AR15 gurus. Difference between DI and piston

tootall

tootall

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I'm trying to understand EXACTLY what happens during the firing cycle of an AR-15.

I think that in a standard DI one, it works this way:
The exhaust is fed deep into the bolt carrier and bolt, which forces them apart.
The bolt cannot go forward, as it is already bearing on the rear of the barrel.
The carrier is free to move backwards, so it does.
As it does, it rotates and unlocks the bolt lugs from the barrel extension
The carrier momentum drags the (now free) bolt rearward with it, against buffer spring.
And forward again...
like this.
http://4.bp.########.com/-2F9CKeP9gy8/UiWmMSeO6vI/AAAAAAAACHs/5LL1OKKdupo/s1600/023.gif

In a piston version AR, the bolt does not get gassed up.
Instead, the piston rod smacks the bolt carrier, and punches it rearward.
The carrier motion cams the bolt open and then carries the bolt rearward as above.

So far so good...

MY QUESTION IS
When I look at how the forces are applied, it seems to me that:
In the DI version, the bolt is forced forward against the rear of the barrel. So the bolt lugs are bearing on their FRONT surface.
Contrast that to the piston system where the carrier moves rearward first, and the bolt hangs up on the barrel extension lugs before rotating out of lock due to the camming action of the carrier.
So the bolt lugs are bearing on the REAR surface.

Is this correct?

Oh BTW, apparently the term DI is not actually correct for Stoner's design.
http://en.wikipedia.org/wiki/Direct_impingement#History
It is incorrectly believed that the AR-15/M16 rifle designed by Eugene Stoner uses a direct impingement action. In the Stoner system covered by U.S. Patent 2,951,424 Stoner specifically states that the action is not direct impingement saying ″This invention is a true expanding gas system instead of the conventional impinging gas system.″[1] Gas is routed from a port in the barrel directly to a chamber formed in the bolt carrier. The bolt acts as the piston and is sealed with small automobile-style piston rings. It is a gas piston system without an operating rod.
 
DI and Piston ARs work in a similar manner with regards to moving the BCG. One just uses a piston to drive the carrier rather than direct gasses.

DI dumps hot, dirty gasses into the carrier group. As a result, there's more fouling in the BCG area and it tends to run hotter hence why they say DI guns like being run "wet" ie well lubed.

Piston dumps hot, dirty gasses in front part of the rifle. As a result, the BCG area is much cleaner and runs much cooler. However, the front piston part of the system gets all the dirty gasses that would have otherwise ended up in the BCG area.
 
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TT

you are forgetting that the case pressure is also pushing rearward during the firing process. you have lock up with both. hence the need for the cam slot.

I would suggest that the term should be DGI - direct gas impingement. As impingement occurs with both a traditional and piston set up.

BG
 
tootall said:
Oh BTW, apparently the term DI is not actually correct for Stoner's design.
http://en.wikipedia.org/wiki/Direct_impingement#History
It is incorrectly believed that the AR-15/M16 rifle designed by Eugene Stoner uses a direct impingement action. In the Stoner system covered by U.S. Patent 2,951,424 Stoner specifically states that the action is not direct impingement saying ″This invention is a true expanding gas system instead of the conventional impinging gas system.″[1] Gas is routed from a port in the barrel directly to a chamber formed in the bolt carrier. The bolt acts as the piston and is sealed with small automobile-style piston rings. It is a gas piston system without an operating rod.
Click to expand...

This is very much correct. The conventional M16FOW operates more as a piston than a true DI system.


Trinimon said:
DI and Piston ARs work in a similar manner with regards to moving the BCG. One just uses a piston to drive the carrier rather than direct gasses.

DI dumps hot, dirty gasses into the carrier group. As a result, there's more fouling in the BCG area and it tends to run hotter hence why they say DI guns like being run "wet" ie well lubed.

Piston dumps hot, dirty gasses in front part of the rifle. As a result, the BCG area is much cleaner and runs much cooler. However, the front piston part of the system gets all the dirty gasses that would have otherwise ended up in the BCG area.
Click to expand...

Lots of misinformation here. "DI" guns vent the excess gas out of the bolt carrier. Reference the holes located at the relief groove for the ejection port cover. The main reason that "DI" guns tend to be dirtier is that they operate faster than "piston" guns. Because of the quicker operation, this tends to create a vacuum and sucks the gas used to propel the cartridge out of the barrel, back into the barrel. Roughly the same thing happens on tanks, and is the reason that some have a bore evacuator on them.
 
DI push on the bolt and carrier to unlock the bolt in a straight rearward motion. Piston AR pushes on top of the bolt carrier causing the bolt the tilt slightly as it unlocks. So using science and logic, a piston ARs bolt under goes more stress "no ifs ,ands or buts"

IMO piston gun that began as a piston gun design are great but DI designs re designed as piston guns will run into issues sooner or later.
 
MY QUESTION IS
When I look at how the forces are applied, it seems to me that:
In the DI version, the bolt is forced forward against the rear of the barrel. So the bolt lugs are bearing on their FRONT surface.
Contrast that to the piston system where the carrier moves rearward first, and the bolt hangs up on the barrel extension lugs before rotating out of lock due to the camming action of the carrier.
So the bolt lugs are bearing on the REAR surface.

Is this correct?
Click to expand...

Kinda. You are forgetting that the case is pushing backwards on the bolt face. So really a DI system has the bolt pushing forward on the case and not the front face of the bolt. Keep in mind the space between the front face of the bolt lugs and the back of the barrel is quite small ca .005" or less.

The really interesting thing about the DI system is that it is somewhat self adjusting. It takes in the gas it needs and then when the carrier moves a certain distance any excess gas is vented out the side of the carrier through those two little holes. You can see this by manipulating the BCG and watch when the gas rings clear those two little holes.
 
Dsiwy said:
DI push on the bolt and carrier to unlock the bolt in a straight rearward motion. Piston AR pushes on top of the bolt carrier causing the bolt the tilt slightly as it unlocks. So using science and logic, a piston ARs bolt under goes more stress "no ifs ,ands or buts"

IMO piston gun that began as a piston gun design are great but DI designs re designed as piston guns will run into issues sooner or later.
Click to expand...

Exactly.
 
Dsiwy said:
DI push on the bolt and carrier to unlock the bolt in a straight rearward motion. Piston AR pushes on top of the bolt carrier causing the bolt the tilt slightly as it unlocks. So using science and logic, a piston ARs bolt under goes more stress "no ifs ,ands or buts"

IMO piston gun that began as a piston gun design are great but DI designs re designed as piston guns will run into issues sooner or later.
Click to expand...

Yes, that is how I see it as well, due to the return spring being on a different plane than the piston pushrod in a retrofit.
In a piston gun designed that way from the start, the return spring is directly behind the pushrod, so no rotational forces.

Boltgun said:
TT

you are forgetting that the case pressure is also pushing rearward during the firing process. you have lock up with both. hence the need for the cam slot.

I would suggest that the term should be DGI - direct gas impingement. As impingement occurs with both a traditional and piston set up.

BG
Click to expand...

Suputin said:
Kinda. You are forgetting that the case is pushing backwards on the bolt face. So really a DI system has the bolt pushing forward on the case and not the front face of the bolt. Keep in mind the space between the front face of the bolt lugs and the back of the barrel is quite small ca .005" or less.

The really interesting thing about the DI system is that it is somewhat self adjusting. It takes in the gas it needs and then when the carrier moves a certain distance any excess gas is vented out the side of the carrier through those two little holes. You can see this by manipulating the BCG and watch when the gas rings clear those two little holes.
Click to expand...

I had wondered about how the case head bearing on the boltface would factor in, but did not comment on it in the OP. (because I wasnt sure...)
So does the rearward push of the casehead get somewhat cancelled out by the forward thrust of the bolt?
 
Stoner's design is better cataloged as an inline piston system.

One thing folks here are forgetting is in this system, air (already in the gas tube) is compressed and acts on the bolt first of all. Upon firing the gases directed thru the gas port first compress the air in the tube and force it first thru the gas key on the carrier into the carrier body. The bolt has already been pushed back onto the barrel extension lugs upon initial setback (firing) and the pressure on the bolt is somewhat more relieved by introduction of the expanding gasses. The pressure acts on the piston and forces the carrier rearward while the cam path acts on the cam pin to unlock the bolt.

WRT to person (Motiuk) who suggested the Inline Piston forms a vacuum - no -- the same issue happens on piston guns -- with the same cam path, what is happening is the bore still is pressured and when you unlock your creating another opening for the gases and fouling to go -- look at a Hk416 suppressed...

Piston guns really only offer one thing over DI guns, submerged/partial submerged firing - which is IMHO one of those iffy requirements at best (ever looked thru an optic coming out of the water...)

The main way to counter fouling in the operating system is to change the cam path -- while I am not a big SCAR fan - if you look at its cam path, it is longer, and allows more pressure to subside from the bore and thus less garbage is brought back into the receiver.

The best way to look at all of this is with partial cutaways and high speed video -- its educational.
 
tootall said:
Contrast that to the piston system where the carrier moves rearward first, and the bolt hangs up on the barrel extension lugs before rotating out of lock due to the camming action of the carrier.
So the bolt lugs are bearing on the REAR surface.

Is this correct?
Click to expand...
This detail actually takes some careful consideration.
An improperly timed system can result in bolt lug round off. More of an issue then carrier tilt in some piston systems from my experience.
Some systems have a "bolt assist spring ", a little spring that goes inside the carrier behind the bolt to activate camming action as the carrier moves backwards. I figure this spring is n good idea.

I picked up a CMMG piston kit, I had to tune it up a bit for optimum operation.
Works great now, but I can tell if you have too much gas, or too light a buffer, too tight of gas rings, or too weak an assist spring, the rifle could self destruct.


PS. yeah, the CMMG kit requires you to use the gas rings, and if you use fresh tight rings, the spring can't do it's job properly.
 
jackrabbit000 said:
Check this out:

[youtube]H3TVsekcFWo[/youtube]
Click to expand...

So full of BS it was funny.

Dsiwy said:
So using science and logic, a piston ARs bolt under goes more stress "no ifs ,ands or buts"

IMO piston gun that began as a piston gun design are great but DI designs re designed as piston guns will run into issues sooner or later.
Click to expand...

I heard a good explanation as to 'why' that was, direct from the mouth of a Colt Canada engineer:

Basically, the gas in a DI C7/AR acts as a better 'cushion' to opening the bolt, as the pressures between the bolt and carrier group are highest upon firing, venting off as the gas rings on the bolt pass the holes in the carrier. To simulate this, H&K put a spring inside the carrier behind the bolt to recreate this initial opening force. However, the force to open the carrier/bolt now comes from the piston rod, above that expansion chamber, and the spring can't simulate what the gas was designed to do in the first place. Not without a strong spring, and a stronger buffer spring to close the whole deal up... and so on. Either way, the end result is the steel bolt cam shredding the aluminium cam clearance on the upper, wearing it out at a much faster rate because of the increased/misplaced forces on the bolt carrier.

At least that was with the early versions. AFAIK. I hear H&K is refitting/repairing a bunch of these with a fix, but I have no firm idea as I don't follow news from H&K... maybe someone in the biz can enlighten us.


http://www.hkparts.net/shop/pc/diagram.asp?wid=27
Have a look and you'll see what I mean.
 
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Suputin said:
The really interesting thing about the DI system is that it is somewhat self adjusting. It takes in the gas it needs and then when the carrier moves a certain distance any excess gas is vented out the side of the carrier through those two little holes. You can see this by manipulating the BCG and watch when the gas rings clear those two little holes.
Click to expand...

Thanks, I always thought the holes were there to oil the rings. Learn something new everyday here.
 
The way I see it is that they both break in different ways.

The big question is: At how many rounds and at what firing schedule does the system fail? Failure means the weapon ceases to function.

The other way of asking the question, does it pay to buy a "reliable system that wears out within a known period of time" or a "system that lasts a long time but it will fail fast once it is used beyond its limit".

Let's look at it this way, if the receiver of a piston gun receiver progressively wears out in 20,000 rounds and a DI bolt has a high probably of snapping in under 6000 rounds under an intensive firing schedule, it is better to toss away a piston gun ( or send it back for overhaul) every 20,000 rounds than snapping a bolt without knowing when it will happen. If I am running a SOF kind of organization, that is a pretty good trade off for a small fleet of weapons.

On the other hand, for a large organization who does more traditional infantry or LE enforcement with a less demanding firing schedule, parts are not changed until they are broken and the entire inventory is maintained as a "fleet" on a set schedule. In this scenario, a good DI gun will probably last a long time and it does not pay off to buy a piston gun and pay for the weight penalty. If you look at it this way, you get more bang for the buck with DI guns due to the scale of economy in both upfront cost, lower maintenance skill needed, and parts replacement. The number of failure due to excessive usage can be predicted based on organization history and as a project manager you will just factor those incidents into the total life cycle cost.

For a regular joe weekend warrior, there isn't really a need to buy a piston gun. And for those who shoots a lot, buying a 90 dollars bolt every 6000 rounds or until failure is still cheaper than buying an expensive HK piston or LWRCI gun. On the other hand, if I am in an organization where I need to fire fast and furiously and it must be reliable/durable only in a predictable a set number of round count(more than 6000), I will be exploring some other options other than DI guns.

But if you really like a piston gun, buy it. It is not a choice of economy - I buy guns because I like guns. And deep inside everyone loves good "fit and finish" even if we deny it in the forum. :)
 
KevinB said:
WRT to person (Motiuk) who suggested the Inline Piston forms a vacuum - no -- the same issue happens on piston guns -- with the same cam path, what is happening is the bore still is pressured and when you unlock your creating another opening for the gases and fouling to go -- look at a Hk416 suppressed...
Click to expand...

Correction. I did not intend to imply that only DI guns create a vacuum. When any firearm discharges, the pressure in the bore drops and will form a partial vacuum (ie rarefaction)


To further comments here, piston guns do have the potential to be less accurate. Typically, the piston starts moving before the bullet has left the bore, and the moving mass is off the axis of the bore. This tends to accentuate the flex of the barrel outside of the primary vibration.

Also, Stoner's design has no camming action for primary extraction, and instead relies on pressure forcing the bolt forward, which unloads the locking lugs to allow free movement with little contact pressure. On piston conversions this does not happen, and may lead to increased wear on the lugs, and accelerated headspace growth.
 
Can people explain pws ar15 long stroke system like in the ak47 ? what the difference between between it and piston or DI
 
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