Here are official regulations regarding black powder, ordnance, and explosives.
US Government list of manuals -
Federal Explosives Law and Regulations
State Laws and Published Ordnances
Black Powder Exemptions:
http://www.info-central.org/regulatory/orangebook/subparth.html
(The manual online above is from 1982 but it says the same in the 09/2000 edition.)
b) Black powder. Except for the provisions applicable to persons required to be licensed under subpart D, this part does not apply with respect to commercially manufactured black powder in quantities not to exceed 50 pounds, percussion caps, safety and pyrotechnic fuses, quills, quick and slow matches, and friction primers, if the black powder is intended to be used solely for sporting, recreational, or cultural purposes in antique firearms, as defined in 18 U.S.C. 921 (a)(1 6) or antique devices, as exempted from the term "destructive devices" in 18 U.S.C. 921(a)(4).
It should be noted that most Civil War artillery is field artillery and contains far less than 1 to 3 pounds of black power. It should also be noted that more than half of all artillery is solid in nature (i.e. never contained any powder but was used as a battering ram) or possessed a wood fuse adapter which over the years of being in the ground has subjected the shell's powder charge to moisture and contaminates.
Destructive Devices:
ATF P 5300.4 - Federal Firearms Regulations Reference Guide 2005 (Revised - 09/2005)
Section 5845(f) – Destructive device – The term “destructive device” means (1) any explosive, incendiary, or poison gas (A) bomb, (B) grenade, (C) rocket having a propellant charge of more than four ounces, (D) missile having an explosive or incendiary charge of more than one-quarter ounce, (E) mine, or (F) similar device; (2) any type of weapon by whatever name known which will, or which may be readily converted to expel a projectile…..
The term “destructive device” shall not include any device which is neither designed nor redesigned for use as a weapon; any device, although originally designed for use as a weapon, which is redesigned for use as a signaling, pyrotechnic, line throwing, safety or similar device; surplus ordnance sold, loaned or given by the Secretary of the Army pursuant to the provisions of section4684(2), 4685, or 4686 of title 10 of the United States Code; or any other device which the secretary finds is not likely to be used as a weapon, or is an antique or is a rifle which the owner intends to use solely for sporting purposes.
Section 179.25 Collector’s items.
The director shall determine in accordance with 26 U.S.C 5845(a), whether a firearm or device , which although originally designed as a weapon, is by reason of the other characteristics primarily a collector’s item and is not likely to be used as a weapon. A person who desires to obtain a determination under that provision of the law shall follow the procedures prescribed in …..