Re: 220 Swift Improved?????
There are a variety of improved swifts, and the 220 Weatherby Rocket that GoodGrouper mentions is one that has enjoyed some popularity.
I owned a 220 Swift Ackley improved with 40 degree shoulder in the mid 1980's, and the Winchester .220 Swift cases which I used had a capacity of 50.4 grains, and a capacity of 55.5 grains after fireforming.
Although I have only tried one "improved swift" , I believe that ballistically there is probably not a lot of difference between the various versions. Consequently, I would not enter a debate into which one is the best.
I agree with GoodGrouper suggestion, and also recommend you use it for bullets of 80 grains (or more).
Now that Sierra have introduced a 90 grain MatchKing, and with Richard Graves introducing an excellent variety of Wildcat bullets bullets in 80 - 100 grain weight, I believe that an "improved Swift" with a 1 in 7 twist barrel, would be a good platform for those bullets.
However with cases of this capacity (or larger), if you fire 3 shots in about 3 minutes the barrel is hot, and you should then wait for it to cool before firing again. So if the rate of fire is likely to exceed that, then I would choose a smaller case. Regards, Brian.