So I have a quick question regarding piston operated uppers.
In a normal D.I. upper, as gas is imparted into the gas tube, it travels down through the gas key, into the cavity in the carrier, and pushes the bolt itself forward. This eases extraction while also thrusting the BCG rearward. So essentially the gas is used to force the carrier rearward, bolt forward, rotate and unlock the bolt from the breach, then extract the round as it travels rearward.
However, in a piston system, the gas key is replaced with just a flat piston key which the short stroke rod impacts and forces the BCG rearward… so there is no forward motion of the bolt to help unlock and ease the stress on the lugs right? It just rips it right out… which to me sounds very detrimental to the system… it extracts the round before it may be at the optimal size to extract ( forward motion of the bolt by a D.I. system provides sufficient time for the case to cool and shrink) and it just shoves the carrier to the rear and basically just rips the lugs out, causing them to sheer against the extension lugs until they eventually unlock and allow the bolt to leave.
Basically piston systems just use brute force to rip the bolt/shell out of the chamber… seems to me like carrier tilt would be a drop in the bucket compared to the stresses that type of action has on the rifle, so the various retro-fit kits available would be about as beneficial as using a filled diaper to lube your gun…
The gas rings on the bolt are replaced by a spring, constantly pushing the bolt forward. What you’ve stated may very well be the reason behind it.
Someone else may know better/more…
in my writeup back in feb 2010, this is what i wrote:
“The new PWS 5.56 AR Piston Bolt is heat treated to mil spec and MP tested. The bolt has no gas ring groove, but instead has a modified shoulder that is designed to accept a spring behind the bolt. The spring provides a slight amount of pressure that ensures that the bolt is in its most forward position in the receiver extension when in battery. One of the reasons why I left the gas rings on the bolts of my piston guns was to provide some drag inside the carrier, to help reduce the cam pin slamming into the carrier. During the cycle, a bolt without gas rings is free to ‘bounce’ back and forth a minute amount, and the cam pin will slam around in its channel and at the end of its movement.
When in battery, there is still a minute bit of slop in the bolt, and at the time of firing, it may not be in its most forward position. On a DI system, the gas expands into the space between the tail of the bolt and the bolt carrier, and pushes the bolt and carrier apart, ensuring that the bolt is pushed forward into the receiver extension. On a piston gun, the bolt is just unlocked by the backward motion of the carrier when the piston/op rod moves back. If the bolt isn’t in its most forward position, the cam pin can make premature contact with the cam pin recess on the inside of the receiver, causing more wear than with a DI gun. The wear might be self-limiting and not an issue, but it also implies more stress and wear on the components than if it was prevented or reduced. The PWS piston bolt with spring is designed to reduce this by keeping spring pressure on the bolt so the slop is taken up. The bolt is for use in piston rifles only, and will not work in DI rifles.”
Piston ARs are like the rotary engine of the car world but of the rifle world. In theory they’re better…but in execution not so much. There is no “free lunch”. If you want a piston rifle buy one that was meant to be one (AK, FAL, M1a, SCAR).
how do they go about keeping the bolt forward during extraction? Or do they not worry about it and simply just use brute force of the piston pushing the BCG rearward?
Although the purpose of the firing pin spring is to push the firing pin back in order to activate the firing pin safety after firing, the firing spring also spring loads the bolt. If it pushes it forward, I can’t really say, since I can’t see inside the gun…
Anyways, there is no sign of excess stress on my BCG, barrel extension or inside the upper on my gun. No signs of carrier tilt either. I have probably close to 10k through it…so I guess the design works, huh?
I doubt you could use any spring in a piston AR bolt carrier that would provide as much thrust on the bolt without pushing the carrier out of battery as a DI AR does with gas pressure. We are basically talking thousands of PSI vs a spring that could only supply at most 12 pounds of force before it starts to overcome the action spring.
As to whether the gas pressure behind the bolt actually pushes it off of the barrel extension lugs, I really couldn’t say, but I’m sure it at least cuts down the load on the lugs during unlocking.
Huge +1 to this. I love the people who take an AR and then make it piston driven and get .300 AAC… isn’t that just the AKM? 125 gr bullet, 2300 fps, piston gun…
Keep your ARs ARs - Direct impingement and 5.56. That’s my opinion, anyway… not that I know anything
While the gases do push against the bolt, they cannot push the bolt forward! Although the bullet has exited the barrel by the time the expansion chamber fills with enough pressure to start the carrier moving, the pressure in the bore is still greater. Also, the spent case has expanded enough to take up what little slack there was and resist any forward motion of the bolt.
It’s also a fallacy to believe the AR does not have a piston. It’s piston is part of the bolt and the carrier is a cylinder. The piston is inline with the bolt and bore, reducing muzzle climb induced by off center recoil forces and reciprocating mass.
The AR does not feature primary extraction, a mechanical camming action that starts extraction as the bolt begins unlocking. Many other designs do, such as the M14 and most bolt actions. Whether the AR uses the original inline piston or an aftermarket offset piston relocated to the gas block, it must rely on “brute force” to yank the spent cartridge from the chamber
I have one of each. I like my DI. I wanted a piston gun(just because). I like AR’s not ak, fal etc. etc. blah blah. So…I built a piston AR! and I shoot the shit out of em both and don’t worry about how many milliseconds longer the damn piston bolt lugs are taking to unlock from the exstension lugs than the DI bolt is or how forceful one process is over the other!
And how exactly did you come to that conclusion after reading MistWolf’s post?
How is the unlocking process of the bolt in the M16 FOW different than in an op-rod design, considering that the bolt cannot move forward?
Also, I just spoke to the main armorer here, and he reiterated that there is no difference in the unlocking process between the two systems; the gas pushes on the carrier and the op-rod pushes on the carrier. The cam pin follows the cam track as the carrier moves to the rear, and this causes the cam pin and bolt to rotate until the lugs on the bolt are no longer in line with the locking lugs in the barrel extension.
There will be wear on the locking lugs on both systems.
and I shoot the shit out of em both and don’t worry about how many milliseconds longer the damn piston bolt lugs are taking to unlock from the exstension lugs than the DI bolt is or how forceful one process is over the other!