http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=46506 - minimum bore diameter: .236", though the manual says 8mm, or .315"
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=46703 - minimum diameter is 7.2mm, or roughly .28".
Or, for $30, contact Wally Siebert in Issaqah, WA, and order a "mini-borescope". It's a 5/8" diameter,best guesstimate) brass tube with a single lens in it. Take the bolt out of the rifle, and run the tube in, lens first.
I've only used the Seibert unit - it's good for finding stuff that is easy to spot, such as gross pitting, and uneven rifling wear.
Anotehr option is to go to e-bay and type "Borescope" into the search tool. If you thought a Gradient Lens,makers of the Hawkeye) product was expensive, you ain't seen nothing yet!