Answer to the gun stumper.

First off:

Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff Quaff... buuuuurrrrrp!

It is something old - if 1945 qualifies as old.

Here's the original question and the answers.

It's not a muzzle-loading cannon. That would be one hell of a counterbore if it was! (For those who don't know - a counterbore is when you drill in from the muzzle, making the bore larger than the rifled portion. There are many reasons this is done, but in black powder weapons it was mainly done to prevent cracking. It can also be done to repair muzzle damage, and that is often done in small arms. The russian arsenal rebuilt Mosin rifles are sometimes found with counterbores.)

Those who guessed/deduced artillery - you are correct, as far as it goes. There are too many lands and grooves for a small arm. They are also too pronounced. And, too flat at the breech. There was no forcing cone - which should have given it way that it was not a small-bore arm, either. The 'rusty' portion is flat. That would be one odd looking cartridge - for a rifle or a tank or artillery piece. Another option would be separate loading artillery - but their breeches have some other tell-tales, such as the 'swiss groove' - and they too still have a forcing cone for the initial ram and seating of the projectile.

There is really only one weapon that fits the evidence (and you had to be able to figure out that there was no forcing cone): a recoiless rifle. Where the cartridge sits unsupported in the breech and the rifling is pre-engraved on the rotating band of the projectile.

I told you you were going to have to be a geek to get it. There were some very informed guesses. I was impressed. Calliope - your brother made a good guess too - it does look like the sleeve that would engage a prop shaft. And John, you were correct - it uses an abbreviated form of interrupted screw breech block.

The 'dirt' in the bore? The rust-colored stuff is cosmoline. The crunchy particulate matter is welding residue from when the weapon was dewatted and the bits carefully rewelded to meet ATF specs to still be a "non-gun."

Here's a picture with a slightly different POV.

The weapon in question? A 57mm M18 recoiless rifle. Made by Firestone, in Canada (note the "C" serial number), in 1945.