If you have a 1:9 bbl the conversion may be pretty accurate with it. Just like with with heavy bullets, some 1:9 bbls shoot .22 very well, others, not so much. Just have to try it and see.
If you'd like to shoot cheaper, but still get realistic AR triggertime (double taps, follow ups, etc), you could look into a 9mm.
Or, depending on how much money you have, get both. I have one of each.