It is just a bit bigger -- you can look the two up on the internet to get "sorta SAAMI" sizes -- but the .221 Fireball offers just about the same thing, except the Spitfire is a semi-exotic wildcat. If it's the latter you want, have at it. Felt that way myself from time to time. But if you want the ease of getting it going, the Fireball is a factory round.
From Ken Howell's book, RCBS makes form dies for the Spitfire. I would imagine they also make the other needed dies. You can look up cost, but when I made a .30 Herrett, I was stunned about how much RCBS wanted for a two-die set for an oddball cartridge. For forming dies, the set Butch Lambert makes (if he still does) will make just about anything. These are essentially the design used by Skip Otto earlier, & I imagine others was well. Worth having if you do any wildcatting.
Back to Ken Howells' book: He gives water
displacement (not capacity) as 28 grains water for the Spitfire. For comparison, the standard Hornet displaces 21 grains water, and the .221 Fireball 33 grains water.
Going on -- the K-hornet (RCBS version) is also 21 grains, but the Hornet improved (Niedner, 25-degree shoulder) is listed at 22 grains. There is probably an Ackley version, a bit bigger still.
The .22 Remington Jet (Factory) displaces 28 grains, but the rim is .440. The Standard 218 Bee is given at 27 grains water. 218 Bee brass is going to be better than the Hornet; I use to shoot a .32-20 in Hunter Silhouette, and it did quite well. The Bee, of course, is just a .32-20 necked down by the factory. The Bee's rim s a little bigger though, .400 versus .350 for the Hornet, and .36 for the Spitfire; .378 for the Fireball. The rims of rimless cartridges can be turned smaller of course, as the guys who shoot a 6BR (or wildcat based on it) with a PPC boltface have to do. May be other issues with any of the the slightly larger cases, but it'd be the same for a Spitfire, I'd think. Ruger owners would know about your particular rifle.
Then there is the .22 Flea at 13 grains water (made on a .32 ACP case). At .358. the rim fit is good.Smallest case I heard of is the .22 Epperson Cricket, based on the .25 Auto case. Rim is just .302.
A good source for wildcat info would be Cheechacko, who posts here from time to time. Also Todd Kindler, the VarTarg guy, might also be a resource, at
http://www.woodchuckden.com/
Hope this is at least interesting, if not useful.
Edit:
There is also the .22 CCM (Cooper Centerfire Magnum), but the brass is proprietary & might be a bit risky if they quit making it. Displacement is 15 grains of water -- bigger than the Flea. I have no idea how it fared with shooters. Much smaller rim.