... who else?  There seem to be numerous manufacturers making short-stroke piston conversions, but besides PWS (and a few very proprietary AR-derivative systems), does anyone else make a long stroke system?

PWS use to sell a conversion if I recall, but now it is only rifles and uppers.

Adcore is a long stroke. It has a solid rod as part of where the gas key would be on a DI.

No adcor is a short stroke. It has a really long op rod but that op rod is tapped by a piston located in the gas block. I would know I own one.

The Faxon Firearms ARAK-21, while not a traditional AR-15 upper, is a long stroke system.  It's a self-contained upper that mates with AR 15-lowers.  All parts, save the firing pin, are proprietary.  It also doesn't need the buffer/spring/tube to function.

So it works like the Saiga shotguns?

Being as I've never owned a saiga or looked at them in any real detail I couldn't tell you. But I can tell you and show you how the BEAR system works

In the gas block you have your piston, piston spring, and regulator.






The gas block mounts into the upper receiver via a pin which is secured via a retaining pin held in place by the removable lower handguard. The gas block interfaces into a gas key that comes off the barrel in an L-shaped fashion.





The OP rod extends off the BCG through a guide located in the upper and sits directly behind the gas block containing the previously mentioned components. The OP rod is staked in place on the BCG.




When you fire the weapon gas travels into the gas tube into the gas block where it pushes against the piston tapping the OP rod starting the operation cycle. As the piston cycles it passes two vent holes at the bottom of the gas block where gasses are expelled and the piston return spring pushes the piston back against the regulator. As this is happening the OP rod and BCG cycles backwards unlocking the rifle, ejecting the spent casing, stripping a fresh round from the magazine, chambering the fresh round, and locking, while cocking the hammer readying the system to fire again.

That's how it works. Nothing more to it.

TNW used to claim their piston system was long stroke.  It is not clear to me if it is or not:

http://www.tnwfirearms.com/product-p/gpsx-cplt-ar15-xxxx-xkit.htm

I was going to say that I had forgotten about the Adcor BEAR, but it appears that it actually wasn't an omission from this list ...



So, instead of the op rod being fixed to the back of the piston head and pushing a key on the carrier, to which the rod is not attached, the piston head pushes on the op rod, to which it is not attached, which in turn pushes the carrier, to which the rod is attached.  The piston still only strokes a small portion of the distance that long-stroke system does, but the disconnect is further forward.  Good to know.  

It actually makes sense that they have the separation where they do given their forward charging handle.  It can then push on the op rod without moving the piston.  Out of curiosity, do you have pics of the charging handle internals and how it interfaces with the op rod?

I'll have to dig around. I can't take new ones right now for obvious reasons.

They are all short stroke.  Even if the piston is attached to the bolt carrier (i.e.  Yes, AK is a short stroke)

Long stroke is as the name says, the bolt unlocks after a long distance.
Long stroke designs have not been popular since world war 1

Of course, clip = magazine now days, you can call long stroke if you like for any short stroke with gas piston attached to the carrier.

Er, long-recoil designs haven't been popular in light automatic weapons since the ChauChat, but if the piston itself reciprocates with the bolt carrier, it is by definition a long-stroke gas system.  If the piston reciprocates a shorter distance than the bolt carrier, then it's a short-stroke system.

"Long stroke piston operation:

 In this system the piston is connected directly to the breech block
and controls the position of the block at all times. This sort of
arrangement is found in by far the greatest number of machine guns. Typical
examples are the FN MAG, the USA M60 and the Russian PK. The
piston tends to be long and heavy and the recoil mas is considerable."



"The short stroke piston:

 Here the piston moves back a distance that can be as short as a fraction
of an inch but it imparts its energy to an operating rod which forces the
breech block to the rear.
 This way of arranging things is suitable for rifles and is found on the very
great majority of gas operated rifles. (A notable exception is the Russian
Kalashnikov system which is a long stroke system.) The operating rod
impinges on a bolt carrier and the movement of the carrier first unlocks
the bolt and then carries it to the rear. The operating rod and bolt can be
quite light and so there is not an excessive change in the centre of mass."

Source: Jane's Infantry Weapons 1975 p. 6

You're confusing long recoil and short recoil with long stroke piston and short stroke piston.  

In long recoil the bolt and barrel both travel fully rearward, at the end of their stroke the barrel is unlocked and returns forward, once the barrel has returned forward the bolt then follows.  In short recoil the bolt and barrel travel rearward for a short distance, the barrel is then unlocked and remains stationary, the bolt continues rearward.  Upon returning forward the bolt captures the barrel again and they both move the short distance forward into the fully locked position.

In a long stroke gas piston, the piston travels with the bolt carrier for the full motion of the carrier.  In a short stroke gas piston the piston strikes the bolt carrier or op rod and then returns forward, inertia continues to carry the carrier rearward.

The Remington 11-48 was a long recoil operated shotgun.  Almost every semi-automatic pistol uses some form of short recoil with the Browning system being the most common.  

The AK-47 uses a long stroke gas piston wherein the piston is connected directly to the bolt carrier.  Upon firing gas enters the gas block and forces the piston rearward.  The PWS piston AR uses a long stroke gas piston as well.  The Dragunov uses a short stroke gas piston.  Upon firing gas enters the gas block and forces the piston back.  The piston strikes the bolt carrier, driving it rearward.  The piston then returns to its forward position while the bolt and carrier continue rearward.

If you want my opinion short stroke is the way to go. I've fired a number of long stroke piston guns including the SAW, M240B, AK47, AK74, PSL, and a few others I can't remember off the top of my head.From an engineering standpoint is long stroke technically more reliable? Yes. In practical application I've never seen any problems with short stroke piston systems. Additionally long stroke has a lot more mass and I speak from experience you do notice it a lot more when trying to stay on targeting or firing in fully automatic. I slapped my BEAR upper on an M16 lower once. It was really controllable.

The Adcor might be a short stroke, but it still has the reciprocating mass of a long stroke.

The recoil impulse still feels softer and more controllable versus my M6 IC. My adcor is a 16" and my IC is a 14.5" so that may have something to do with it too.

Probably has more to do with gas port size than it does operating system.

The Faxon ARAK is a long stroke.

Doh! Just saw it already mentioned.