Sorry, all federal "gun charges" are felonies. Ask Randy Weaver, or anybody else who has run afoul of federal gun laws, and survived.
There are certainly bigger charges than that to fry these two with, but as far as I have heard, the feds can only trace the rifle from the factory to the West Coast wholesaler. I'm beginning to suspect that the feds feel that there are questions that should not be asked, and are afraid of something getting out in trials in the states on murder.
You bet they are. I think it's a darn good bet that one of the alphabets was running him as an agent/informant and he outmanuevered them. There are just too many oddities in his known past to be explained away by any other explanation in my book.