PRC IS LARGER DIAMETER. kinda end of story IMO. turn down the prc to fit a LRM chamber? i size down but have only expanded the case with fireforming and have never have never shrunk the base and rim.It looks like the 7prc case is bigger in diameter than the 7lrm so that wouldn't work. 7prc is .532 at the base and 7lrm is .528 and the end of the case 7prc .518 and 7lrm .511. Is your 7lrm brass hornady? If it is you will probably only get 2-3 loads before the primer pockets are loose. I could never get more than 3 loads on two different rifles with that brass.
How much would the chamber need to be set back to overcome the diameter issue?