Size is everything. There used to be kits for doing tiny amounts of lost wax casting in a microwave.
I’ve not tried but a regular set of brazing hearth parts and torch should make up an egg cup sized crucible furnace.

Only last week I smelted a load of zinc for anodes. I used the bottom half of an old gas cylinder (may be a 7Kg ) lined it with 1″ cramic fibre blanket, cut a tangential hole near the bottom to put in the end of a burner. An A4 crucible. The important thing to consider is the need for secondary air for the burner. Almost any metal vessel will do even a biscuit tin. I have melted brass at about 1020c by using 8 K23 bricks in a circle and a sievert burner. Small name plates were successfully cast.

In answer to the question –  YES ! For gun metal you will need about 1100*c +. Small items will need higher temperatures, solid lumps lower.

In 1995 I used a spindrier casing, salvaged kiln bricks, the blower from a hotpoint washer/drier and the variable speed unit from a hoover vacuum cleaner to build the furnace I still use. The burner was made from bits and had a 1.8mm nozzle. A pot of brass in about 20 mins bronze/ gunmetal takes a little longer.

Good luck.  Noel.

PS if you need more info just ask. As for gas bottles see the tread on silver soldering torches and the Bullfinch chart. N

You can buy complete 6kg capacity propane furnace for little more than £70 on our favourite auction site. Not sure about safety standards but model engineers are a resourceful lot and well used to operating supposedly dangerous or hot items!

Air blower (e.g. hair drier) and coal or charcoal should get hot ennough. You need a crucible that will take the heat. Surround and cover the crucible with charcoal. This pot mender is melting iron https://www.youtube.com/watch?v=rzOgxx-YG8w