You're confusing the inlet and the design a little, in hopes to help. BDL and M5 are bottom metal inlets. While yes most BDL are internal boxes and most M5 are mag fed…you can find the opposite in each. Examples, HS Precision makes a very good (but expensive) BDL inlet mag system and Hawkins makes a very nice BDL bottom metal Obendorf BDL system. I'll probably get some disagreement but at this point, all stocks should be inletted for M5.
The action being cut for internal mag versus external is the key here. An external mag will be better set up for a switch barrel when you change case head sizes. Reason being, you buy a 308 cut action and the 22 Creed would work perfectly but when you switched to the 7 saum you'd lose the ability to load from the top. You'd always have to load the bdl from the bottom. Can it work, yeah but it's not ideal and likely require some tuning. You would also have to change box and follower (as mentioned above) with each barrel swap.
The Hawkins Hunter setup is what I've built my switch off of but I find I don't switch often. Good luck!