Kroil is good...
as a penetrant--it worms its way into microscopic spaces (like between 2 close-fit gun parts for example) so as to lube them when they move. Some benchrest shooters do clean with Kroil. However, their guns aren't "really in need of a severe bore cleaning."
You could try Kroil, in the "Can't hurt, might help" department. Having bought a can of it, you'll find it useful as an excellent light lubricant, if nothing else.
For really dirty bores my go-to cleaner is Blue Wonder gel. Put it in the bore, leave for the recommended time, run a brush into the bore several strokes, then patch out the resultant gunk. Usually works wonders.
Lately I've been having good luck with the foaming bore cleaners, also--spray it down the bore, wait the prescribed time, brush & patch it out.
Something else that sometimes works is the old-style military bore cleaner (not the kinder, gentler stuff they use now) but I can't suggest where to lay yr hands on a can of it.
Anyhow, good luck, Skidoo Man 603. A really old, fouled, pitted, dirty bore (I'm guessing this is an old milsurp Mauser you have) can be a real challenge to get bright.