I understand the concept that DI is the source of moving the bcg. What confuses the issue is that chamber deposits are clear evidence that the gases also exhaust from the action end of the barrel.
For them to have no effect would be from timing delayed enough to have no propulsive force against the bolt. My anecdotal impression is that blowback on the bolt still occurs and is a factor, regardless of what the action is.
It’s hard to conceive there is no effective propulsion at all in DI when the same force is the actual method of cycling in other firearms. Taking advantage of it through appropriate timing would be more efficient.
A blowback or roller lock delayed firearm is cycled exclusively by propulsion from gas pressure against the bolt face. Having DI simply unlock the bolt at the appropriate time would be sufficient. Changing port sizes will certainly affect the amount of gas flow, but it also changes the timing pulse.
I appreciate the concern that DI may be depicted inaccurately, which is why I hoped someone would come in with data and time pressure graphs to illustrate. I sell auto parts and constantly deal with questions from a completely uneducated public. I’m no stranger to the concept that “Just because I say so.” lacks sufficient detail to communicate information. I explain it anyway, and sometimes get something across. I also know I don’t know everything, and to question common assumptions, like “that’s the way it is.”
Raising questions about how the DI system works has shown me it’s not the jammomatic many believe it to be, including myself. While of limited value to the public, how it works exactly is beneficial knowledge, just like understanding that you can’t charge a battery fully in less than an hour. More like twelve. Having the right operating concepts makes related decisions more appropriate.
Let me get this: DI has no related gas propulsion affect on the bolt face at all?
From people much smarter than me…actually the reverse is true,
When gas reaches the expansion chamber in the BCG the pressure works in all directions within which causes the parts that can move to…well, move…
The carrier pistons to the rear with enough energy to complete it’s cycle, but the gas pressure also works against the rear of the bolt.
Initially, the bolt is driven back by the pressure within the case working against the head of the case, firmly pressing the rear of the locking lugs on the bolt against the front of the lugs in the barrel extension.
When the gas enters the expansion chamber, the pressure working against the rear of the bolt literally pushes the bolt forward since the area that the gas pushes on the bolt is greater than the area that the same pressure pushes against the head of the case.
That forward movement of the bolt is critical to the longevity of the bolt and to the operation of the DI system.
The forward movement greatly relieves the shear forces working on the numerous (but relatively small) locking lugs on the bolt as the bolt turns to unlock as the carrier moves rearward. Relieving the shear forces helps the bolt locking lugs live longer since they are relatively weak to that lateral force.
It also significantly reduces the force needed to turn/unlock the bolt so the system has more energy in reserve to overcome other potential issues when cycling (residue buildup, lack of lube, dirt/grit, etc…).
This thread starts with a reply to an ongoing discussion, if things aren’t clear, bear with.
The short version is “What propels the bolt carrier group with enough energy to cycle it? Gas from the tube or case pressure on the bolt face?”
I’m coming from the perspective the gas in the bcg is to unlock the bolt, and the case drives it back.
Blowback and delayed roller lock work off case pressure against the bolt. This includes most auto pistols and the HK/CETME rifles.
Timed unlocking is to prevent damage from an uncontrolled bolt from calibers bigger than .380/9MM. The TM calls out a serious caution that the bcg should never be assembled without a cam pin, as the results could be lethal. The bolt wouldn’t lock and the carrier would be propelled backward with 50% of the energy meant for the bullet.
There is a opposite opinion that the bcg is forced back by the pressure delivered by the gas tube. I asked if there was any case pressure involved, or none.
Since two types of semi auto firearms are exclusively operated by case pressure against the bolt face, I think it entirely possible it’s the same process with DI. The gas tube delivers enough to open the bolt, the case drives the bcg back.
One symptom of case pressure is that the cam pin is forced to the side of the upper channel, creating friction and wear. I believe that happens during extraction/ejection, and is the cause of the TM’s instruction to generously lube the channel. On the return, only chambering the round would cause pressure, at a much lower value.
This same concept is what I believe is happening with a piston gun - it moves the bolt carrier to unlock it, the case propels it to cycle.
In the case of DI, the cam pin gets a lot of wear - and I don’t think its coming from being pulled back during the cycle, but forced back. The size of the pin also seems to indicate that, too.
What’s missing are some pressure graphs and data to show which contributes how much and when in the cycle, and I’m still looking.
I believe that the DI is the major cause for the bolt/carrier group moving back. I bet case pressure helps some, but I suspect it is only a negligible amount.
As the DI opens the bolt, it also pushes the bolt away from the bolt carrier, propelling the BCG backwards. This happens quickly. The BCG has enough momentum to keep going backwards.
I’m guessing by the time the DI has unlocked, most of the case pressure is gone.
I remember seeing a dual AR-15 full auto that had the gas tubes crossed from one upper to the next. It was made by Tromix. If case pressure was necessary to the operation of the action, this could not be possible.
Most auto pistols are not blowback. Most are recoil operated.
With the recoil and delayed roller locking system the cycle of unlocking starts at the moment of firing. The rollers delay unlocking until the pressure has dropped and the bullet has exited the barrel. The momentum impaired to the mass of the bolt then completes the cycle and extracts the case. The key is that the momentum starts before the pressure has dropped. If the pressure has dropped then there is nothing to initiate the cycle.
On the DI system, gas from the barrel initiates the movement of the bolt carrier. The inertia of the carrier then completes the cycle after the pressure has dropped and the bullet is out of the barrel.
Please correct me if I’m wrong, but it appears to me that both of those army times animations are incorrect as they show the bolt rotating and unlocking before the BCG moves. The animations also don’t show any movement of the bolt relative to the BCG. As I understand it, the bolt lugs are locked behind the chamber lugs when the bolt is closed.
When the round is fired, the bolt can not move or rotate until the BCG moves rearward from the DI gas pulse, so the initial impulse of the cartridge base against the bolt face is effectively nullified. As the BCG moves to the rear, the cam pin rotates the bolt and the bolt unlocks. Momentum of the BCG (and possibly residual barrel pressure) then move the bolt rearward, ejecting the spent shell, then stripping a new cartridge from the magazine as it move forward again.
As the bolt moves forward the bolt lugs go past the chamber lugs, and as the BCG comes forward the cam pin rotates the bolt, locking the lugs for another cycle.
I am an expert, so I am respectfully asking: Base on the above, does a piston driven AR15 have more wear and tear on the bolt (minus the extra gas that allievates pressure on the bolt/chamber lock up)?
In my understanding, yes to a degree…especially in systems that begin turning the bolt while there is still anything much more than atmospheric pressure in the barrel (e.g., barrels with a long dwell time, suppressed systems, etc…)
Also to my understanding, just increased wear is not so much the problem per se…it is much more the increased shear forces on the rear of the bolt lugs.
As explained to me, as the bolt turns while being pressed against the lugs by residual barrel pressure, the forces will be increasingly concentrated on the trailing side of the rear of the bolt lugs. Those forces can over-stress a weak and/or fatigued lug causing it to fracture and flake off a chip potentially creating a malfunction or leading to more lug failures and catastrophic bolt failure as the remaining unbroken lugs have to carry more of the load.
The gas that is sent down the gas tube into the gas key of the BCG is what sends the BCG rearward. Rather than the gas moving a piston which in turn moves the bolt to the rear (like a connecting rod on a rocker arm or piston in a car engine) the gas is directly (hence DI) forced against the BCG.
Pressure on the bolt face by the cartridge should be minimal at the time of unlocking and extraction. Like others have posted if there was significant pressure at the time of unlocking the locking lugs would get a good deal of wear.
I wish I had all the pictures from ADCO, but the details of what physics are occuring during unlocking and the transition to extracting the case are poorly understood by a lot of folks.
What makes the DI system a bit unique is that the gas being directed through the gas tube and gas carrier key enters the chromed chamber inside the bolt carrier (functions as a piston), which not only drives the bolt carrier back, but applies pressure forward on the bolt (the bolt tail is in this area, the gas rings on it provide a seal, which is why they are so important to cycling), this force being applied is important, and off-spec systems will cause the rear face of bolt lugs to wear even faster - those failures are usually a combination of this, and mostly suspect metallurgy.
The bolt face remains forward while the carrier travels back, and during this motion the cam pin functions to begin unlocking the bolt - any suggestion that case pressure is require simly isn’t the case, as the amount of force required to cam the bolt out of locked position in the amount of time it does means there is enough momentum in the bolt carrier group to cycle the action.
Once the camming motion of the bolt and cam pin is completed, the cam pin reaches the end of it’s helical track, and the bolt is accelerated rearwards. This quick bit of impulse is why bolts tend to break right around the cam pin hole, and why cam pins demonstrate so much wear in so many distinct areas. The channel the cam pin follows is secondary in function - the bolt will unlock through a combination of the cylindrical part of the cam pin in it’s helical channel as well as the top part contacting the upper receiver.
Further evidence to me that case pressure isn’t involved is that when stripped cases occur in the chamber, the BCG generally cycles and loads the next round creating double feeds.
As the bolt reaches its fully forward position, the holes (two, or three in the case of the LMT enhanced BCG) then allow excess gas pressure to be vented out via the ejection port, and as the bolt carrier group continues rearward travel, the spent case is ejected.
The extractor will rip the rim off a case stuck in the chamber. Gas pressure alone is enough to operate the action of an AR.
Cut the gas off and you have a manually operated rifle. If anything, the back thrust of the case on the bolt face serves to keep the bolt from unlocking until the carrier can perform that function.
A question- how does the gas get from the carrier key to the rings? Is the carrier key hollow all the way through? Or is it from spillage as the key separates from the gas tube? If it’s spillage, how is the gas contained until it gets to the rings? Unfortunately I don’t have my rifle with me to study
The gas key directs the gas down into the bolt carrier (the front cylindrical hole in it is hollow, with just gas vents on the starboard side). The bolt tail with the gas rings fits into this cylindrical hole, the gas rings make a seal around the outside, so when you disassemble the bolt carrier, you’ll see this as a lot of open space, but the tail of the bolt usually fills it, and it’s chromed because the bolt reciprocates and rotates inside it, with the gas rings making the seal against the bolt carrier, and it’s chromed because the majority of gas used to cycle the action is routed through here.
I found this on vuurwapen.blog. Microsoft offers a free viewer for ppt files.
There is residual pressure on the case forcing it against the bolt face. It some instances it’s the only thing that extracts a case.
DI operates the cycle all by itself. This has been the accepted dogma long before I served, but like a lot of proprietary data, never demonstrated. Thank you TACOM.
This certainly amplifies what many of you were saying.