No break in, just shoot and clean. No problem as a deer round if you use bullets designed for deer hunting. Most who advise against it have only seen the results from varmit bullets which are not adequate. The 40 gr bullets are not deer hunting bullets and are probably too light to shoot well. But I'd try them, just not on deer. Sometimes stuff works even though it is not supposed to.
As to accuracy, I'd be surprised if it needs anything other than playing around with some different bullets to see what it likes best. My 1:7 guns shoot best with 60 gr and heavier bullets. I've used bullets as light as 50 gr with acceptable results, but nothing lighter.
There are only 6 states where rifles are legal and 223 is not at the last count I had. Several others have recently changed their laws or are considring such. There are a few other states that are shotgun only so all rifles are illegal, including 223. The laws prohibiting 223 were written years before it existed specifically to outlaw the 22 Hornet. With good bullets the 223 is a very effective deer round.