I was thinking the other day about a homemade slam fire shotgun when the thought occurred to me that I could make this into a sub caliber device in a reproduction anti tank gun maybe the size of the WWII M37.  It would look like an M37 and the barrel would be a piece of tapered tube with the homemade shotgun barrel inside of it.  Would this be a destructive device?

From what I understand, as long as the firearm part is legal, the rest is just decoration.

So that would mean your subcaliber firing portion would have to be permanent, not a chamber insert like normal.

I've been wanting to make a 50 bmg version of this for a long time;

When I went through AIT at Ft. Polk in 1975 we went to a range (just once, but it was fun enough I'd like to have done it more) and fired those old 90MM recoiless rifles - with a 7.62X51 barrel device inside them.  They worked exactly like the 90MM would have, but we'd insert that single tracer round in the chamber, close the back, rotate the handle, tap the guy who was firing it on the helmet and watch the tracer fly down range to hit (or miss) the truck/tank/apc that was our target.  Then we'd repeat.

Something like that would be neat.

I'm actually having a blank firing PAK38 constructed... My plan is once the company is done building it, I will build a subcaliber firearm to be able to fire 50 cal or some other kind of projectile from it... Nothing crazy that will need an active recoil system involved... But I do want to be able to hit a target rather than just fire blanks with it.

You could probably get some good ideas from here.  It's what I started with on my 30mm GAU build:

http://users.multipro.com/brentandloriann/