"Best" BCG

[Pasta123]

The load on the cam pin hole is governed by the bolt carrier velocity, which is governed by the piston pressure and buffer weight.

The biggest gain in a longer cam track is increased reliability from easier extraction. it's not intended to be a part longevity improvement.

Bolt life really is kind'a over blown for most folks, unless you do a lot of supressed full-automatic fire . . .

IMO, the reputation of the AR-15/M16 being “unreliable” have a lot to do with it. Most people still believed that piston is a stronger system regardless what they actually perform.

I’m building my very first rifle since a little more than a month, with little time to search a lot of posts/data. I’m sure I’ll made mistakes but also learned a lot. What I thought was right weren’t true at all.

[Pasta123]

If anyone interested, I found LMT's patent for their enhanced bolt on Google (Previous design that's expired).

[lysander]

IMO, the reputation of the AR-15/M16 being “unreliable” have a lot to do with it. Most people still believed that piston is a stronger system regardless what they actually perform.

I’m building my very first rifle since a little more than a month, with little time to search a lot of posts/data. I’m sure I’ll made mistakes but also learned a lot. What I thought was right weren’t true at all.

Well, the AR design does have a reliability issue regarding extraction.

Since the piston is the bolt and inside the bolt carrier, you cannot use gas pressure from the piston to assist extraction.

If you look at all the short stroke piston designs, like the FAL, Tokarev, or AR-18, after unlocking is completed the piston still has about a quarter inch of throw to help snatch the case from the chamber, even if it is slightly sticky.

In the AR-15 design, the carrier is propelled backwards, but as soon as the cam pin bottoms out in the cam track the gas pressure is pushing as much forward as back. The only thing extracting the case is the momentum of the bolt carrier. And the buffer weight does not help as the buffer cannot pull.

This makes the AR-15 design more sensitive to carrier weight and bolt velocity. From an extraction point of view, it would be better to run standard weight buffer, but a heavy carrier that has the extra weight of the H2 or H3 over the standard.

[Pasta123]

Well, the AR design does have a reliability issue regarding extraction.

Since the piston is the bolt and inside the bolt carrier, you cannot use gas pressure from the piston to assist extraction.

If you look at all the short stroke piston designs, like the FAL, Tokarev, or AR-18, after unlocking is completed the piston still has about a quarter inch of throw to help snatch the case from the chamber, even if it is slightly sticky.

In the AR-15 design, the carrier is propelled backwards, but as soon as the cam pin bottoms out in the cam track the gas pressure is pushing as much forward as back. The only thing extracting the case is the momentum of the bolt carrier. And the buffer weight does not help as the buffer cannot pull.

This makes the AR-15 design more sensitive to carrier weight and bolt velocity. From an extraction point of view, it would be better to run standard weight buffer, but a heavy carrier that has the extra weight of the H2 or H3 over the standard.

Well, TIL. I see why OBC makes sense. While longer cam path are always better for extraction, you can only have so much room to work with, but one can always add a counterweight on the tail. Wonder why nobody thought about that before.


Also, I've spent sometime thinking about the gas system, realizing that issue regarding bolt life might be overblown. If my guess is correct, an internal piston system (or semi-DI) will always be better here thanks to the longer dwell time, which also means less bulk for the same bolt life. While the bolt might be slightly small for the job, given how treatment & (reasonable) design process have progressed, it's not impossible to improve upon it.

[Clint]

This would be true of the other designs if the length of piston travel under power exceeds the length of carrier travel until bolt pickup.

In other words, if the piston is still under power when bolt pickup occurs, the piston does help extraction.

For reference, the AR cam track is ~.325".

Conventional short stroke pistons like the FAL and AR18 seem to have piston strokes around 1".

These would seem to be powering the carrier during bolt pickup.

Many of the "tappet" style systems like the M1 Carbine, SCAR, AUG and HK416 seem to have much shorter piston strokes around .250".

These don't seem to be powering the carrier during bolt pickup and would essentially be the same as the AR from that standpoint.

The other point to consider is even if the piston is technically powered during bolt pickup, how much force/pressure is actually present this late in the pressure curve?

Thoughts?

Well, the AR design does have a reliability issue regarding extraction.

Since the piston is the bolt and inside the bolt carrier, you cannot use gas pressure from the piston to assist extraction.

If you look at all the short stroke piston designs, like the FAL, Tokarev, or AR-18, after unlocking is completed the piston still has about a quarter inch of throw to help snatch the case from the chamber, even if it is slightly sticky.

In the AR-15 design, the carrier is propelled backwards, but as soon as the cam pin bottoms out in the cam track the gas pressure is pushing as much forward as back. The only thing extracting the case is the momentum of the bolt carrier. And the buffer weight does not help as the buffer cannot pull.

This makes the AR-15 design more sensitive to carrier weight and bolt velocity. From an extraction point of view, it would be better to run standard weight buffer, but a heavy carrier that has the extra weight of the H2 or H3 over the standard.

[lysander]

This would be true of the other designs if the length of piston travel under power exceeds the length of carrier travel until bolt pickup.

In other words, if the piston is still under power when bolt pickup occurs, the piston does help extraction.

For reference, the AR cam track is ~.325".

Conventional short stroke pistons like the FAL and AR18 seem to have piston strokes around 1".

These would seem to be powering the carrier during bolt pickup.

Many of the "tappet" style systems like the M1 Carbine, SCAR, AUG and HK416 seem to have much shorter piston strokes around .250".

These don't seem to be powering the carrier during bolt pickup and would essentially be the same as the AR from that standpoint.

FN SCAR has a little more travel than .25". While venting starts almost immediately, the pressure in the piston remains high enough to keep a descent amount of force for extraction. One of the reasons for the large piston area, force = pressure X area.


The same is true with the Steyr AUG, while it only travels a short distance before venting it takes a few milliseconds to drop the pressure to ambient. For example, the AR carrier cavity is about 2,400 psi just before uncovering the vents, but after 1.25 milliseconds the pressure is still 350 psi and producing a force of about 100 pounds, an the AR has two 1/10 inch diameter holes, about three times the vent area of either the FN or the Steyr.


H&K's piston looks to be short, but it isn't. Again, while the piston begins venting as soon as the little nipple pops out of its bore, the vent orifice is only 1.5mm so it take a while for the pressure to completely drop down to where it's not doing anything.


The M1 Carbine does not vent externally, and has a piston stroke of 3/16 inch. The Carbine is similar to the AR, in that it extracts on momentum alone, but it does have some camming action by the locking lugs to increase the power for initial loosening.

The other point to consider is even if the piston is technically powered during bolt pickup, how much force/pressure is actually present this late in the pressure curve?

Thoughts?

This is the pressure-time curve for an AR-18. It can be taken as typical for a 5.56mm rifle with a 0.078" diameter gas port 12 inches from the base of the cartridge. Assume we have a piston the is 5/16 inch in diameter, that gives us a piston area of .0767 square inch, and venting, proper starts at that little bubble at 725 psi (0.5 X 10^7 Pa). The residual force on the piston at venting is 55 pounds, so there is enough force to be helpful.

[Clint]

Thanks Lysander!

Excellent info.

On the SCAR, venting is immediate because the vent is a forward facing hole in the gas plug, correct?

Any idea on the FN vent size?


Apparently on the HK416 10.3" barrel, the front vent is 1mm (.039 ), as this nice cutaway from Nefarious Arms shows.

FN SCAR has a little more travel than .25". While venting starts almost immediately, the pressure in the piston remains high enough to keep a descent amount of force for extraction. One of the reasons for the large piston area, force = pressure X area.


The same is true with the Steyr AUG, while it only travels a short distance before venting it takes a few milliseconds to drop the pressure to ambient. For example, the AR carrier cavity is about 2,400 psi just before uncovering the vents, but after 1.25 milliseconds the pressure is still 350 psi and producing a force of about 100 pounds, an the AR has two 1/10 inch diameter holes, about three times the vent area of either the FN or the Steyr.


H&K's piston looks to be short, but it isn't. Again, while the piston begins venting as soon as the little nipple pops out of its bore, the vent orifice is only 1.5mm so it take a while for the pressure to completely drop down to where it's not doing anything.


The M1 Carbine does not vent externally, and has a piston stroke of 3/16 inch. The Carbine is similar to the AR, in that it extracts on momentum alone, but it does have some camming action by the locking lugs to increase the power for initial loosening.


This is the pressure-time curve for an AR-18. It can be taken as typical for a 5.56mm rifle with a 0.078" diameter gas port 12 inches from the base of the cartridge. Assume we have a piston the is 5/16 inch in diameter, that gives us a piston area of .0767 square inch, and venting, proper starts at that little bubble at 725 psi (0.5 X 10^7 Pa). The residual force on the piston at venting is 55 pounds, so there is enough force to be helpful.

[Disciple]

This makes the AR-15 design more sensitive to carrier weight and bolt velocity. From an extraction point of view, it would be better to run standard weight buffer, but a heavy carrier that has the extra weight of the H2 or H3 over the standard.

The theoretical separation of the buffer and carrier actually has never been mentioned in any modeling exercise I have read and I don't think it really happens, and since all the models leave it out yet still match experimental data, even if it does occur its effects are negligible.

Out of the few plausible times that the buffer face doesn't maintain tension to the carrier, it couldn't really apply when talking about sane carrier velocities for 99.9% of users with an A5 during extraction. The internal piston of the BCG pushes the carrier back, resistance to unlocking and extraction does not stop or really slow down the carrier at that point in a normal system. If the system runs into more resistance in those points, the action will not have full travel in its range of operation in a balanced system.
Using a higher ratio of buffer mass can improve control carrier bounce and other items, given the same total reciprocating mass.

I am confused. Is a light carrier and a heavy buffer a problem for extraction? When?

[lysander]

I am confused. Is a light carrier and a heavy buffer a problem for extraction? When?

Not necessarily a "problem", just two things working at cross purposes.

When the gas enters the carrier cavity, it pushes the bolt forward against the rear of the barrel (or balances it against the residual chamber pressure), and here the bolt velocity is zero. In the back end of the carrier cavity the carrier gets pushed backward by the gas pressure. When the cam pin bottoms out in the cam slot and the bolt is as far out as possible, no more energy can be added to the system through the gas pressure. Note that the bolt, and the attached empty cartridge case are still motionless. The only way these can be accelerated rearward is by robbing momentum from the moving bolt carrier.

MV = mv

M = bolt carrier mass
m = bolt mass
V = bolt carrier velocity
v = bolt velocity

The buffer momentum at this point is useless as the buffer is not attached to the rear of the carrier and cannot pull.
Because extraction is only accomplished through the momentum of the bolt carrier, a lighter bolt carrier will require a higher velocity to make-up for the lost mass. If you increase the buffer mass you will tend to decrease the carrier velocity, which works against your minimum extraction momentum.

[DwayneZ]

FailZero BCG's. Never an issue. Which can be said for 50 other BCG's. FZ has been good thus far, so that's the goto item.
And yeah, FZ is treated toolcraft, but the two vendor hands-on does have a rather decent end product.