Was shooting an old 303 SMLE today using cast bullets. Barrel is a bit rough and I usually give it a scrub with JB to get all the lead out.
Ran out of JB but had some Brasso, nothing to lose in this barrel so I gave it a try.
Worked very well, at least as good as JB and perhaps better, all the copper streaks are gone, with much less work than JB, probably because of the ammonia content of Brasso.
Did a google search and didn't find much, just a few positive comments when used in similar situations of old barrels.
One bad comment in this forum but really referred to using Brasso on the brass and bullet.

Anyone have any definitive answers whether Brasso is safe in good barrels or is it too abrasive.

Don't suppose as you say it mattered in the old smellie, but maybe I would jib at anything else of any worth, as you may have noticed Brasso will dry out if given a few minutes, It then becomes really hard to shift without some other lifegiving liquids/esthers being put back with it, It might be better knowing what the grit size is first before condemning it out of hand? Steve.

Didn't let it dry. Used it just like JB, on a patch on a PH type jag, short stroked the barrel and then wiped it all out with Eds Red. Cleanest that old barrel has ever been in my hands.

I bought some of the cheap wal-mart brand stuff in the blue & white bottle...because they were out of hoppes,any way that stuff had a lot of ammonia in it and i didnt know it,I cleand all my guns and put them away,checked in a couple weeks and doggone they were all full of what looked like rust! I bought fell out,I grabbed the rod and oil can and started running patches down them,cleaned them best i could,shot each one a dozen times,then cleaned with hoppes and oiled them up good,they clean up like a brand new dime/no harm/& have never shot as well,what ever happend helped them and i was proud my barels were not damaged,i think it brought all the crude to the surface & turned it brown?anyways thats some strong stuff,try it,just dont wait a long time like i did before re- oiling your bores after use...That stuff flat out works& is cheap..Hope this helps