i agree with john doe. hand tight is plenty good enough. remember this weapon was designed, more or less, to be field stripped by marines in the field.
I just crank mine down as tight as I can by hand. I think hand tight is good enough personally, I have not had issues. Maybe if you tighten it with a pair of channel locks might cause some damage, but I would not suggest you use any tools to tighten the cap. Hope that kinda answers your question. Do you have any damage to your cap?