Off hand, and in Ohio to boot, check with Steve and the fine folks at ADCO up in Sylvania (outlying area of Toledo).
They generally don't do much in the way of barrel chambering aside from opening .223 Remington and .223 Wylde chambers up to 5.56 NATO spec, but that doesn't mean they don't have the tools and can't help you out. Its worth asking.
Also in OH, and certainly one of the best, is Mark Gordon at SAC in Wellington, OH (NW of Akron/Medina IIRC).
I'd check those sources first and if you don't have any luck there...then you can concentrate on any number of highly competent smiths here on the Hide that can do that work for you.
Out of morbid curiosity, what kind of barrel and chamber are you dealing with now? Generally, unless it is some sort of hybrid/match Grendel chambering I've not heard of, or it was just cut wrong from the get-go, I haven't seen one that was too tight yet or otherwise wouldn't handle the longer, heavier pills suitable for use within the confines of your average AR mag.
