Semi Auto Design question.???????

E

Eric Larsen

New member
This is just a curiosity thing....Which design uses more of the spent rounds energy to cycle the action...locked breech/blowback?

I would think the blowback uses less...less things to move. But I dont know...thus the thread.

Shoot well and thanks for any and all replies.
 
But, alas, the blowback design uses a proportionately heavier slide to retard the slide's movement instead of the locking devices. Both use heavy springs.

I doubt that there's proportionately/relatively much difference. Basic physics: equal and opposite reactions.
 
Walt,
Does that question sound familiar? LOL..I understand the physics part of it and the slide weight...thanks for your reply.
I thought there might be an alan retentive gun nut with too much time on his hands that has figured this out, somewhere....
Shoot well
 
I am not sure of the reason for the question. If you are asking which action has the most reduction in velocity due to pressure being required to operate the gun, the answer would be neither. In both types, the breech should remain closed until the bullet exits the barrel, in which case nothing more can affect its velocity one way or another. The blowback action is operated by residual gas pressure after projectile exit; the conventional locked breech (M1911, Luger, etc.) remains locked until projectile exit and is operated by recoil from the bullet movement, not from the gas pressure itself.

In theory, a gas operated action, since it taps off gas before the bullet leaves the barrel, would affect bullet velocity more than either of the other types. As a practical matter, the difference in velocity between, say, an M14 and a bolt rifle of the same barrel length, is negligible.

Jim
 
Jim:

Your comment that the barrel is locked up until the bullet left the barrel would seem to imply that a longer-barreled gun with the same action and caliber would have to cycle slower than a short-barreled version. That doesn't seem to be the case...

That would also seem to say that different weight bullets would cause the action to work at different speeds.

A semi-facetious question: how does the gun's action know that the bullet has exited the barrel? [I've amended this, after coming back and re-reading. I understand how the pressure drop when the bullet exits the barrel on a locked breech will cause the barrel to become unlocked, but I don't see how the same phenomenon can occur in a fixed barrel/blow-back design. That's my point of ignorance.]

I got the impression from my reading -- but I'll acknowledge that I've never done anything like scientific testing on this subject, or seen photos that demonstrate the process -- that the recoil/extraction process started almost immediately, before the bullet left the barrel. And while the breech is locked, things ARE still moving to the rear...
 
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In a blow back action, the case releases it's grip on the chamber walls when the pressure drops enough. That's how "delayed" blow-backs with grooved chambers are delayed: the grooves give the case more purchase in the chamber, thus delaying release.
 
Hand_Rifle_Guy:

Thanks. That make good sense.

The whole process is relatively delicate and subtle, regardless of the lockup method...
 
Eric, as others have touched on, what you are looking at is a cycle of a very rapid (but not instantaneous) pressure increase, then decline over a period of time that is rapid, but not instant. The total cycle time and pressure profile varies with the powder type and quantity; the weight of the bullet; the length of the barrel; and other variables like the lubricant on the bullet, the actual manufactured shape of the barrel (bet you thought they were all straight and true, right?); the shape of the muzzle; and condition and tolerances.

The "loss" of velocity caused by a gas system is inconsequential. High power rifles especially, but also pistols have erosion at the muzzle caused by gas blowing past the bullet before it exits the barrel. That's one of the reasons why you have a barrel recrowned. Whether the gas is used to cycle the action or blows past the bullet and out the barrel is moot.

So, take it from the designer's perspective. You are given a graph of time and pressure. You have to design a mechanism that will cycle the action at the correct time. Too early and you rip the bottom off the cartridge: too late and you get FTF because you lack the energy required to work it.

So the design should be efficient, flexible, and easy to manufacture. The "flexible" part comes about if the design has plenty of power to spare even if low-powered rounds are fired, while functioning properly if hot loads are fired as well. Some pistols show more sensitivity to this than others, although changing springs can resolve it.

We take these two approaches for granted, but that wasn't always so. The first model of what became the M1 Garand was primer fired - it used the displacement of the primer in the cartridge to wor the action. This was considered unworkable because the Army very understandably wanted standard ammunition.

The second model was the so-called "Gas Trap" version. These models had a collector around the muzzle. As the bullet exited the muzzle, the pressure wave was collected by the "trap" into a cylinder which pushed on the end of the "Operating Rod" to unlock the bolt lugs and cycle the action.
After successful field testing it was felt this system would be too difficult to maintain in the field. Garand then developed the Gas Port system used on the WWII Garands. He continued to evolve the system and the M14 has a much more compact gas system.

Pistols differ from these rifles in that the cartridges are much less powerful. So a rotating bolt lockup is not needed and Browning's system of locking lugs in the barrel and slide is adequate. It so happens that smaller pistol cartridges lend themselves to a blowback system, which really just takes advantage of the "push" those cartridges create. More powerful handgun rounds require some sort of lockup (or they'd have to be a lot heavier!)to gain the correct timing cycle.
 
Hi, Walt,

The whole problem is that if the breech is allowed to open with high pressure in the chamber, the brass case will fail and harm the gun or the shooter.

But I was probably over-simplifying. Yes, initial motion of the breechblock in a blowback system begins when the powder gas builds up pressure. But the breechblock is balanced so that its own inertia keeps it from moving very much if at all until the bullet has left the barrel. With low power cartridges, and a longer barrel, as in a .22 rifle, the breechblock is balanced to keep from opening before pressure drops, even though the bullet may not actually have left the barrel. Some designers have taken advantage of the initial blowback motion to create a sort of delayed blowback system, such as the one Pedersen used in the Remington Model 51.

In a locked breech pistol, the longer barrel adds weight. Since the barrel is locked to the breechblock, the entire mass resists the recoil force, so a longer barrel balances out. Of course, if the barrel is too long (heavy), the recoil will be insufficient and the gun will not work. This is what happens when someone tries to put a 16" barrel and shoulder stock on a 1911 type pistol; the result is usually not reliable.

How does the gun know the bullet has exited? It doesn't of course, and the pressure drop has nothing to do with it. The gun designer balances the gun to operate within a rather narrow set of ammunition parameters. If someone, say a handloader, goes outside those parameters, the gun may malfunction or be destroyed, depending on whether the load failed to reach the minimum parameters or exceeded the maximum.

Can a powerful cartridge be used in other than a locked breech mechanism? Yes, Astra proved it with the 9mmP and 9mm Largo a long time ago, and High Point does it with both 9mmP and .45 ACP, cartridges that would normally require a locked breech. But you don't get a free lunch; Astra springs would do justice to a Ford truck, and the High Point slides are massive, making the guns awkward and heavy.

Jim
 
Thanks guys. This has been one of the better message chairns I've seen on The Firing Line, and there have been a bunch of good ones.

I especially appreciate Jim and VVG's responses.
 
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