Gunsmithing Help validate my logic/math (please)

[BurnOut]

Short story long, I am in the final planning stages of getting a bolt gun currently chambered in 450 Bushmaster re-chambered to 45 Raptor. The cartridge diagrams are below. The 'smith that I contacted about re-chambering said that he'd do it, but that expanding the base of the 45 Raptor case (spec diameter .478") to the base dimension 450 Bushmaster (spec diameter .500") via fireforming is a lot of case expansion. I agree, and set about to do a bit of math to see what kind of impact that would have on the (Raptor) case wall thickness.

So, comparing the case base circumferences (circumferencii?), the Raptor case circumference is 95.6% of the Bushmaster base circumference. Assuming that the case expands uniformly, and using a Raptor case wall thickness of .013" (45 Raptor case mouth OD of .478" less a projectile diameter of .452" (.026" difference) divided by 2), does that mean that the theoretical wall thickness of the fireformed Raptor case will be 95.6% of .013"?

 

[BurnOut]

After thinking about it, and digging waaaaaaaay into the dusty parts of my noggin, I figured out a method for getting the actual answer here. I decided to calculate the area of the Raptor case wall by calculating the area of a .478" diameter circle (outside of the Raptor case) and subtracting from it the area of a .452" diameter circle (inside of the Raptor case). Using that figure and working backwards to a (fire formed) Raptor case OD of .500, I was able to determine that if I started with a Raptor case that had an OD of .478" and a wall thickness of .013", I would end up with a fire formed case with an OD of .500" and a wall thickness of 0.0123973905867194". This of course assumes uniform expansion, uniform case wall thickness, etc...

Looking at my original quick-and-dirty calculation resulting in a case wall thickness of 95.6% of .013 (0.012428"), I can see that I didn't quite hit the nail on the head with my circumferential ratio comparison, but it was pretty damned close. Close enough for practical purposes, anyhow. In case you're wondering, the difference in case wall thickness between the two different calculation methods is 0.0000306094132806".

So, I said that to say this... if you find yourself in a position of wanting to calculate case wall thickness after fire forming and know the beginning and ending OD, and beginning case wall thickness, then determining a workable (theoretical) post-fire forming case wall thickness is as easy as calculating the ratio between pre- and post-fire forming case OD, and applying that ratio to the pre-fire formed case wall thickness.

You may now return to your regularly scheduled actual interesting conversations.

 

[chevy_man]

The base of the case is hardened, unlike the annealed neck.


I'd put money on case separations unless you anneal the entire case, then harden the base after sizing with a die/mandrel.

If you shoot them soft they'll stress crack. If you shoot them hardened they'll stress crack.

 

[bschneiderheinze]

This Most fireforming is done in the upper part of the case I see it unlikely you fireform the whole case

 

[Ledzep]

BEGGING for trouble.

If you were the right tooling you could use conservation of area. To do that you'd have to know the thickness of the wall down at the base. If it's .013 you're fucked. It's not though-- I'd put money on .060-.090". Let's say .075 for the purpose of this demonstration...

.478/2= .239
(.478-.150)/2= .164

Pi*(.239^2 - .164^2) = Pi* (.25^2 - X^2) (finding the area of the big circle (OD) minus the small circle (ID).)

Solve for X and you get .179, or an ID of .359 or a wall thickness of .0705".

However, you're not using the right tooling. This is happening under gas pressure in milliseconds so you're probably just going to balloon and split cases. Longitudinal splits if you're lucky... case head separations if you're not.

 

[BurnOut]

Hmm. Shit.

May have to make my own cases from .284 brass, as suggested by the 'smith...

 

[Easy_E]

Here in lower Mi we are stuck with 1.8" or shorter straight wall cartridges so something better feeding than the Raptor interest me .

 

[Supersubes]

I know Jack about these cartridges except I shot a bushmaster once. Thought it was pretty cool.

What’s the purpose of going to the raptor?

 

[BurnOut]

I know Jack about these cartridges except I shot a bushmaster once. Thought it was pretty cool.

What’s the purpose of going to the raptor?

The Bushmaster is a ~35k psi cartridge, while the Raptor is basically 460 S&W magnum with a rebated rim (a 62k psi cartridge). Depending on the projectile, we're looking at 300+ FPS difference out of a similarly sized cartridge.

 

[BurnOut]

After taking in the feedback provided above and chewing on it a bit, I think I'm going to spend some time working up some 450B loads to see how close I can get to 45 Raptor MVs. The 450B was designed as an AR15 cartridge, and pressure was limited in deference to that rifle's bolt design (pretty weak compared to most bolt-action designs)... so it's theoretically possible to crank the pressure up and start getting noticeably improved performance out of it.

 

[Ledzep]

To a point. Bolt actions are better at containing this, just watch out for exposed case head out of the breech.

In a pistol, for example, if you load a .380 case with a 9mm charge and seat a bullet to 9mm length and shoot it in a 9mm (ignoring headspace and reliability issues), you're liable to blow the case apart. The .380 cases are thinner at the base because they run at lower pressures and don't need to be so thick. Anyway, pistols have much more unsupported case head material because of feed ramps etc.. Just a word of caution. The fired cases will let you know when they're not happy anymore.

 

[Easy_E]

You can load more than max and get some good velocity with the 450 . We shoot 400 yard steel all the time getting good groups . My furthest kill on a white doe is 307 yards it worked great .