I have learned that cast bullets can be sized down to smaller calibers for which there are no or few moulds. As long is the lube grooves retain a certain depth, apparently.

But what about gas checks?

Lets say I build a rifle with a 0.423" bore. I could get a .44 mould and size the bullets down to 0.424" or whatever, but what would I do about gas checks? Can sizers squeeze them down too, or would I have to make my own out of soda cans or whatever?

For bullets: try Tom at accuratemolds.com
If you can draw it he can cut it.

For gaschecks: try this fellow = http://castboolits.gunloads.co...s-Check-Forming-Dies

Pretty well standard practice amongst us 423" bore users of cast bullets is the use of 44 cal gas checks. These size down easily in a sizer die. My own Lyman 450 die is .424" sizing and crimping on Hornady 44 cal gas checks in one pass.

Annealing them makes sizing a lot easier.