Reaction of Luger barrel to torque of firing?
 

I had an interesting email today...

Rich -

A Luger collector told me that one theory regarding Lugers is that over long periods of time the barrel can unscrew slightly due to the physics of the bullet going down the barrel.

In other words, if you fire one enough, it will start to unscrew the barrel from the receiver. And supposedly this accounts for the witness marks on the barrels and receivers which often do not line up properly.

In your experience, have you ever heard of such a thing - on a Luger or any other gun with a rifled barrel that is screwed into a receiver?

Sounds a little far fetched to me, but I thought in your experience you might have heard of such a thing.


I had never thought about it...but since I had a bare receiver and a loose barrel sitting on my bench, I tried a little experiment. I oiled the threads, screwed the barrel in 2 threads, and ran a cleaning rod with 2" square patch folded twice on it down the barrel from the rear (as a bullet would be traveling). I held the rod steady so it wouldn't turn. It actually screwed the barrel *in* a quarter turn, not out.

The Luger barrel is threaded 20tpi with a right-hand thread, like a bolt you might find in your local Ace Hardware store. It screws in in a clockwise motion.

I suppose if the barrel was loose to begin with, the vibration of shooting *might* unscrew it...

Looking from the rear, the Luger rifling seems to twist clockwise. I'm guessing this is what is called a "right hand twist" in the specs...I don't have a book detailing a lot of barrels, just Nonte's 'Pistolsmithing', and it lists 18 pistols (mostly revolvers) all having right hand twist (the Luger is one of them).

It is interesting that although I was applying pressure down the barrel (rear to front; direction of bullet travel), the barrel actually turned in *against* the pressure...


Comments on the collectors theory are welcomed...Flames welcomed...cries of "burn the heretic!" welcomed... :D

Rich,
You ought to appear on "Myth Busters"!:) A great illustration that dispells a common misconception.

There are some other forces taking place we are not considering.
As a cartridge is fired pressure increases forcing the brass case to expand and seal against the breach, the bullet then slams into the rifling, I think it is at this instant that the reverse force is being applied. Once the rifling breaks in this force will be reduced considerably. Basically I wonder if the smoothness of the forcing cone on a new barrel is the determining factor of barrel back out and then only at the extreme earlyness of barrel life?

Good Post Rich!!

Vern

Indeed. There is heat, for instance. Not just the heat of combustion of the propellant, but heat of friction. A quick firing of a full magazine may induce enough heat to loosen the threads...

(I know; heat itself is not a force, but the expansion & contraction it induces can be so considered...)

And there is Newton's 3rd law of motion. There is a constant rotational pressure forcing the barrel into the receiver as the bullet travels forward; Newton's law tells us that when the pressure is suddenly relieved, there will be a reversal of pressure in the opposite direction - the "equal and opposite reaction" (actually, an oscillation of opposing pressures until they gradually die out)...

Since the pressure reversal is the last force acting on the barrel (the bullet has exited), and the heat is still present, it could be argued that the combination of these two might impart an unscrewing action on the barrel...

I've never seen or even heard of a barrel of any pistol/rifle unscrewing from use (assuming it was properly machined, fit, and torqued in the first place), and I'll maintain a level of skepticism in this regard... :rolleyes:

(Although you can see from the progression of S&W revolvers that S&W must have had some reason for pinning their later model barrels...)

I have been hand loading a 460 S&W for a while now, I have been pushing that gun pretty hard, the first set of scope mounts sheared completely off and on the higher end, the side plate screws nearly fell out of the gun after only 5 rounds, requiring lock tite to be used.

No barrel movement, but it is obvious that some interesting forces take place when a gun is fired.

Thanks for posting

Vern

I've taken the shot out/corroded barrels off of a couple of beaters to replace with after market barrels. I had to put relief cuts in the flanges to get them to budge.

If they can shoot loose, I would like to know how?


Ron

I wondered why my FAL barrel fell off at the range the other day....now I know. IMO whoever came up with such a goofy theory needs to collect and intellectualize (is that even a word???) a little less and pull the trigger a little more. But then, I'm a cretin.

P.S. I am being nice and I am playing well with others. But the fact is that shooting your barrel loose IS crazy talk!! :)

I tend to agree, there have been many combination guns made that only require a hand tightening of the barrels. They don't shoot loose even in the bigger calibers.

If it does happen, it must be a special circumstance, that is why I suggested initial forcing cone roughness, just a WAG, no proof.

Still, there are a lot of lugers that show a slipped barrel, it might also be that this was done to bring it to point of aim. Even though we have very clear testing procedures not everything is written down. Knowledge of the craft is used in almost every profession, its just impossible to write every thing down that may be required to be done.

Vern

Postino, guys
The experiment that was made, actually demonstrated the result that I think will be obtained. It is quite logical for me, that the firing of a shot wil actually tighten the barrel. I will try to explain the logic as follows.
The rifling is to the right (clockwise) observed from behind the pistol
The barrel screws in to the right (clockwise) observed from the front of the pistol
The barrel screws in to the left (anti clockwise) observed from behind the pistol
When a shot is fired, the bullet at first do not want to rotate, because it is in a non rotational state, and to get it rotating, some external force has to be applied. In order for the bullet to rotate clockwise, the rifling of the barrel apply a force on the bullet in the clockwise direction. The bullet on the other hand tries not to rotate and exerts a force in the opposite direction - which is in the anti- clockwise direction and thereby tightening the barrel.
This rotational force (torque) that is applied by the rifling will be great in the beginning of the bullet's travel because it has to start spinning very fast, but once spinning, the only force that will try to decrease the spinning or rotation, will be the friction in the barrel plus the rifling.
The two forces (the rifling in a clockwise direction) and the bullet's "unwillingness" to rotate in the opposite direction, is not equal to each other. The rifling plus the grip that the person exerts on the pistol overcomes the sluggishness of the bullet to rotate, and is therefore a nett force in the clockwise direction which spins the bullet clockwise.
I can speculate about the rotational force that the hand on the grips have to exert on the pistol in order to keep it from turning in the hand - but I will leave it at that - and first think about it!
Piet

Piet, your logic is irrefutable. :thumbup:

I was thinking about the torque that will be applied on the hand(s) when a shot is fired.
Before I come to that - what is the effect of the rifling (turns per inch or cm)? I will argue that the more turns the rifling has per cm, the greater the force applied to the bullet in order to get the spinning started (the accelleration of the spinning or rotation is greater) and the bigger the torque on the barrel/pistol. I will also argue that a heavier bullet and or a hotter load wil have the same effect.
What will happen if we could "suspend" the pistol in the air and pull of a shot? Will the pistol rotate clockwise or anti-clockwise (seen from behind the pistol)? Anyone wants to deliberate?

Newton's Third Law applies. "Every action has an equal and opposite reaction."

Anti-clockwise. Same direction the barrel was twisting. (As seen from the rear). ;)

I believe the rotational forces applied by a projectile on a gun would be minimal considering the weight of the gun itself. Also, the linear forces applied by the propellant on the projectile are going to be orders of magnitude greater than the rotational forces applied by the projectile on the bore; even small calibers can generate high pressures (e.g., a factory loaded .32ACP can be 21,000 psi [SAAMI]).

IMHO the high pressures (generating the linear force) driving the projectile down the barrel (linearly), along with the weight of the gun, are going to overcome any rotational force applied by the projectile itself.

Steve

Using this logic, every time I do a full mag dump with my trusty M3A1 my barrel should launch itself into the suset and end up in a different zip code.

...Only if the barrel has a left-hand thread and the rifling has a right-hand twist...

...Does it???... :rolleyes:

Mr. Postino, RH twist barrel is pinned to a large barrel nut with RH threads with a spring detent. Tell you what, next time I take 'er out I'll cram something under the detent to keep it from contacting the barrel nut, tighten the barrel almost hand tight and see if any counter- rotation occurs. Good enough?

Take pics... :D