Thread: Shotgun-9mm thing

Hint, work out the area of each case head.............................PSI stands for Pounds Per Square inch.

Originally Posted by veitnamcam

Hint, work out the area of each case head.............................PSI stands for Pounds Per Square inch.

And the calculation Toby should be simple for a fellah like you. Just remember Big Ben Pies are Square.

Hopefully I've got this right

area for 12g - 0.6165"
"""""""".308-- 0.1750"

I can't do the litte 2 off my phone to show its sqaured

So thrust or pressure applied to the action for each is?

Originally Posted by veitnamcam

So thrust or pressure applied to the action for each is?

VC you are subtly becoming quite the physics teacher. Keep it up and Toby will thank you for it one day.

Starting to think me ditching physics for computing was a silly move on my part

I found this

http://en.wikipedia.org/wiki/Bolt_thrust

Also got the formula



6.96645 is a number I got for the 12g but have a feeling it's wrong and if it is right have no idea if it's N or lbs

I googled and can't find a formula for that. Do you know one I can work off?

Originally Posted by Toby

I googled and can't find a formula for that. Do you know one I can work off?

Its pounds per square inch so

12g - 0.6165" x 11,300psi = 6966 pounds of force trying to break the action apart
.308-- 0.1750" x 61500psi = 10762 pounds of force trying (and possibly succeeding) to break the action apart.

So we can see its in the bad idea bucket to run max power 308 out of the shotty action.

If we take 6966 pounds of force as max and divide by 0.1750 case head diameter we end up with 39,805psi as max safe pressure.
This is right around starting loads for 150gr in 308win.
Data I have here for cast bullet loads(that you cant boot up the arse cos they are too soft) Is 170gr 2000 fps =37700psi, 1600fps= 21600psi.

So it should be safe as houses subsonic or at reduced loads, the issue is making sure it is(if it was made) never fired with a full power load.

12g case head roughly 1.9cm diameter.
1 inch = 2.54cm
1.9/2.54=0.748 inches

Area of a circle
A=pi x r^2
pi x (0.748/2)^2 = 0.439 in^2

12g case head about 0.439 in^2

0.439 in x 11300 psi = 4966 pounds bolt thrust

It only be for subs. If I wanted a full .308 load I'd use the .308

So my calculation is right by vc and wrong by banana? I got the measurment off chuck hawkes which is different to bananas

Originally Posted by Toby

So my calculation is right by vc and wrong by banana? I got the measurment off chuck hawkes which is different to bananas

No doubt that I'm off with the case head measurement. I just roughly measured a 12g with crappy plastic ruler and rounded it to the nearest mm. The working should be solid though.

If you're using 0.6165" as the case head area, seems like you're including the rim of the case in the external case head measurement.
This would give you an optimistically large amount of force that the action could handle. The pressure is only pushing on the internal case head area, which is much smaller than the exterior case head area if you're including the rim.

I dont have one to measure but a quick google suggests 6-7k pounds thrust for 12g

Weld a bit of 6 mm steel to the face of the block and extend the firing pin!

Yes also using outside 308 case head measurment giving larger readings of force.

So I have tracked down a 9mm barrel off cut just under 4" long. So who here can thread it and drill out and thread the chamber insert for me?

@veitnamcam @vietnamcam

Hope one of those is right