Timing and tensions are not mutually the same. What do you propose?
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Timing and tensions are not mutually the same. What do you propose?
Old thread, but a few things:
With the M16 gas tube design
1) From experimental measurements, the time of the first pressure rise in the carrier cavity (the cylinder) is about .25 ms after the bullet passes the gas port. The time of peak pressure is about .75 ms later.
2) From experimental measurements, the pressure in the carrier cavity at the start of unlocking is around 1700 psi, the chamber pressure at the start of unlocking is around 2500 psi. Calculating the respective forces generated by the respective areas shows the forces to be roughly equal. If there is a force imbalance pushing the bolt forward, it would show up as spent cases being shorter than the chamber headspace. Since this is not the case (at least I have never heard of it), it would seem the nets force on the bolt is always against the barrel extension lugs, or the force is well below the force required to resize the case. Either way, there is no 'assisting extraction' from the gas pressure force on the bolt.
3) From experimental measurements, the bolt begins to unlock about 1 ms after the bullet has passed the gas port, and complete unlocking takes about .75 ms. Bullet exit is about .25 ms after passing the gas port.
4) Models based on these experiments and others indicate that the timing of a barrel mounted piston with a diameter of .312" will be approximately the same as that of a gas tube design. The only major difference will be the locking lugs will not see a forward bolt thrust. However, the chamber pressure at the start of unlocking is only 2500 to 3000 psi, with a thrust of 270 pounds. The pressure at completion of unlocking is 1000 psi with an associated thrust of 100 pounds, so lug forces are minimal.
5) The piston thrust from a barrel mounted piston are intentionally lower to keep the initial acceleration of the carrier down so unlocking will occur at the same time as the gas tube design, however the stroke of the piston is double that of the gas tube design (.750" versus .325"), this leads to higher final carrier velocity and higher cyclic rates.
With the M4 gas tube design:
6) From experimental measurements, the gas tube and carrier cavity pressure are roughly the same as with the M16, but everything occurs about 0.25 ms sooner. And, the chamber pressures (and bolt forces) during unlocking are about 75% higher.
7) Even though the moving mass is less (lighter buffer), the timing of events during the first .325" of carrier travel is roughly the same, however, the final carrier velocity is higher.
Steel cases:
The difference between steel and brass cartridge cases is steel does not obturate during peak pressure as well or spring back as much as brass after the peak chamber pressure. This means the force required to pull the case from the chamber (extraction force) is higher. Extraction occurs after the carrier cavity has vented, so there is little gas pressure used during this phase of the operational cycle, bolt carrier momentum is the primary energy source for extraction. This being the case, you either have enough energy to complete extraction with sufficient left over to complete ejection, feeding and locking, or you don't. A higher case taper makes extraction easier, as there is less sliding contact between the case and the chamber wall.
In any case, extraction has practically no influence on unlocking in the AR bolt design as the bolt must rotate around 22 degrees (approximately .020" from completely unlocked) before it can start to move the rear and extract the case, and the extractor slides around on the rim during unlocking. This is because the locking faces of the barrel extension as well as the locking faces of the bolt are perpendicular to the axis of the bore, as long as any portion of the bolt lugs are in contact with the extension lugs, rearward motion is prevented. (There is a .020" chamfer on both the bolt and extension lugs, primarily to ensure that no burrs remain on the corners.)
The less effective obturation of steel cases, similarly, plays no part during unlocking. It does, however, play a part in extraction, but only because more gas and fouling residue is allowed to get in between the outer case and the chamber wall. The residue on the case is no problem as it is extracted and ejected, however, there may be increased fouling in the chamber, eventually leading higher required extraction forces, due to sticking.
EDIT:
All the stuff about barrel mounted pistons (aka "piston systems") is based on the .312" diameter piston, and associated internal passage geometry used in the Adams Arms piston design. Enlarging the piston diameter, increasing or decreasing the gas passage diameters, or changing the weight of the cylinder/operating rod, will change everything, obviously.
Last edited by lysander; 04-25-16 at 13:41.
Thanks for all the Data! Very much a good read, especially for someone like myself, a piston owner.
My Adams Arms will feed everything really well until the chamber starts getting dirty. Finally had to ditch buying Tula as the steel wouldn't extract well once it got dirty. Otherwise, I have never had a hangup from brass before.
After having to manually knock out 4 tula in a row, I decided to try some of my LC M855, fed just fine and worked without issues. After reading your post, I think I have a pretty good understanding of most of the "why" behind that.
As always, thanks for sharing; this bit of advisement will surely be beneficial in the future.
With the Adams Arms, you could have just switched over to the big hole and kept running Tula.
The big hole?
Im not sure i quite understand.
I have a 4 position piston block.
1. No gas, have to manually cycle between each shot.
2. Supressed, reduces pressure due to backflow from can.
3. Unsupressed, or "open". This was what it is ran from.
4. Unlock piston and remove for cleaning.
As mentioned, it runs fine for multiple magazines, but will stop feeding reliably at somepoint. Very well could be extractor issues, but i dont have more than 2k rounds through it.
I hope I dont come off cross, not the case at all, just not sure what you mean.
Oh! I understand what you're referencing then.
I know the rifle will operate just fine on supressed with the M855 and such, when it comes to the Tula, I, honestly, haven't even tried it on the lower setting.
I still have a few hundred on these Tula to blow through (looking back, I can't even imagine how many payday weekends at 3 boxes max per trip I had to even make to accumulate this much ammo) and will see if it was maybe just a particular lot.
I didn't really experience these issues initially, but it's been mostly fed LC M855 from an old Bandolier can as that is my current "go-to-round" as I have that stockpiled even heavier than the Tula.
The tula will work, even in a pinch, for awhile. Even after fouling up, I am sure a quick patch through would reopen it for many rounds. Not sure if my tolerances are just that tight on my AA15, or if the ammo I am using is simply that dirty. Either way, after repeated stuck cases, I decided it wasn't worth all the effort. Especially when the case lip rips off instead of pulling the round during a slam drill.
Last edited by HeruMew; 04-26-16 at 12:42.
My question is this:
Do you think the bled gas system pressure from the gas port is anywhere near the chamber pressure, to the point that gas system pressure in the internal expansion system would overcome even residual chamber pressure?
That's a tough sell from a physics perspective, which is a nice way of saying, "No way."
Op-rod driven designs have to compensate for the increased port diameter and gas volume by buffering and springing up in the recoil system, in order to try to tame unlocking shear, early extraction of obturated cases, and violent ejection.
ETA: I see lysander covered this in great detail already.
Last edited by LRRPF52; 04-29-16 at 23:04.
EDIT:
The above is technically correct, but I forgot to mention there is that 1000 psi residual chamber pressure assisting extraction, and it is pretty important.
Testing has shown that during extraction, the extractor jumps off the case rim, if the residual chamber pressure is too low, or absent, the bolt will move to the rear and loose contact with the case (by the forward push of the ejector, and exacerbated by a dirty chamber or dinged cases). When the extractor returns to its normal position, it misses grabbing the rim and you have a failure to extract malfunction.
This was proven by configuring two M4 Carbines, one above the other with the top M4 gas tube inserted in the lower M4's bolt carrier. The lower M4 was fired, and without its gas system hooked up, it did not cycle but left the spend cartridge in the chamber. Then the top M4 was fired and the gas from the top M4 was used to cycle the lower M4. The lower M4 suffered failures to extract....
www.dtic.mil/ndia/2003smallarms/din.ppt
Last edited by lysander; 04-30-16 at 11:56.
The lower M4 failed to extract with a normal bolt.
What I don't see listed is which extractor spring configuration was used.
The weaker rifle spring (which is generally insufficient) or an upgraded spring / o-ring.
When the Ejector was removed, the lower M4 extracted properly.
So the residual chamber pressure must be sufficient to overcome the effect of ejector tension.
Additionally, we've done some tests with the Extractor removed.
https://www.m4carbine.net/showthread...4-No-Extractor
Some gas system configurations actually "self extracted" due to residual pressure.
Last edited by Clint; 04-30-16 at 13:47.
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