The ruger predator action/barrel model for Grendel has a bottom opening basically set up for an AR mag. I don't have that ruger with me any more but am going off of memory.
When I had one and was calling around, seems as though the mag would need to fit or narrow down to that AR-mag-style of offering up the bullet.
So for a magpul (which I'm assuming has a 308-style AI bottom magwell opening right?)... I believe AI or some AI-style mfr has a 223 version of that Pmag, that would fit into the standard AI-style magwell, so a 223 shooter could use the AI mags. Would that be what you're looking for? If so then prob need to tweak the feed lips some.
I don't know what the 6br conversion kit is, so if this is all on the wrong track for you my apologies.
AI Pmags got split into "2" styles a couple years back, one (original green follower) for the 308-based cartridges, and one (brown/tan follower) for the 6.5 creed type cartridges. Wasn't supposed to be a lot of difference, the tan ones work for me in my 6.5's a bit better than green but they all seem to work, least that's what I recall. I have a 308 and 243 using the greenies. My creeds use the tans, sometimes a greenie in the mix.
edit to add: you're going thru some of the pain I figured I was in for, so I sold the ruger and never got the magpul hunter for it. Now I did get somewhat of success with an mdt chassis, using an AR/Grendel mag but it was still finicky. Just sayin'.
When I had one and was calling around, seems as though the mag would need to fit or narrow down to that AR-mag-style of offering up the bullet.
So for a magpul (which I'm assuming has a 308-style AI bottom magwell opening right?)... I believe AI or some AI-style mfr has a 223 version of that Pmag, that would fit into the standard AI-style magwell, so a 223 shooter could use the AI mags. Would that be what you're looking for? If so then prob need to tweak the feed lips some.
I don't know what the 6br conversion kit is, so if this is all on the wrong track for you my apologies.
AI Pmags got split into "2" styles a couple years back, one (original green follower) for the 308-based cartridges, and one (brown/tan follower) for the 6.5 creed type cartridges. Wasn't supposed to be a lot of difference, the tan ones work for me in my 6.5's a bit better than green but they all seem to work, least that's what I recall. I have a 308 and 243 using the greenies. My creeds use the tans, sometimes a greenie in the mix.
edit to add: you're going thru some of the pain I figured I was in for, so I sold the ruger and never got the magpul hunter for it. Now I did get somewhat of success with an mdt chassis, using an AR/Grendel mag but it was still finicky. Just sayin'.
"Down the floor, out the door, Go Brandon Go!!!!!"