Are you only assembeling the lower or are you going to build your upper as well? I would go 5.56/ .223..... more options with a .22 conversion bolt..... cheaper to trane with.. The fundamentals of marksmanship are the same.....Cant to that with another caliber..
5.45 is cheaper than 5.56. Yet more expensive than .22LR. For $140 you can get 1080 rounds of 5.45. A little cheaper if you buy it by the crate like I did.