The .22-204 vs. 223AI.......I've been shooting the .223AI for about 15yrs or so and like it a lot. Very good accuracy and surprisingly more velocity than the std. .223, more than you'd expect. And 40gr bullets will really put a smile......about 4190 with VV powder. This is accuracy/working velocity, not see-how-fast-it'll-go velocity. Only fly on this cartridge was making it feed smoothly in my rifles. So they use a loading block and work just fine. But I wanted something that would feed with no hiccups. The .204 case is a beautiful thing and I've a couple 22-204's. One is used with 40gr BT's and the load it likes is 2015 powder for 4170, essentially the same as the 223AI. That's cooking right along with a 40. I'm not concerned about whether or not it works in my AR. The case is pretty maxed out and perfect for wildcatting. Years ago I had a 222mag and it was nicely accurate, but working/accuracy velocity wasn't in the same league as either of these two other cartridges. But the case doesn't hold as much powder either.....in a case that small it doesn't take a lot of extra powder to make a noticeable difference, that and a better shoulder angle. So to me the 22-204 is a really good one and definitely worthwhile. The 6-204 is said to be a terrific cartridge and I look forward to having one of those also.
