Well, my guess is that it's a .38-45 Clerke; which was a .45 acp case, necked down to take .355 bullets.
It's hard to tell the diameter by perspective, and that would be a VERY rare and unusual chambering, but it would seem to be the closest fit. I'm pretty sure it's not a .400 cor-bon or a .38 casull; because it looks smaller than a .40, and seems to have more shoulder than a .400 Cor-Bon, and it's got a .45acp headstamp and doesn't take a small rifle primer like the .38 Casull.
What's your guess?
