No.1 Mk3 Bolt Cracks

[Ridolpho]

Rob,
I think at first blush I would have felt the same way about it as you. But putting on my engineer cap (my business card folds into one), I find it helpful to think of the path of least resistance. If one were to strap the action right at the barrel threads to the theoretical immovable object, then yes, it probably would deconstruct itself as you suggest.
But as a shooter, we’re not doing that. We’re placing the butt on our shoulder. So what seems more likely, the forged steel stretching or your shoulder moving and absorbing the shock?
Path of least resistance.
Now maybe I’m missing something too but that’s how I see it.

Think about the few milliseconds after firing- the pressure in the chamber is already several thousand psi but the bullet hasn't begun to move forward and the rifle is not recoiling. As an exercise in "statics" (I can almost remember Eng. 201), with the case head bearing upon the (nicely headspaced) bolt, how can the action body not be in tension? When the Turks wanted to shoot 8X57 from the SMLE why did they weld a bar of steel onto the right body? When the Soviets developed the rear-locking SVT 40, why did they progressively thicken the two siderails of the body? According to Soviet documents, they found body stretch affected headspacing (call it dynamic headspacing as opposed to static). I'm probably missing something here but I went on to be a geologist as opposed to an engineer so.......................

Ridolpho

[Peter Laidler]

We're back to 6th form physics classes now where you MUST accept the laws of physics and stop thinking fuzzy logic.......... Or we go off into megga mission creep. Accept that the explosion DOES act all around up, down, round, back and forwards for a megga millisecond and then dislodges the bullet which travels up the bore. The bolt is held firmly against the locking shoulders whence the loading, in the form of recoil is transmitted rearwards in the form of felt recoil. So far, so good. Simple question. Where is the opposing action that will cause the body to stretch when it is already under compression? Stretching in mech engineering terms is when the material has exceeded the bounds of its elasticity and retains that form....... It's stretched! As would a bolt between two girders. Tighten up the nut to 80ft/lbs. Exceed that and the bolt will stretch or exceed the bounds or limits of its elasticity. Simple test at uni to test tensile bolts, try to run the nut down the bolt shaft and it won't run because the bolt had stretched. We know the reason. No such reason for a rifle body to stretch.

Forget broken bolts, this is nothing to do with that (well, it is but distantly related)

[Terrylee]

Peter, many thanks for your explanation, which for one without an engineering background is most enlightening. However, there is still one thing that I don't understand. If there is no stretch to the receiver, what causes an increase in head space?

[ActionYobbo]

just throwing this in to muddy the waters. So I read the action wont stretch because the shoulder takes the recoil. Now with the shooter using the lead sled where will the recoil go? since its not absorbed by the led sled but just held with brute force will that now cause the recoil to be absorbed by the locking lugs?

to the OP was your friend using a shooting aid like the led sled?

[browningautorifle]

Now with the shooter using the lead sled where will the recoil go?

Into the lead sled. That's what they're for...

[Doco overboard]

No engineer here either but my cave-man thinking makes me believe the opposing reaction is transmitted back to the barrel in a radial fashion through the breeching mechanism and how well the barrel is fit to the action face. The vibration is expended into a node. The mass and tension of the receiver to the stock further control the release of energy and keep it true to form. The receiver (body) and well fitted bolt are heavier than the bullet as it travels through the barrel. The kick or recoil is just what happens as the gas exits the muzzle and it's in a straight line which the system can easily overcome because its rapidly decreasing. The energy from the powder burning and expanding is moving outwards in all directions like straight pins stuck in a tomato.
Head space is controlled by correctly heat treated parts that are fitted properly which can repeatedly with stand or overcome cartridge tension to the chamber walls. The cartridge fit I think does all the real work as long as there's something there to keep it in place for a short period of time.

Dunce cap on

[CINDERS]

A snippet from Physics of Guns;

"When one object exerts a force on a second object for a certain amount of time, the second object exerts an equal but oppositely directed force on the first object for exactly the same amount of time. The momentum lost by the first object is exactly equal to the momentum gained by the second object. Momentum is transferred from the first object to the second object. In this case, if a gun exerts a force on a bullet when firing it forward then the bullet will exert an equal force in the opposite direction on the gun causing it to move backwards or recoil. Although the action and reaction forces are equal in size the effect on the gun and the bullet are not the same since the mass of the gun is far greater than the mass of the bullet. The acceleration of the bullet while moving along the gun barrel would be much greater than the acceleration of the gun (acceleration= force mass)."


So the steel would be opposing the forces applied to it equally in the opposite direction I guess.

If one of the armourers or Ginsmiths (typo but it made me laugh) here would answer if my reply has merit in that the Lee bolts are not a drop in fit and to do that without proper fitment into the action is playing with fire as it is not set up for the correct transference of the recoil wave correctly on firing. TIA for your reply.

[Ridolpho]

This is an interesting topic- thinking about it today while casting Snider bullets and managed to burn my hand! There's lots of literature on the subject but I can't find anything that demonstrates that the compressive stresses on the receiver (due to the peak force acting on the gun during recoil) are enough to fully offset the tensile stresses on the receiver (due to "bolt thrust"). Easy enough to guesstimate the possible forces due to bolt thrust (chamber pressure and area of cartridge head are known although pressure reduction due to strength of the brass is uncertain). There are reports of experiments made to try to determine peak recoil force which suggest something in the 500lbf range (applied very briefly, of course). Isn't that going to be a lot smaller than the force due to bolt thrust?

Ridolpho

[Peter Laidler]

I know the answer to all of the amateur metallurgists, chemists, gunsmiths and would be Armourers. Just ask yourselves this simple question. To stretch a piece of elastic, you pull it between your fingers. To stretch a piece of leather, you wet it and pull it out on a table. To test a piece of steel to test for shear strength, you pull it (stretch don't forget) between two clamps to stretch it until it shears. Pray, tell me how you stretch a rifle or bren gun body by doing NONE of these stretchy or tensioning type exercises? Nope........, me neither!

You see, I'm a simple engineer. I'm biased in favour of gravity, the existence of the moon and the tides, the laws of physics and not standing in the path of oncoming trains.

[GeeRam]

You see, I'm a simple engineer. I'm biased in favour of gravity, the existence of the moon and the tides, the laws of physics and not standing in the path of oncoming trains.