Looks like a 416 Rigby

http://www.gundigest.com/gun-collect...-little-screen

It does, but...

The neck is too long and it appears to be rimmed.

http://www.cotep.org/forum/picture.p...pictureid=1046

I'm going to say it is an Ackley Improved version of the 450/400 Nitro Express 3 1/4".

Here is the 450/400 Nitro Express 3 1/4". Blow out the shoulder and you should have something dang close:

http://www.kynochammunition.co.uk/ca...0-400%20NE.jpg

http://www.cotep.org/forum/picture.p...pictureid=1046

Based on the material and shape of the projectile, which appears to be steel, I would lean towards this being an anti tank / anti aircraft round. Without a micrometer reading.... I'm guessing its a 20mm x 105b anti tank round. Picture below:

https://www.google.com/search?newwin...KQ-I5lzN0KM%3A

Second guess would be the Soviet 14.5mm round (similar to our beloved 50 cal)... but the shell profile isn't as close to the 20mm....

Let us know when you figure it out!

DaFadda

Looks to me like a .458 WinMag that went through the crimping die too far........Notice the neck has no shoulder, the crimping die was set too deep.