Sorry, bad news.
A gunshow is in town and I bought a paper pack of 5.45 out of a spam can. A quick check with my chinese calipers, the bullets measure .219-.220. Sample rounds of prvi match .223 I have on hand, measure .222-.223.
Hmm, change gears maybe? Maybe try a .22 Rimfire barrel as a donor? Pac-nor, mcgowen, and shilen list them @ .222.