I have been pondering what I could/should store in the buffer tube area since getting my 15-22 on the 15th. From this thread I believe I will end up putting some cleaning components in, such as a small bottle of Ed's red, a small tube of grease along with some patches maybe even a bore brush and jag with multiple short sections of cleaning rod. Don't have to worry about losing the plug because I replaced the buttstock with a Magpul MOE SL
My Magpul MOE+ grip will hold a sight adjustment tool, any allen wrenches specific to the gun/scope and an extra battery for the scope/light. Most everything will be in small zip lock bags as to not rattle; works in my other rifle along with a firing pin, retaining pins and bolt rings.