When I checked when this was first posted, all I could find is full-length rifle boxes from Brownells that are 51.5" long. Looks like the OP has a solution, maybe John Boyd from Quality Arms who posts here might have a line on them. Most AR's aren't much longer than a broken down SXS or OU, run 32" with a collapsed stock, so one would think there would be an abundance of options out there.