OK, so here's some very rough guidelines with pics. If you're going to do this, you'll need to be able to make slight adjustments as you go, so it's not gospel. And do so at your own risk!
First disassemble your DBM/CMMG magazine
Split the halves and remove excess plastic around the magazine `channel` Leave a few screws intact. I later trimmed even more of the remaining triangle away to ease fitment. The method I used to remove the plastic was a bench grinder (taken slowing to avoid melting) If you have a detail saw of some sort, then even better.
Reassemble with remaining screws and bolts.
Sand down the outside of the magazine so it's a uniform thickness (an orbital sander makes light work of this). There's no need to go further down than the depth of the magazine at it's thinnest point. Leave it slightly thicker at the top until you can test it in the PMAG. I have used electrical tape to keep the 2 halves together while outside of the PMAG and trimmed approx 1" from the bottom after a test fit in the shell. (You will lose some round capacity)
Take your standard PMAG, remover the spring and follower. Then file down the top to remove the lip, I decided to go just above the first rib and over the bullet. The first rib represents to me where the top of the BDM needs to sits for correct alignment with the magazine catch. Yours MAY be different.
Use a round and flat file to remove the ribs inside the PMAG at the upper most top and bottom of the magazine where the BDM will come into contact with it.
Now it's just a case of test fitments and trimming sections from the top of the magazine to make it fit snugly inside your filed out shell. Once you have it fitted tightly inside, you refit the BDM internals (minus the red bottom tab) and slide the PMAG floor plate over the magazine. Once I have tested them and found them to be working. I will tidy it up by sanding any rough edges and possibly using an epoxy resin to hold it permanently in place.
In all, it took me about 1.5 hours at the bench to do one. It would be if I were to do more. Hopefully some of that makes sense, I'll do my best to answer any questions
