Not sure there is a down side to barrel mounting, other then maybe not being able to use the iron sight. To me it seems the only logical thing to do if you are not good with iron sights. I am kind of surprised these rifles don't come as an option for barrel mound optics from the factory. The only way I would want one is with the barrel mounted scope, and is why I haven't ordered one as yet.
Or better yet as a complete kit option like they do with some of the other rifles, complete with scope, sling and case. Local shop has one of those special equipped lever 30-30's like that and it is cheaper then the same gun with no sling or scope by about $10 and we can not figure out why.