The .221 Fireball is a short range varmint cartridge - Point Blank Range walk around hunting, or watching ground hog holes within 200 yards. It's a nice .22 caliber fit between a Hornet and a .222 or .223. One want's to shoot a bullet "just" big enough to kill the intended game. It doesn't shoot far enough for wind to be more than a minor "hold" issue at any practical wind velocities for hunting varmints. Flat base bullets work just fine.
If I was going to build a .221 Fireball specifically for point blank range (PBR) varmint hunting, I'd go with a 1:14 twist and plan to shoot 34 to 40g bullets in it. There are a bunch of good bullets (Berger, Hornady, Nosler, Sierra, and Midway) in this weight range that work great on ground hogs and smaller animals, maximize the PBR, and are relatively easy to load for.
After some playing with QL and QT, the 34g Midway DogTown would leave a 24" barrel at around 3,500 fps and give a +/- 1" PBR (what I use on chuck rifles) of 16 to 190 yards. My experience with the 34g DT bullet in my Hornet has been very good - it has excellent terminal performance (ground hogs are DRT, drop in their shadow) and shoots better than the 35g V-Max in the same rifle. I've not tried the DT in any other rifle (I have good hunting loads for all my .22 varmint guns and would rather be hunting than wearing out barrels doing load development), but if I was building a .221 Fireball it would be one of the first bullets I'd try for PBR varmint hunting. If I couldn't get the 34g DT to shoot, I'd probably try 40g bullets from Berger, Sierra, Nosler, and Hornady, in that order.
Edited to add: After some further analysis, I think my first choice would be one of the 40g bullets, like the 40g NBT in a .221 Fireball. If my analysis is right, it will add about 11 yards to the PBR taking it to about 201 yards.
Fitch