand they have put a one-piece bolt carrier on skids in the PSX to minimize "carrier tilt" and the associated wear on the upper reciever that results from it.

In any event, I don't think the smith pistons are even out yet, maybe I'll be the guniea pig and try one when they get here.

 

[MCMXI]

In any event, I don't think the smith pistons are even out yet, maybe I'll be the guniea pig and try one when they get here.

Do you know if S&W is using (or is going to use) the POF 1st generation GP system ... that's what Bushmaster uses? The reason I ask is that the gas block and gas plug shown on their web site looks like the one that POF uses ... just wondering.

 

[LoneStarWings]

Do you know if S&W is using (or is going to use) the POF 1st generation GP system ... that's what Bushmaster uses? The reason I ask is that the gas block and gas plug shown on their web site looks like the one that POF uses ... just wondering.

From thefirearmblog.com:

It IS a modified Adams Arms design. I talked to them at the SHOT Show, and Adams supplied the original design with a few mods: “skis” on the back of the bolt carrier to prevent or reduce the propensity of the bolt carrier to wear out the buffer tube, a unitary bolt carrier (no bolted-on key where the rod impacts) and the block will be pinned on. The S&W rep said that the pinning of the block is more of a sales issue than a real mechanical upgrade.

 

[MCMXI]

LoneStarWings, duh ... I completely forgot that you mentioned the Adams piston system in your OP. Thanks.

 

[PandaBearBG]

IMO I think that the piston gasblock (piston block??) looks like it would move your center of gravity forward, maybe throwing off your usual balance? Regardless for a short carbine, the benefits out weigh any cons that can easily be learned and compensated over a short time.

 

[PMROY]

The Bushmaster Gas Retrofit Kit, uses the existing front post, so the added weight is minimal.
I personally do not find my rifle to be front heavy, compared to my LE6920. Also, the Bushmaster gas Piston rifle, compared to the regular Bushy AR is only .28