Hi!
I had the same question when I sent my Merkel 302 16b to
JJ Perodeau Gunmaker
711 S 263rd West Ave.
Sand Springs, OK 74063.
It is a 2 barrel set (16b/9.3 X72R and a 16b/16b)
JJ recommended opening the slug bore of the 16b/9.3X72R so there would be no concern of damaging the barrel when I fired Brennek 2.5" 16 slugs.
He did excellent work and also made claw rings for the scope, sighted in the gun for deer hunting.
I have been very happy with his work and honesty
Jim