On my Century C93 the rails are welded shut, just as they should be. It’s built on an SW receiver (they used several types) This along with taking my feeler gauges with me when I bought it is something I looked for. I looked at several till I found the one I wanted. This is why I wouldn’t buy one sight unseen.
As far as bolt gap, HK specs is .004-.020. Once your BG settles (they all settle some when new) if +4 rollers put you anywhere the middle of HK specs you should be good for many many thousands of rounds. I know a guy over on HK pro who bought a C93 just to put his NFA registered auto pack in and after the initial settling using +4's he is at .009 BG and has since run thousands of rounds in full auto and it has held there steady so far. In a semi auto my .011 with +4's will stay in HK specs longer than I could ever spend on ammo.
As far as bolt gap, HK specs is .004-.020. Once your BG settles (they all settle some when new) if +4 rollers put you anywhere the middle of HK specs you should be good for many many thousands of rounds. I know a guy over on HK pro who bought a C93 just to put his NFA registered auto pack in and after the initial settling using +4's he is at .009 BG and has since run thousands of rounds in full auto and it has held there steady so far. In a semi auto my .011 with +4's will stay in HK specs longer than I could ever spend on ammo.

