Action type and physics

[Vonderek]

I have wondered how much energy is required to cycle a blowback-type semiauto action and how it affects ballistics. Has anyone ever measured the velocity of the same load out of similar barrels with one test gun being a semiauto and the other a bolt action?

 

[General Geoff]

There aren't too many straight blowback actions on full power rifle cartridges; there are a few roller-delayed blowback types, though. The energy required to cycle them depends directly on the recoil spring strength of the bolt. The stronger the spring, the more energy it takes to compress it. The idea is that the spring must be strong enough, in tandem with the bolt being massive enough, to delay the bolt from coming out of battery until chamber pressure has dropped to a safe level.

There is likely a (very) small effect on muzzle velocity in a blowback vs bolt action design, but it's probably no more parasitic than a gas operated action. The recoil force is just being harnessed directly by the action instead of it all transferring straight to the shooter's shoulder.

 

[Vonderek]

I was thinking more in terms of 22LR but then it got me thinking about pistol cartridges...obviously there are no bolt action pistols but I was thinking the same physics would apply.

The recoil force is just being harnessed directly by the action instead of it all transferring straight to the shooter's shoulder.

This makes sense. Thanks.

 

[ants]

Just conjecture, but I would expect other characteristics of a particular gun (rifling depth, number of lands, barrel twist, minor and major bore diameter, bore fouling, etc.) to make a bigger difference in robbing energy.

After all, if you can rack the action of a blowback 22 rimfire using just two fingers then it certainly doesn't take much energy away from the system.

 

[oneounceload]

obviously there are no bolt action pistols but I was thinking the same physics would apply.

Actually, the Remington XP-100, Savage and Weatherby were all bolt action pistols

 

[General Geoff]

I was thinking more in terms of 22LR but then it got me thinking about pistol cartridges...obviously there are no bolt action pistols but I was thinking the same physics would apply.

There are a few bolt action pistols out there, but most are chambered in full power rifle rounds. Bolt action doesn't doesn't lend itself to use for pistols with pistol cartridges. There ARE a few bolt action rifles chambered for pistol cartridges though, such as the De Lisle Carbine.

As for .22lr, a bolt action vs blowback autoloader, variances in barrel length and rifling twist ratio will probably make more of a difference than the action type in terms of muzzle velocity. Indeed, standard deviation between individual cartridges is probably larger than any velocity differences you'd see in two different actions with the same barrel length.

The bolt action is potentially more accurate, however, because the action is locked during the time the bullet is travelling down the barrel (and thus there is little to no reciprocating mass which can throw off your aim). This difference is VERY small, however, and you'd need a pretty tricked out set of rifles and a world-class shooter to measure it.

 

[deadin]

I seem to remember reading about a 30/06 blowback rifle designed and submitted to the trials by Col. Thompson when the Army was looking for a semi-auto service rifle. (The trials that ended up with the Garand being selected.)
Anyway, apparently the ejection was a bit daunting. It was reported that it was sticking empty cases into an oak door off to the side of the test area.

I think he was trying to use the Blish lock that didn't work very well in the submachine gun either.

 

[eye5600]

Blowback action: I've wondered how far the casing moves before the bullet is out of the barrel. Anyone know?

A key part of the design is to slow the movement of the casing with springs, and with the mass of the slide.

 

[Oro]

Blowback action: I've wondered how far the casing moves before the bullet is out of the barrel. Anyone know?

I have seen slow-motion video of a 1911 style pistol being fired in a fixed rest. The bullet is clear of the barrel (but still visible in the frame - only a few inches away) before there is any appreciable rearward movement of the slide.

 

[Oyeboten]

Yes...in any of these, far as I know, the Bullet has left the Barrel before the Action is in motion...so, there's no loss of FPS...

 

[General Geoff]

The action DOES begin to move as soon as the bullet begins to move (simple physics). But the action moves less than a millimeter before the bullet leaves the barrel.

 

[rcmodel]

Regardless of what type of action, or what caliber you talk about.

The bullet has to be out of the barrel and bore pressure dropped to near atmosperic before extraction can take place.

If the action opened before bore pressure had dropped off and the case relaxed it's grip on the chamber, the gun would blow up in your face.

rc

 

[jerkface11]

I have seen slow-motion video of a 1911 style pistol being fired in a fixed rest.

A 1911 fires from a locked breach it isn't a blow back action.

 

[Uncle Mike]

Thank You rcmodel....

If one of you would like to get a blow back 22 and a Thompson Center break action pistol...it would be interesting.

 

[Vern Humphrey]

I once did a test, using a Ciener Conversion Kit on an M1911. I fired .22 ammo from the same lot, half with the gun as issue, half with two 1-lb weights clamped to the slide. There was no significant difference in muzzle velocity.

 

[rcmodel]

get a blow back 22 and a Thompson Center

Hey! Anyone with a cronograph can test it out of the same gun.
Which is the only way to test it and get meaningful results.
No big trick to hold the bolt shut on a .22RF.

Shoot a string semi-auto and another string holding the action closed with your thumb on the operating handle or slide.

I'd bet my booties the SD of the ammo used will be more then the differance between locked & not locked.

rc

 

[RBANNON]

I do not know for sure, but I think trying to hold the bolt or slide shut with one's thumb, finger, or any other body part might not be the best thing to do. Having done it a couple of times many years ago finally "cured" me of it!

 

[chrissmallwood]

If anyone has a Springfield 87a or one of the Savage/Stevens version that would probably work best for testing this out since you can fire it in semi-auto then lock the action closed and shoot it like a bolt action.

 

[Oro]

A 1911 fires from a locked breach it isn't a blow back action.

Yes, it is. It's a "delayed blowback," - but it certainly is a blowback action.

The action DOES begin to move as soon as the bullet begins to move (simple physics). But the action moves less than a millimeter before the bullet leaves the barrel.

No, the physics are more complicated than that. From video evidence, it certainly does not move even a millimeter before the bullet is free from the barrel. Consider:

1) The interaction of the slide/cartridge base is, for the period the cartridge is trapped in the chamber, an "inelastic collision" in classical physics.

2) It's not a "pure" inelastic collision - there is tension besides the mass of the slide in the form of pressure from the recoil spring.

3) once the cartridge is allowed to ricochet off the ejector, you now have an elastic collision, and a different set of equations to apply.

3) peak pressure is not reached with the cartridge still in the case, it comes later, when the bullet is travelling already.

There is no single "simple physics" equation to deal with this complicated interaction, you have to use several and then integrate them. It is entirely within the physics of the situation that the slide in a given gun does not reach a point where its inertia and that of the spring are sufficiently overcome to start the motion with the bullet free of the barrel.

 

[1911Tuner]

From video evidence, it certainly does not move even a millimeter before the bullet is free from the barrel.

Sorry, Oro. It certainly does move before the bullet exits. It's Newton's 3rd Law at its most basic.

In order for an action/reaction event to occur, there must be a vectored force between two objects. If the bullet is gone before the slide moves, half the equation...the "action" half...is missing.

No action = no reaction.

For the record...The 1911 is almost a delayed blowback. The only thing that keeps it from being a true blowback is the fact that the slide doesn't move independently of the barrel...and even straight blowbacks operate from recoil forces.

Let me go find a link to a video that'll let you see it happen, and I'll be right back.

 

[1911Tuner]

Try this one. It'll take a few seconds to load.

http://smg.photobucket.com/albums/v323/sv1cec/videos/?action=view&current=hispeedteaser_mid_w-1.flv


There are several guns firing in slow motion. The last 1911 is the one to watch closely. It's a stop-action clip of the gun firing. The first thing you'll notice is a tiny bit of slide movement rearward...then a light wisp of blowby gasses...and the slide moving. Then, you see the bullet nose start to peek out of the muzzle...and the slide is still moving. Finally, the bullet exits, and the fireball follows.

The slide moves and pulls the barrel backward with it. At about 1/10th inch, the linkdown phase begins. At that point, or just before the linkdown phase starts...the bullet exits.

Watch closely.

Interesting...what?

PS

Kuhnhausen's "Balanced Thrust Vector" description is utterly wrong.

 

[proud2deviate]

Wow, thanks for the vid, Mr. Tuner. I always wondered what was causing that little thumbnail-ish ding on my brass. Now I know

 

[1911Tuner]

How 'bout the bouncin' bolt on that AR15?

 

[proud2deviate]

I was reminded of a marble on a cake plate (don't tell mom!)

 

[Oro]

Sorry, Oro. It certainly does move before the bullet exits.

I certainly can't tell from that video when the bullet exits and any perceptible slide recoil happens. I have seen this video, it was what I was referring to above. It certainly makes it clear no slide recoil is happening before the bullet exits the barrel:

http://www.youtube.com/watch?v=tiHTZzE1NdA&feature=related

It is certainly possible for one item in a collision to have a major vector change before there's perceptible movement in the other party to the collision. It's very basic and involves inertia and momentum. If you compare the mass of the slide to that of the projectile, as well as the added force of the spring holding it in place. Sometimes I find it easy to understand and explain these concepts with extreme examples:

Imagine throwing a marble from a dock at a cruise ship passing by. Did you see the marble bounce and change direction? Did you see the cruise ship shudder or recoil? The lack of apparent change in the ship doesn't mean there wasn't an interaction between them.

Another example is a battleship firing a broadside. Even firing all guns at once, a WWII era battleship suffers no displacement laterally from it. Does that mean no work was done on the ship by the massive energy absorbed by it in firing 20,000 lbs of projectiles at basically high-powered rifle velocities (I believe around 3000fps)?

Those are extreme examples, but they help make it clear that you can have a collision or "action/reaction" where the vector change on one body is dramatically quicker than upon the other body.