For another anecdotal reference point...
I built a 6.5PRC a couple years ago using a Bighorn SR3 short action.
The stock I bought (used) had an M5 cut, so I bought both a flush bottom metal and an M5 BDL from Hawkins (along with the Wyatts 6.5PRC kit).
The Terminus magazine that I used in the flush fit bottom metal worked the first time and every time.
The BDL kit needed some massaging of the follower spring to get the back of the cartridge up where the bolt would grab it. When I first installed it the first round would always pick up, but then the base of subsequent rounds were not popping up high enough. I thought I had it fixed, but it cost me a follow up shot. So you really need to check it and do some testing with live ammo incase the shells are moving in the magazine.
Good Luck,
Ross
