What paperwork do you need to have a functional T-34 gun ?
Are you a museum or ? Gun dealer licensed?
"Destructive Device" as defined by Federal law:
Any firearm with a bore over 0.50 inch except for shotguns or shotgun shells which have been found to be generally recognized as particularly suitable for sporting purposes. (Many firearms with bores over 0.50" inch, such as 20-gauge or 12-gauge shotguns, are exempted from the law because they have been determined to have a "legitimate sporting use".
Also, actual explosives count as DDs, in which case everything I've mentioned below will apply, with the addition of having a proper magazine to store the explosives in.
Destructive devices require you to submit a "Form 1" if you are building one or a "Form 4" if you are buying one. Also, remit $200 tax. This tax is paid every time the gun is transferred. So if you ever sold it, either you or the person buying it would remit $200 to the ATF in tax. You also have to get your local chief law enforcement officer to sign off on it (some States require their cLEOs to sign off on NFA stuff, I think) but this can be difficult as some chiefs and sheriffs will refuse you. Also you have to submit a passport photo and fingerprints.
Alternatively, you can skip everything but the tax by having your Trust (for which you will be both the grantor and the settlor/trustee) buy and own the NFA item on your behalf. This has been an effective way to bypass recalcitrant police chiefs and sheriffs in the past, but the window of opportunity may be closing as the ATF is basically rewriting the law by administrative action.
Black powder muzzle loaders don't generally qualify as firearms, hence, why you can legally build or buy a a civil war cannon without any sort of registration or paperwork.