One Round

If you have the means and the world hasn't fully embraced rapid fire from a magazine...

GURPS: Magic has a spell called Cornucopia which supplies unlimited quantities of ammo... the catch is you have to pull each round out one at a time and they disappear after a minute or if handled by someone else before being fired.

Energy cost to enchant is 50 times the cost of the round in G$.

So a little pouch that makes .30-40 ammo is 40 energy to make which means G$400.

Pistol ammo is cheaper, .38 ammo for your M1894 is just 10 energy and just G$100!  .45 Colt is G$250.

Cornucopia's for handguns are going to be common.  The G$300 cornucopia for .44-40 is going to be highly prized by the Winchester armed Sioux brave.

The Indians use magic and know how to do enchanting, why don't the Sioux make their own?

A quirk of the spell is it being based on the value of the item.  .44-40 ammo is TL5 stuff and it costs G$0.60 per round in a TL5+ setting.  Indians are TL0 stone age savages.  The price of a round of .44-40 is doubled for each TL difference.  32 times in this case or G$19.20 per round.  That's 960 energy for an Indian to make a .44-40 cornucopia.  The good news is it's just G$8.25 per energy for TL0 societies who start with G$250.  The bad news is that's still G$7,920 or 31.68 times the starting wealth.

They do make cornucopias, an arrow is $G2 and that's 100 energy, G$825.  Steep, but bearable for a wealthy warrior.

I just realized what a forensics nightmare such rounds would be.  No bullet or cartridge case remains.  Maybe not even powder residue.

For the full explanation as to why 60 energy or less is G$10 per point and 61 or more energy is G$330 each, refer to GURPS: Magic p. 21.