How hard is it to replace missing barrel keels? Any substitute that might work?

I make them when needed. Not hard but it takes some time. In my view, anything short of steel soldered in with rosin flux will likely cause a bigger issue later on.

Do you mean the keel behind the fore end hanger, or the fillets that fill in the holes at the muzzle. Some people call those keels too.

Muzzle end

Years ago when I was stupider I shortened the barrel on a 16ga Stevens 5000 for a slug gun. When I was done there empty spots where keels should be. I rosin fluxed the voids best I could and then jammed in a filed piece and some plumbers solder and gently heated until it melted. I put enough in that it was proud of the muzzle and then just filed it down...

Thanks guys! First time stretching my wings, not great but it works for me! Still have some filing to do!

If you put a ball of degreased steel wool in the void it will keep so much solder from running down.

I would have a look at that left barrel whilst your working on that gun.

Yeah that’s on the list!

I always just filed a triangular piece from a round bar to a good fit . Leave it about 1/8" longer . Make sure the insides are clean , tin the piece ,flux the insides then solder in with a large iron , flat off then tin over the ends and polish off and any solder that may have got into the muzzle ends . May be advisable to wire the barrel ends together .