Why is a 3/4" Rocket Called a "One Pounder"?
The following is a quote I lifted from rec.pyrotechnics written by Mike Swisher and posted here with his permission:
The ounce and pound system for sizing rockets initially related to the weight of a lead or sometimes an iron ball having the same OUTSIDE diameter as the rocket's case. The practice had the same origins as that of describing the bores of guns by the number of bore-fitting balls that could be made from a pound of lead. For example, a 12-bore shotgun is so called because 12 balls of 0.729" dia. - its calibre - weigh one pound.
This was a function of rockets being rammed in moulds (sleeves or supports). In the seventeenth and eighteenth centuries, firework- making was considered a subdivision of the artillerist's profession. Gun barrel makers were among the few mechanical craftsmen that had the capacity to bore true cylinders of any desired diameter or length. Rocket moulds were made by such workmen, and in the old illustrations they even look rather like cannon barrels.
The ID of a rocket for purposes of calibre designation was considered to be 2/3 of its OD. Accordingly, a rocket of 3/4" bore would have an OD of 1-1/8". In the original (English) system of nomenclature, this was called a 4-oz. rocket. A 4-oz. lead ball is actually 1.056" in diameter and hence an actual 4-oz. rocket, having this OD, would by the formula have a bore of 0,704" rather than 0.75", but the latter was considered an accepable approximation. There may also be some confusion here between the "regle de plomb" (use of lead for the standard ball) and the "regle de fer" (use of iron for it), which would have resulted in a somewhat larger diameter.
In this English system, a 1/2" bore rocket was called a 1-oz., 5/8" was called 2-oz., 3/4" 4-oz., etc. These were close, but not accurate, given the above assumptions. In the American nomenclature these diameters are respectively called 4-oz., 8-oz., and 1-lb. In other words, the British designation has been multiplied by 4. I do not know how long this has been the American system, but it antedates 1930. The current U.S. identifications of rocket ID with nominal calibre by weight are pretty much the same as those given by Weingart, the first edition of whose book was published in that year.
Why the American nomenclature is 4 times the British, we can only speculate. The late Ray Anderson told me that at the old Wells plant in Rialto, California, which was run by a scion of the British family of pyrotechnists of that name, there was a custom of referring to the stars or inserts that fit roman candles as, for example, "4-oz." if they fit into a case having the British dimensions for a 4-oz. rocket. Thus a star or insert that fit a 3/4" bore tube was called a 4-oz. star. He hypothesized that with the lapse of time and the loss of knowledge about the practice's origin, the OD of the insert came to be assumed to refer to all articles having the same OD, hence a case having 3/4" OD (which would have a 1/2" ID) became a 4-oz. case.
I'm not sure whether Ray's speculation was correct, or whether the practice simply originated in commercial puffery. A "one pound" rocket, after all, sounds more impressive than a four-ouncer. Weingart himself wrote, before giving dimensions for cases, that -
"Of late, the diameters and lengths of Roman candles and rockets have been changed and reduced so often that no standard of sizes can be given, but the following will be found to be useful for average work and may be used accordingly."
Whatever may be the explanation, the American system of designations is not a 'fudge' (as the old British system was) - it's completely arbitrary, without direct reference to any objective standard of measurement. It also probably owes its continued existence solely to firework hobbyists, since skyrockets haven't been domestically manufactured for the consumer market in decades, and the Chinese rockets I've seen for sale as consumer fireworks in this country have not carried ounce or pound designations. The only reason it's used is that the hobby suppliers have sold (for example) 3/4" ID cases 7-1/2" long as appropriate for "one-pound rockets." The terminology becomes even more absurd when such a case is cut down for use as a whistle or strobe rocket, and that is then described as "one-pound." The only positive thing that can be said for such a practice is that most American firework hobbyists will understand, on hearing the description "one-pound rocket", that a 3/4" ID case of some undetermined length has been used to make the article.