Question on AR platform, DI vs piston

Status
Not open for further replies.
Also to clarify - the gas piston MUST be in motion before the bullet leaves the barrel. Simple physics. We need the piston to move, meaning it has to accelerate, and against spring tension. So the gas pressure (force distributed over an area) has to accelerate the piston and start it moving while the bore is still pressurized - once the bullet leaves the barrel, that pressure drops to zero, meaning the force the gas exerts against the piston drops to zero, and it can no longer accelerate - move - the piston. The bore and piston “cylinder” must be under pressure to apply force on the piston, so the bullet still has to be holding pressure in the bore. Once a force is exerted, the piston accelerates - starts moving - and it can only remain moving after the bullet leaves the bore and the force/pressure subsides by its own inertia (state of movement, current velocity when the force stops acting). So it’s patently false to say “the bullet is gone before the piston starts moving.”

I don’t know how many times whether it be ARs or semi-auto handguns I’ve heard people say the bullets left the rifle/pistol before the recoil or movement occurs.
 
Last edited:
I have plenty of practical experience with both, and I'm not a huge fan of piston systems. For one thing, there are many different designs, and they are all proprietary. This matters when we are talking about compatibility and interchangeability for upgrades, repairs, etc. The piston guns also cost more. Where the piston guns shine are in military units or agencies where users are firing a lot of rounds between cleanings. I had DI guns the whole time I was in the military, in a unit where a M4 barrel rarely lasted more than 2 years due to the amount of rounds we fired. I never felt at a disadvantage for not having a piston gun, and with the proper maintenance, M4's and similar guns are very reliable. Remember that the entire mideast is covered in a talcum-powder like moon dust.
 
that only applies to the receiver itself, and the further machining is important, as all the important dimensions are machined, not forged. then all the extra pieces parts, including BCG are different.
and just as important is the care taken in assembly. some "big name" makers were notorious for using barrels of red loctite when higher dollar manufacturers were using anti-seize and proper torque, and staking, depending on the parts. i suppose they're a lot better now, but it used to be pretty easy to spot shoddy assembly just by looking at the nuts on the gas key.
20 years ago, COLT was the ONLY AR to get, then FN, later a smattering of name brands joined in, now a days it's almost entirely commodditized, with almost every brand getting their parts from just a handful of big suppliers. I will say there are still a few smaller operations making higher quality parts and assembled guns, but they run 2-3 times what the cheaper guys do, and most shooter will never see the difference.

If money is no object, I'm staring at Daniel Defense or Aero Precision, but I think 99% of us are on a little tighter budget.
 
Also to clarify - the gas piston MUST be in motion before the bullet leaves the barrel. Simple physics. We need the piston to move, meaning it has to accelerate, and against spring tension. So the gas pressure (force distributed over an area) has to accelerate the piston and start it moving while the bore is still pressurized - once the bullet leaves the barrel, that pressure drops to zero...
The Army did studies and confirmed that the bullet is indeed clear of the muzzle and several feet downrange before the gas pressure builds up enough to get the piston moving. This is true even of the M14 which has several inches of barrel past the gas port and the piston right the in the gas block. Slow motion video confirms this.

The pressure in the bore does not drop to zero as soon as the bullet uncorks the muzzle. It takes awhile for the pressure to drop. Just like when you blow up a balloon, then release the neck. The balloon doesn't deflate instantly. Watching slow motion video clearly shows the the gas continues to flow out of the barrel long after the bullet has exited. Piston driven gas operated rifle work off the residual pressure left in the bore after the bullet has cleared the muzzle.

If the action were to start moving while the bullet was still in the bore, suppressors would not cause over gassing because the action would be moving before the bullet got to the suppressor. Suppressors cause over gassing because they delay the pressure drop in the bore. This delay means the residual pressure in the bore hasn't dropped quick enough for the correct flow through the gas port.
 
The same videos have been criticized for years as red herring evidence, as it effectively promotes the idea if you’re pushing hard on your car, but can’t get it to roll, it’ll start rolling when you don’t push as hard. Acceleration and perceived velocity aren’t always the same thing.
 
Physics is physics. The time the bullet is in the bore from gas port to muzzle is measured in fractions of a millisecond. It takes time for the gas to travel through the gas port and pressurize the piston. It takes longer to travel down the gas tube and pressurize the expansion chamber. It takes time for the passenger to get out, walk to the back of car and curse the driver for not filling the tank the day before.

There it's an official paper put out by the Army on the subject. Armalite use to have it posted in the tech section of their website.
 
PS- If a piston driven, gas operated rifle didn't run off the residual pressure, the Garand would operate at all.
 
If you want to get a piston gun, get one that was designed from the beginning as a piston gun. HK and PWS actually resigned the upper reciever to fit their pistons and didn't just shoehorn the op rod through the gas tube hole. Those two guns tend to be really reliable.

That being said a good DI AR is a very reliable weapon. Military Arms Channel ots currently doing a long term reliability test on a BCM rifle. They cleaned it abs lubes it initially and now they are shooting it without any cleaning or extra lubricant. At the latest video he is just over 6000 rounds. Its starting to get sluggish and ejection is starting to get erratic, but it's still cooking.

Look up the famous Filthy 14 rifle. It was a loaner rifle at a training facility that was lubed but not cleaned. I believe it went something around 15000 rounds before it started having serious issues which prompted a good cleaning.
 
Agreed. Which is why the acceleration can’t and doesn’t start AFTER the peak force subsides.
Are you maintaining that the action begins moving before the bullet departs the barrel although the Army study found otherwise?
 
For clarity, I'd like to add that I'm not talking about the blow back, or the recoil operated systems. The actions of both the blow back and the recoil operated systems start moving as soon as the bullet does.
 
I didn't buy a complete BCM rifle. I picked up one of the complete BCM lowers that Primary Arms sells then later bought a light weight upper from them when they were on sale. I have to say that is a very good lightweight rifle. It is lighter than most piston alternatives will be. I highly recommend their rifles if you can spend the money. As an alternative, the light weight rifle kit from Palmetto State Armory isn't too bad. It actually is fairly light and shoots decently. I wouldn't put it up there with the BCM rifle, but it is better than I expected.

I have heard piston actions are better for short barrel applications so it may depend on what you want.
 
Physics says it must even if its a barely perceivable amount.
Not from gas pressure acting on the piston, it doesn't.

I'd also like to point out to Varmint that peak pressure occurs before the bullet comes close to reaching the gas port.
 
...I have heard piston actions are better for short barrel applications so it may depend on what you want.
Shorty ARs have advanced enough over the last several years that this is no longer true. In fact, the direct gas system is now a better choice for short ARs and suppressed ARs. Much easier to tune the gas drive properly.
 
Not from gas pressure acting on the piston, it doesn't.

I'd also like to point out to Varmint that peak pressure occurs before the bullet comes close to reaching the gas port.

If there is pressure in the bolt body the carrier will be moving. There is no delayed response in physics. It might be .010" it might be .0001", but the carrier will be moving.
 
What could be happening is that the inertia of the entire rifle recoiling is essentially accelerating the rifle rearward at a faster rate than the carrier is being accelerated rearward for a moment giving the appearance that the carrier is not being acted upon until the bullet has left, but the reality would be they are both being accelerated rearward by separate forces. So essentially it might just take a moment for the gas pressure in the carrier to accelerate it off the rest of the reciever which is itself also being accelerated rearward by a separate force. Hadn't thought about that before
 
It is the dwell time, the period of time between when the bullet passes the gas port and the bullet leaves the barrel that determines pressure that moves through the gas tube and acts on the piston. By the time the bullet leaves the barrel the tube is pressurized. Depending on the many factors the bullet may be out of the barrel by the time the bolt carrier moves perceptually. But the pressure is already in the tube and peaks as the bullet leaves the barrel however it takes time for pressure to drop in the barrel and tube. If dwell time is not long enough the rifle may not have enough pressure to operate until hotter loads like NATO 5.56 are used. This was the case in a Dissipator I had. Of course this varies by port size. The pressure in the tube must overcome the inertia of the bolt carrier to vent so the bullet may be out of the barrel before that happens. It accelerates so fast that pressure is bled off as the bolt spins and the extractor begins to pull out the bullet. There is still pressure in the barrel and if it doesn't drop enough by the time the extractor moves you could have an extraction problem. This also happened to me.
 
I would go with a DI AR. While it does have a gas piston I know what you're asking and LWRC or PWS would be the best bet there, imo. I hear good things about POF too but not first hand really.

For the price of a piston you can get a nicer DI. What is your budget?

For serious use stuff I like:

Sons of Liberty Gunworks or BCM
DI LWRC(do not own one but handled and shot one)
Colt.
FN15 Tactical(I own the FDE version)

Get used to the platform before assembling your own , imo. It helps you figure out what you want or need.

For budget stuff in liking Aero Precision though. Seems pretty hardy albeit basic. Just my 2 cents.
 
Status
Not open for further replies.
Back
Top