The P-38 can opener is 38mm long and has been alternately called the "best Army invention ever" or the "John Wayne". If I could have found the training video w/ The Duke opening his C rations with this thing you know I would have included it...
GeorgiaOutfitters has a detailed page with the complete history. #recommend. They seem to have become beloved by ex-Army types and bushcrafters/bugout/EDK types as evidenced by the below comment from PatentsPending.
I still have the first one I grabbed out of our first box of C-rations, in Army boot camp in 1977. The only place a GI could get them was in a case (12) of C-rations, where they appeared to be thrown in as an afterthought, 4-5 to the case. It's on my keychain. I still use it to open cans when camping (car-camping, that is). I've used it to adjust my carburetor, get a grip on a splinter, clean my fingernails, pick my teeth, adjust rifle sights, let air out of a tire, rip open taped boxes, and open cans of all sizes to the gasps and amazement of lookers-on. It shows no sign of wear, other than the hole where the keyring has carried it for 28 years.They have them at Ball and Buck in the North End. $1.
[thx to @thewilderthings for the inspiration]