piston or gas

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the premium on a piston ar is what? roughly ~$400-$500

And a good BCG costs what? ~$200?...that's 2 bcg's you can burn up with DI and still be money ahead. SO, the economic standpoint is a wash, b/c by the time you shoot enough rounds to eat up 3 BCG's, your lower and barrel are gonna be pretty sloppy anyway, no matter which system you choose. My .02

to me, it boils down to not liking to clean a weapon = I will pay the premium.
 
I have a lot of experiance with the M16 in Vietnam. It sucked.

I just put a Ruger SAR556 (piston rifle) on lay-a-way. I still don't like the 5.56mm for combat but for a zombie rifle it should be okay.
 
DI has worked well for half a century. It's lighter and cheaper. I'll stick with it. YMMV.

Once the lugs unlock, the main thrust of the bolt comes from the cartridge itself, which still has the residual pressure of the gas that hasn't escaped from the muzzle
Click to expand...

That statement couldn't be more wrong. Inertia is what keeps the bolt (or slide, on a pistol) moving rearward after the breech unlocks. If you still have chamber pressure after the bolt unlocks, your firearm is broken and dangerous to use.
 
Its not an accuracy issue with the piston, its an accuracy issue with the shooter not being able to hold the rifle as consistantly on target with the gas piston system acting on the barrel. The DI system is so broadly used for the simple reason that almost anyone can fire accurately with one, the piston system adds a percieved increase in recoil, it adds another moving part not in line with the bolt, barrel, and stock, so therefore theres more barrel rise.
I remember reading about the first impressions with the H&K416 that the special forces were using, they were initially having problems with accuracy, the rifle has a short barrel, a piston moving above the barrel out of line with the barrel, and took getting used to.
For the total difference of around $300 I got a gas piston rifle over another DI rifle, the fact that it runs 10,000 times cleaner is just a pleasant side effect, the fact that it runs so much cooler amazes me still.
If I lube a DI rifle down like I normally do, then add a bit before shooting, the bolt carrier is dry, cruddy, hot in a few rds.
Heck I dont mind cleaning them, its part of the fun of shooting them, but the fact remains that I can fire 200-400 rds in a day and spend 20 minutes taking my time cleaning my gas piston rifle, then work on another firearm I was using that day.
If I fire 100 rds through my DI rifle, or any other for that matter, and I clean it using the good old fashion way I used on my piston rifle (not using my powder blast or any other spray cleaner) Ill be busy for a good part of the evening.
Gas pistons were made for the AR rifle system for the simple reason that the DI system has serious shortcomings, one of those is the hours of cleaning and constant oiling required. Why did our military ask for a much more reliable system to be designed for them? Simple when it comes to the nasty gritty dirty desert, and faced with long drawn out firefights and attacks that can come at any time our soldiers werent always able to rely on the M4 to work every time.
Like I said before, if you religiously clean and oil an AR/M16/M4, between uses, while in the desert while not using every day, between every sandstorm, after every trip down a dusty road on a vehicle, you may not have a single problem. But the enemy doesnt wait for you to clean your weapon then reasseble it multiple times a day, they would much rather know when you clean it to attack you.
 
^ I thought that was what the SCAR was for? A real, purpose-designed, piston-operated weapon?

Compared to the SCAR, a piston-driven AR is not much more than lipstick on a pig, right? Seems to me they are reinventing the wheel, when the wheel worked fine (insert plucked-from-air high percentage number here) of the time is areas not containing sandstorms.
 
H & K 416 is piston driven. It is one to be replacing the M4. Firing rate is around 800 rounds/minute. Piston is in the block itself and is tilt-type. It has burst capability as well as semi-auto. Full breakdown and demonstration of the piston-drive (in slowmo) can be seen on the Military Channel feed where I viewed it. They are high dollar at GunBroker and several can be found. Class3.com out of Florida has them for sale last I heard.

Unless you truly need full-auto capability. It dont make sense to have a piston-driven AR that toggles at the block like the H&K, in lieu of an inline gas blow-back at the bolt AR-15. You cant fire enough rounds in semi-auto to make a meaningful difference. But, if the H&K 416 fits your eye and wallet. Piston-driven AR's are found to be more reliable when pitted against their counter-part top line in-line gas-blowback AR-15's.
 
Piston drive ar's typically have parts that can be touched by hand, after firing multiple rounds; you could never do this with any part of a gas/di AR.
Click to expand...

Piston ARs also have parts that can't be touched. The heat is just in a different location.

http://www.youtube.com/watch?v=6auXTiqNtEo
Watch time at: 16:43
You should watch it all, but that section talks about the heat.
 
SHvar said:
If I lube a DI rifle down like I normally do, then add a bit before shooting, the bolt carrier is dry, cruddy, hot in a few rds.
Heck I dont mind cleaning them, its part of the fun of shooting them, but the fact remains that I can fire 200-400 rds in a day and spend 20 minutes taking my time cleaning my gas piston rifle, then work on another firearm I was using that day.
If I fire 100 rds through my DI rifle, or any other for that matter, and I clean it using the good old fashion way I used on my piston rifle (not using my powder blast or any other spray cleaner) Ill be busy for a good part of the evening.
Click to expand...

This article describes all you need to keep a direct impingement AR15 functionally clean and it doesn't take more than 15 minutes to do it - even after 400 rounds.

The instructor who wrote the question now has a direct impingement rifle (BCM midlength) that will exceed 30,000 rounds with NO cleaning of any kind. All they did was add more lube periodically. He has several more that are in excess of 10,000 rounds.

BobaFett said:
The heat is just in a different location.
Click to expand...

Yeah, typically it seems like gas piston manufacturers like to vent heat either right in front of my optics or right where my offhand is.
 
IdahoLT1, if you're trying to make an "apples to apples" comparison between DGI and GP ARs in terms of accuracy, wouldn't it make sense to shoot some groups with a DGI AR, then retrofit a GP kit and shoot some more groups. At the very least, compare an LMT, Bushmaster etc. standard DGI AR with one of their GP variants. Then you might have some meaningful data. As it stands, that list is meaningless. Good grief, most rifles weren't shot in the same month let alone on the same day, some not even in the same year!! How about ammunition used? What about barrel differences such as twist rate or type of manufacturing.

:)
 
boba fett; you are right, like the muzzle/flash supressor, and the piston itself.
but the piston cools down very fast, and neither are what you would consider, very likely to break down from heat/filth, anytime soon!!!!
 
SHvar said:
Gas pistonswere made for the AR rifle system for the simple reason that the DI system has serious shortcomings, one of those is the hours of cleaning and constant oiling required. Why did our military ask for a much more reliable system to be designed for them? Simple when it comes to the nasty gritty dirty desert, and faced with long drawn out firefights and attacks that can come at any time our soldiers werent always able to rely on the M4 to work every time.
Like I said before, if you religiously clean and oil an AR/M16/M4, between uses, while in the desert while not using every day, between every sandstorm, after every trip down a dusty road on a vehicle, you may not have a single problem. But the enemy doesnt wait for you to clean your weapon then reasseble it multiple times a day, they would much rather know when you clean it to attack you.
Click to expand...

The M16/M4 does not need to be cleaned that often. We would do a quick wipe down after a mission, 5 minutes, and a good cleaning every week or so. My unit never had problems with our weapons, save for a few of the M249s.

With a mag in and the dust cover closed, very little dust will get in the rifle. During the invasion we actually fought during the huge red sand storm and two days after. Our rifles still worked.

Also the rifle doesnt have to be inspection clean to run. Wipe the bolt and carrier down and clean out the chamber and the lugs. Wipe down the inside of the upper reciever where the BCG rides. You dont need to clean the carbon in the rear of the reciever above the charging handle for the weapon to run well.
 
Soldiers are always sensitive to criticism of their equipment. They are issued a weapon, get used to its faults, and they don’t get to use the alternatives.

The direct impingement gas system of the M16 is an evolutionary dead end. It will be continued to be used in the Civilian sector (AR-10 copies for example), but no service rifle in the world is using the system. Neither is the system being used in squad automatic weapons.

It was not best of its class, it will never be best of its class, it is sticking around because it is too costly to replace, and too politically difficult to overcome the Colt monopoly on Capital Hill.
 
That statement couldn't be more wrong. Inertia is what keeps the bolt (or slide, on a pistol) moving rearward after the breech unlocks. If you still have chamber pressure after the bolt unlocks, your firearm is broken and dangerous to use.

Inertia is driven by the cartridge face pushing the locked open BCG. On DI or roller locked guns, there is NO operating rod to do the job. ONLY the cartridge face pushing the bolt backwards can do it.

Dispensing with blot lugs entirely, blowback from the pressure in the cartridge case is the only thing that makes .22's, .32's, and .380's semi autos work at all.

The 9mm AR pistol and carbine conversions dispense with the gas action entirely. Straight blowback. There is no DI system, a special bolt, and the dedicated uppers don't have the gas tube hole in them.

AR's not ejecting or cycling correctly are sometimes diagnosed as "short cycling." The cause of that is ammo with low powder quantities that produce sub optimal gas pressure. There's enough to unlock the bolt, but not enough left in the barrel to push the cartridge case against the bolt to fully cycle the action. According to pressure graphs, that pressure is an initial peak that tapers at the bullet travels the barrel and exits. The bolt doesn't unlock until then - and the residual barrel pressure in the cartridge case then moves the bolt backward to cycle the action. That's because gas will exert pressure on 360* of the interior of the object containing it.

Pistons don't change any of that, properly designed, they just move the bolt differently in the unlocking phase. Whether it's worth $400 to excuse cleaning a rifle properly is bogus. It's a long standing myth that they need extensive and intimate cleaning on a demanding schedule. That old chestnut has is based on a 45 year old problem long ago solved when the ammo was changed to the correct powder, cleaning kits were issued one per rifle, and Ordnance regained their directive position - telling one and all to clean the weapon periodically - AND LUBE IT.

I can buy more lube for $400 than I will use in my lifetime of shooting AR's. Piston conversions are a marketing exercise exploiting the technical ignorance of a gullible public.
 
Inertia is driven by the cartridge face pushing the locked open BCG. On DI or roller locked guns, there is NO operating rod to do the job. ONLY the cartridge face pushing the bolt backwards can do it.

Dispensing with blot lugs entirely, blowback from the pressure in the cartridge case is the only thing that makes .22's, .32's, and .380's semi autos work at all.
Click to expand...

Still wrong. There is still some pressure as the case begins to extract. But by the time the case mouth clears the breech, the bullet has left the barrel. No more pressure, just momentum of the slide.

In roller delayed, short or long recoil operated locked breech, or gas operated weapons, the bullet is out of the barrel by the time the action unlocks. The bolt has already begun moving rearward before the lugs unlock, and that momentum continues to move the BCG after the pressure has dropped. What happens when the bolt opens before the bullet has left the barrel can be seen in any instance of out-of-battery firing, in which the case usually ruptures and the action cycles very violently. You also end up with a face full of debris that was spewed out of the action because it opened too soon, while there was still pressure in the bore and chamber.

If it were only the rearward force of the cartridge case under pressure that moved the bolt backward through it's entire range of travel, we wouldn't need extractors. There are a handful of small caliber blowback guns that will cycle without extractors (Beretta 950 BS, for example), because they are timed to operate that way. But for most blowback, and ALL locked breech weapons, the pressure is gone by the time the action opens, and the case will not push itself out under gas pressure. It must be extracted, and this is done by the mechanical extractor attached to the BCG or slide that continues to move with inertia alone.
 
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That statement couldn't be more wrong. Inertia is what keeps the bolt (or slide, on a pistol) moving rearward after the breech unlocks. If you still have chamber pressure after the bolt unlocks, your firearm is broken and dangerous to use.
Click to expand...

All semiauto mechanisms are designed to unlock when there is residue pressure in the chamber. The unlock must occur at a pressure below the rupture strength of the cartridge.

This is why case life is so low in Garands and M1a's, the case is being stretched during extraction.

The residual breech pressure is measured and the action is timed to open to make the maximum use of the energy available.

This also explains why there are warnings about port pressures and slow burning powders.

In this Army Design manual they measured the residual pressure curve of the 308 and 30 Carbine because they needed this information in the design of the weapons.


Pressuretimecurve762NatoAMCP706-260.gif

30CarbinePressuretimecurveAMCP706-2.gif
 
Slamfire-

We're not debating that there is some residual pressure as the case begins to extract. The subject here is that it is not that pressure that causes the action to cycle after the bolt unlocks, but the momentum of the BCG that is already traveling rearward from actions induced when the pressures were still high.

Anyone who doubts that it is strictly the momentum of the BGC that provided the thrust need only remove their extractor and witness that there is insufficient pressure to even push the case out of the chamber, let alone move the bolt.
 
I have heard many people say that the Piston guns run cleaner than the DI guns and I have a question....

The same amount of gas/gunk/etc comes out the gas hole in either case. That stuff has to go somewhere, so either it goes in the piston, or the upper.

Is that true?
 
 
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for a short answer, DI is lighter and just as good - that makes it preferable for most purposes.

for long answer, see above.

Now if I may be fighting in dirt, dust, & mud for long periods of time without cleaning, then I'm gonna go with a longstroke Kalashnikov action (i.e. a Robinson XCR), not a piston AR using a short-stroke piston system.

Since I don't 'fight' at all (fingers crossed), let alone in dirt, dust, & mud for long periods of time without cleaning, the DI guns (AR15 types, AR10 types) are *perfect* for me (and most people).
 
Depends, at one time I did fight in the desert, and I never had a problem with a DI full sized rifle, didnt have a carbine to compare it to then. I cleaned daily, and yes it depends where you are at the time you can see sandstorms almost daily (depends on the weather and time of year), in fact depends how nasty it got I cleaned 2 times a day and kept as much of the rifle covered as possible (guess what sands still gets all through it).
Heck I had a malfunction in training once with the M16, it was after almost a month of training in the mountains, while the OPFOR attacked, I cleared the problem, and continued, but it was due to long days, and not enough maintance (I was getting my cleaning kit out to clean right before it happened). Keep in mind it was not in the sand, it was actually nice calm weather, and very little rain in that time.
By the way when you fire a piston AR the piston assembly gets hot, but cools down fast, and its not an area of the rifle that is affected by the heat, grit, carbon, burnt oil, and wont cause it to malfunction.
Also when I fire my gas piston AR the hot gases dont effect my optics, or sights, nor does it heat my hand, or the handguard up.
Yes a GP AR is more difficult to fire as accurate, but thats not a problem in the rifles design, or function, nor will that ever cause it to malfunction under strenuous conditions.
The primary reason DI rifles are used in so many cases in the military is the fact that they are so easy to fire accurately, this allows you to make use of more soldiers with less stringent training, allows you to qualify higher numbers in a shorter amount of time.
Theres nothing wrong with DI rifle if used corectly, if maintained like your life depends on it. The GP is just an option that allows for a more broad range of uses under more extreme of conditions.
They are fun to shoot, Im glad I bought one.
 
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