Roller-delayed blowback system?

[Southern Raider]

The HK/CETME roller lock system opens at a constant rate, REGARDLESS OF CHAMBER PRESSURES.

F=mA should clearly tell you that's not true. Also, the fact that HK makes a different locking piece for every bullet weight, use of suppressor, barrel length, etc. should be another hint.

Now you can make a claim that the net velocity of the carrier group when it opens is roughly proportional to the integral of the pressure curve, and thus by controlling the pressure curve, you can achive a similar carrier velocity. However, the instantaneous velocity at any point in time will be different.

 

[AZ Jeff]

My bad. I meant to say is opens at a constant rate for a given bullet weight, REGARDLESS OF CHAMBER PRESSURES

 

[Tony Williams]

I'll also wager that before this happens, the flutes have largely vented the chamber, so we can't depend on gas pressure to properly expel the fired case.

The only way to prove that would be to remove the extractor from a gun and fire it to see what happens to the case.

 

[Southern Raider]

My bad. I meant to say is opens at a constant rate for a given bullet weight, REGARDLESS OF CHAMBER PRESSURES

Still not the case. Think more along the lines of conservation of momentum. The bullet momentum roughly equals the rearward momentum of the carrier group at any point in time. If you can change chamber pressure without changing the momentum of the projectile over time, then what you say is correct. I don't think it works that way. ;-)

 

[AZ Jeff]

Still not the case. Think more along the lines of conservation of momentum. The bullet momentum roughly equals the rearward momentum of the carrier group at any point in time. If you can change chamber pressure without changing the momentum of the projectile over time, then what you say is correct. I don't think it works that way. ;-)

I think you have to integrate the area under the time/pressure curve in order to calculate momentum correctly. The secret is that, if you change the shape of the pressure curve without impacting the total area under the curve, you can effectively alter the chamber pressure at the moment of "unlocking" without affecting the imparted bolt momentum.

You can thus say that you can control the relationship between unlocking time and chamber pressure while maintaining a constant bolt momentum.

 

[ceetee]

I think you have to integrate the area under the time/pressure curve in order to calculate momentum correctly...

And if you can translate that into English, I can have a cold one waiting for you next time you make it over here to the right coast...

 

[Southern Raider]

I think you have to integrate the area under the time/pressure curve in order to calculate momentum correctly. The secret is that, if you change the shape of the pressure curve without impacting the total area under the curve, you can effectively alter the chamber pressure at the moment of "unlocking" without affecting the imparted bolt momentum.

Mathematically absolutely correct, and what I was implying from my other posts. If you can cause the bullet to exit with the same momentum, then the carrier group momentum will be the same. However, you can't do what you describe and not affect the instantaneous momentum of the system. The end result may be the same, but the path there is different.

Nice to see someone else was paying attention during their basic physics class.

 

[Alphazulu6]

I like how scientific this thread has taken.... all this talk about the delayed blow back system makes me want to go buy an HK and shoot it today!

 

[heron]

A physics lesson! Cool!

 

[BigG]

I think this is a great instructive thread. The minds on this forum all interacting serve to teach so much.

1. A roller locked breech (or nearly any retarded or delayed lock action) is not locked at all. It is force working against a mechanical disadvantage calculated to counteract the back thrust of the cartridge just enough to let the bullet exit before opening under dangerous pressure.

2. A mechanically locked breech, like an AR15 or an M1 has a positive mechanical lock exactly like a bolt action that must be physically unlocked before the breech can open.

3. You can test either of these assertions by taking a cleaning rod and bearing down on the action. You can move the roller lock. You cannot move the mechanical lock unless you move the linkage first.

 

[LeibstandarteAdH]

I don't think it does. The flutes reduce the amount of contact between the brass and the steel chamber and allow it to be extracted without tearing the case rim. The "gas floating" stuff is hype and BS. They use that to explain the black soot. The breech is really opening when there is high pressure, imho.

If you take the extractor out, the case will NOT still be stuck to the bolt face. As i have done this. If Southern Raider were wrong, then the bolt shold stay in contact with the case throughout the recoil and then possibly reclose on and chamber it provided their is not a magazine in the gun, and that it was fired strait upwards (to remove gravity from simply making the case fall off ofthe bolt face.) The fact that the carrier IS moving faster pulls the bolt face away from the case head for a moment when the extractor has been removed. Long enough for the gas/wind pressure to disorent the case enough to "stovepipe" but not completely eject because it is not moving fast enough to stay with the bolt face, due to the fact that the rearward energy of the faster moving carrier was not transferred to the case via the extractor not being there.

 

[stubbicatt]

The history of Auto Ordnance would have been different had JT Thompson et al known of chamber fluting. Combined with the Blish lock this could very well have put a select fire infantry arm into the hands of grunts as early as 1918.

Tearing brass in half is a phenomon of timing of the action. The fluting in the barrel does in fact float the case, leaving only the small section of brass near the fire ring to seal the chamber. The SVT40 also has fluting in the neck portion of that firearm to assist in extraction.

 

[Tony Williams]

1. A roller locked breech (or nearly any retarded or delayed lock action) is not locked at all. It is force working against a mechanical disadvantage calculated to counteract the back thrust of the cartridge just enough to let the bullet exit before opening under dangerous pressure.

Most of that is true, but it implies that all roller locked breeches are blowbacks, whereas the MG 42 machine gun family do have genuine roller locking as part of a short-recoil system.

 

[BigG]

Most of that is true, but it implies that all roller locked breeches are blowbacks, whereas the MG 42 machine gun family do have genuine roller locking as part of a short-recoil system.
Yesterday 09:44 PM

100% true, Tony. Sorry. I meant of the HKxx type. The MG 42 has the short recoiling barrel and therefore the roller/seat geometry is different and provides a true mechanical lock.