Sounds like a fun project. I would be glad to help....
An idea as a thought starter: Moderator please move the following to a correct forum if not appropriate here.
The WW2 British Home Guard Manual had detailed instructions for making improvised slug loads from birdshot rounds. Remembering that in UK ball or slug loads were not manufactured for civilian use. Buckshot loads were scarce and relegated to military and law enforcement, so the Home Guard was taught to make do.
The easiest method by far was simply to open the crimp, pour hot wax over the shot to fill up the case and when cooled the assembly works like a big Glaser Safety Slug. Most of the shot goes out the bore in one clump, whereas some of the pellets disperse randomly in a pattern around it. This waxed shot load was said to be effective to 40 yards or more from a choked bore. .
The alternate and preferred "improvised" slug method, if you don't happen to have a round ball or actual slug mold of the correct size, was to use a piece of dowel or broom handle. Whittle and sand it down until it is an easy sliding fit into the muzzle of the tightest choke of your common double gun. Mark with a pencil around its circumference at a depth EQUAL to but not exceeding the dowel diameter, so that you don't make the slug too heavy. You want a slug weight from 1 to 1-1/8oz, or 440-480 grains (30-31 grams) for a 12-ga. Use the rounded wooden mandrel to press into foundry sand to make sand molds. Melt whatever scrap lead water pipe or battery plates you have as well as any lead shot you have emptied out of the shells, then pour the molten lead into your sand molds to cast hemispherical, flatbased slugs.
After you have removed the cast slugs from the molds, file off any flashing at the base, then clean them well with oil and steel wool to remove any sand imbedded into the surface of the casting. For safety sake these oiled, cast slugs MUST be tried for fit in the gun muzzle and should fall entirely through the barrel of their own weight. If only a wee bit tight, such that they can be pushed into the muzzle with thumb pressure only, the oiled slugs can be "sized" by tapping through the choke with capped length of half-inch pipe and mallet until they fall out the breech. Oversized slugs which cannot be forced into the muzzle with thumb pressure only MUST be remelted and cast again, lest you burst the barrel!
Properly sized, oiled slugs can then be inserted back into the charged and wadded shell and cemented in place by dripping melted wax or tallow over it. It is unnecessary to recrimp the end of a paper shell, just trim it off with a knife. This also serves to identify improvised slugs from bird, duck or goose shot.
Waxed-shot loads were said to defeat a residential hardwood door at 25 yards, whereas solid lead cast slugs would shoot through both side doors of an automobile, or through the "boot," penetrating through the seats and dashboard, entering the engine compartment. They also deflate automobile "tyres" if hit.
Accuracy was supposedly adequate to hit a charging silhouette target at 50 yards, firing 6-8" groups using both barrels, given a little practice. "A well managed shotgun wielded by a resolute Home Guardsman invokes terror in the enemy parachutist!"
Primer on loading cast lead round balls in a 12-ga.:
With 16-ga. balls (0.662" or 0.678") loaded into 12-ga. plastic shotcups I get best results using a 16 or 20 ga. nitro card wad (or two) in the bottom of the shotcup then a small 9mm case scoop of Cream 'O Wheat on that then the ball.
A 0.662" ball will safely go through any 12-ga. choke, but is less accurate in any except a full choke.
A 0.678" standard 16-ga. ball is a good fit in most standard 12-ga. shotcups and safe in full choked barrels.
In a rifled 12-ga. gun the 0.678" RB inserted into plastic shotcups gave 4-inch groups consistent groups at 50 yards. The 0.678" RB and shot cup combination is easiest to load. Use a nitro card wad or two and a scoop of corn meal or polenta under the ball inside the shot cup to provide support and keep the bottom of the shotcup from trying to wrap around the ball.
A fast-burning target load shotgun powder is best, using load data for 1 oz. slugs or shot. A 0.678" RB's is about 1 oz. so common field and target load data can be used, just use equivalent weight birdshot load data. "
