What is this?

I have no specific knowledge of this subject matter, but if there's anything in those disassembled parts that isn't metallic, you need to be real careful how you handle 'em.

The first picture of the projectile looks like an Oerlikon round.

WWII aircraft carriers had scores of those guns lining the decks for anti aircraft defense.
The second picture looks like a more modern 20mm dummy round.

Obturation said:

By gosh i think you're right. I did a google search and that appears to be what it is. Does the cartridge you have unscrew in to pieces? I would think that the military wouldnt want anyone to be able to tamper with the insides, but i guess that it must have been possible. Thanks! The wealth of knowledge on THR never ceases to amaze me.

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How do you think they were assembled? You had a fuse, booster charge, and explosives in it. I'm betting assembled rounds had a pretty powerful thread sealant applied.

Mine is just a drill round, never tried to remove the fuze cap.
You can find a schematic breakdown of the fusing system of that round online. The old 20's had a centrifugal weight and spring that compressed when the round spun out of the bore, thus arming a firing pin and a cap to fire the bursting charge.
I've never heard if the new 20mm rounds still use the same system. There is not much room in the projectile for any more complicated arming system.

I didn't mean to sound like too much of a "Nervous Nellie" back in post #2, but many years ago I acquired a "deactivated" bringback Chinese RPG-2 round that still contained the fuse/detonator assembly.

You do have to be careful with any unknown ordinance. I had a citizen bring in a brown lunch bag to the PD front counter. He said he just bought a home in town and found "this" in the attic. I opened the bag and found a Chi Com potato masher grenade. Still had the screw cap on the handle. I unscrewed the cap and found the pull string still intact. I put it in a cinder block dog kennel and contacted EOD to come by and pick it up. Turned out it was inert, but still had some flaked TNT on the walls of the head.

Another one was a also a bring in at the counter that was a WWII Japanese grenade. Those had picric acid in the detonators and were notorious for being unstable. This one was was also found to be deactivated after being checked out by the local bomb squad.

Occasionally, I've come across a live 20MM round. In WWII, several countries used 20MM cannons and they all had their own color coding. Some had HE in them and can be very dangerous to be tossing around.

Were 20mm Oerlikon shells too small to have been fitted with proximity fuses?

DocRock said:

Were 20mm Oerlikon shells too small to have been fitted with proximity fuses?

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Yes.

Given the technology of the times, they couldn't have fit a proximity fuse into that small a package, and even if they had, it wouldn't have been very effective because it doesn't carry enough payload (explosives and fragmentation) to be effective against aircraft.

Keep in mind that a US WWII proximity fuse contained a radar transmitter, a radar receiver and safety devices to keep it from blowing up during handling or before it had traveled a safe distance from the muzzle after firing. Each shell was also fitted with a self destruct mechanism to keep the enemy from learning how to make them.

Considering that this was done with battery powered vacuum tubes made out of glass, it's almost a miracle that they made 'em small enough to fit into a 90 mm shell.

Correct me if I'm wrong. I believe that the new 20mm shells that the Navy now uses have a new "set back" type fuzing that arms the projectile during the recoil of firing. My search to determine the answer has been thwarted. Maybe the government wants to keep it a secret? On the other hand the space available for an arming device of low cost behooves the manufacture of centrifugal devices.

Correction- the WWII Oerlikon type FFM (which you have a projectile and I have a picture) does not use a fuze arming system in the nose. That projectile is a nose contact fuse. In other words there better not be any kind of plug in your barrel or it would blow up the gun!
(reference "Janes Ammunition 2000-2001)

total recoil said:

In other words there better not be any kind of plug in your barrel or it would blow up the gun!
(reference "Janes Ammunition 2000-2001)

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If you plug the bore on a blowback operated 20 mm cannon, it's gonna blow up no matter what kind of projectile you shoot.