Hey Guys,

Just pondering......if you look at the .284 Win and the .350 Rem Mag, you'll find these two cartridges almost (but not quite) identical in size with the exception of the rebated rim on the .284.

Now....assume we make a wildcat ..... say a .375 cal ...... on both cases. We use the same case length, and same COL for our wildcat thus removing almost all the variations in original brass dimemsions. We load identical charges of the same powder, with identical primers and identical bullets into our two slightly different cases. Which cartridge has more bolt thrust, the 350 Rem case with its full size belted head, or the .284 Win with its rebated rim.......or no difference?

Lobo in West Virginia

Total bolt thrust, or pressure on the bolt face per square inch?

Total bolt thrust, or pressure on the bolt face per square inch?

Good question.

Have been looking at if from the point of view of the REAR of the bolt, were the locking lugs are...not really caring about the differecnes in application to the front of the bolt, but rather working out the load appled to the locking system.

So...as an example just picked out of the air...if a force of 30,000PSI is appled to a .6 sq. In. area at the bolt face in example 1, and the same force is applied to a .8 sq. in. bolt face in example 2, is there any difference to the force applied to the locking lugs (and thereby trnasfered to the action)?

Mike,

As I think about it, it must be total bolt thrust, and as related to ribbonstone's thoughts. Can the locking lugs handle the pressure.

There has been a fair amount of wildcatting on the new short, fat, non belted cases. I'm thinking, "Why not cut a rebated rim on the case so it fits the Marlin 336 bolt, and take advantage of the fatter powder column." Think .416/.375 Beartooth done up on a WSM case feeding through what was a 30-30. I've read concerns about the force against the bolt with these fatter cases.

Would these be within parameters, or am I trading acne for warts?

Lobo in West Virginia

Everyone is concerned about pressure when bolt thrust is the issue, at least until you get to the point of catastrophic failure of the barrel.

Bolt thrust is determined roughly from the largest cross sectional area of the interior of the cartridge case multiplied by maximum chamber pressure. As an example, a basic head cartridge (.30-06 size) develops aprox. 8835 pounds of bolt thrust. The caliber does not make the least difference, a 25-06 will develop the same amount of thrust as a .400 Whelen at the same pressure level. Changing the exterior dimension of the case head will make no difference in this figure when you are only considering pressure against the locking system.

The only place that area of the cartridge head will make the slightest difference is in pressure per square inch against the bolt face itself, and this is in inverse proportion to the case head area. In other words, the larger head will exert fewer PSI against the bolt face than a smaller head, but the net bolt thrust will be identical! Since the brass of the cartridge case is much softer than the steel of the bolt, the only problem to this would be that the cartridge case itself would fail in compression if the head area and resulting PSI exerted against the head exceeded the elastic limit of the brass.

In short, if your theory had any merit we could make a gun in which you could conceivably hold the case in with your thumb against the force of firing if we could get the head area small enough!

If I were building rifles, would certainly build them to handle the worst possible rather than the best. Not so much what a rifle can stand when everything is clean, but what it can stand when the case and chamber are dirty or oily...the standard divation of the powder/primer/case/bullet all happen to fall at the high end of that deviation at the same time...a fouled bore...and high heat from both external temperature and internal heat from rapid fire. Things we try to avoid, but have to rtealize may well happen under field conditions despite our best efforts.

The more I think about it, the more I lean to the British ideas...use a big case at low pressure to allow for the most safety factor while still providing the perfomance level need to take the game cleanly.

ALK,

" Bolt thrust is determined roughly from the largest cross sectional area of the interior of the cartridge case multiplied by maximum chamber pressure."

So, from this relationship, the smaller the cartridge diameter, the less bolt thrust, and the larger the cartridge diameter the more bolt thrust. How then does one arrive at the max amount that the locking system on a Marlin 336 will safely handle? Will the larger diameter case have to be "down loaded" so as not to exceed the safety limits?

Ah, well, Marshall's .416 Beartooth is a known quanitity.

Lobo in West Virginia

[QUOTE=El Lobo]
(1)So, from this relationship, the smaller the cartridge diameter, the less bolt thrust, and the larger the cartridge diameter the more bolt thrust. (2)How then does one arrive at the max amount that the locking system on a Marlin 336 will safely handle? Will the larger diameter case have to be "down loaded" so as not to exceed the safety limits?

Part one, you now seem to have a pretty good grasp of the situation. The assumption in this situation is that both cartridges produce the same pressure. For part two, you really don't given the amount of information that is usually available.

Remember, the model 336 includes the 1895's which are the same basic action with a few modifications and, we have been led to believe, a different heattreat. They also include the .356 and .307 Winchester chamberings which also were heattreated differently. To really make a determination it would be necessary to know the specific alloy of all stressed parts, (frame, bolt, locking block) their specific heattreat and Brinnell hardness. This information would allow a calculation of the modulus of elasticity for the respective parts which would determine the stress they could take before permanent deformation occurred. Of course, the weakest part would determine overall strength. This is the reason you will often see advice to stay with a cartridge of the same class as originally chambered when doing such custom work on an existing rifle.

The method usually taken by such experimenters as Dick Casull and John Linebaugh is to take the direct route and test guns to destruction to make this same determination. They are willing to go to this expense as part of their development work.

If you are designing a gun it is best to calculate bolt thrust based on the maximum internal diameter of the chamber,. I know, theoretically only the inside diameter of the case matters but that assumes you never get a bad case. A case head rupture would change everything as the gas leaks out to the full chamber diameter and the case head becomes a piston driven rearward based on chamber diameter. It is just wiser to make safety calculations based on worst possible scenarios rather than on 'normal' conditions.

ALK,

" Bolt thrust is determined roughly from the largest cross sectional area of the interior of the cartridge case multiplied by maximum chamber pressure."

So, from this relationship, the smaller the cartridge diameter, the less bolt thrust, and the larger the cartridge diameter the more bolt thrust. How then does one arrive at the max amount that the locking system on a Marlin 336 will safely handle? Will the larger diameter case have to be "down loaded" so as not to exceed the safety limits?

Ah, well, Marshall's .416 Beartooth is a known quanitity.

Lobo in West Virginia