If the face of the breech-plug is polished and case-hardened, then re-polished bright, it'll reflect light very well.
The case-hardening stops the face getting scratched up with a cleaning worm or whatever.
I have a 'jaeger' I did this way, and you can see the bore very well in daylight with no bother at all, even specks on the breech-plug face if any are present.
Seems (in England at any rate,) breechplugs were hardened, and polished, so it's a historically 'correct' thing to do...
All best,
Richard.