I'm sure that's true, that Ruger would have to order muzzle caps with a slightly larger opening to make .358 or .35 Whelen RSI's. I'm guessing that whoever Ruger sources them from would be happy to supply a couple thousand or so to spec, so I'm not sure that's a big obstacle on a thousand-dollar rifle.
On the other hand, if
you want a .35 Whelen or .358 win RSI, it's only a new barrel away. A sanding drum on a Dremel tool will create all the clearance you need
.
Dennis
