I've watched several vids on YT about modding mags for 350 Legend, but one thing that never seems to be addressed is the follower.  As my parts for my build are trickling in, I need to get some mags and I would like more capacity than the 350L specific metal 5 and 10 rounders (although I am getting a couple of those just to have feed consistency in case modding p-mags turns out less than great).

P-mags are cheap enough that I don't mind experimenting with them, but I was wondering perhaps if the mags for 300BO might be a better choice for modding?  Is the shape of the follower for 300 different than for 5.56?  I can't seem to find a good picture to see if there is much if any difference.  I see Magpul also has a pack of "enhanced self leveling" followers, but they're for 5.56, and apparently only for replacing the aluminum follower in the UGSI 30-round mags.  I'm guessing they won't work in the newer Gen 3 p-mags?

Dura Mag(C-Products) makes 20rd mags.

Skip making your own mags. They don't work

You can Mod P-mags. same follower. You need to remove the two opposing ribs at the front of the mag.  Wood chisel or aluminum burr used for cylinder head port work.  Ive modded many before they came out with mags.

Yeah, I'm aware what needs to be done to them.  All my pieces parts are here, except the lower build kit, but PSA fINALLY shipped it and it is due to be delivered today.  Anyway, I have a 30 round gen3 P-mag sitting right here.  I also have two CMMG 20-round metal mags.  I initially ordered Duramags from Midwest Gun Werks, but they cancelled those when they shipped the p-mag and the Fab Gradus pistol grip because they didn';t actually have them in stock (even thought their website said they did), so I had to re-order and get the CMMG mags.  

I have the tools on hand to make the modifications to the p-mag, and I will also take it a step further.  I've been a wood worker for over 30 years, so I have a lot of different kinds of sandpaper and abrasive cloth.  One in particular I have is called "Micro Mesh" (but it's not mesh at all), cloth backed, with an extremely fine cushion layer with the abrasive on top of that.  It goes from about 1500 grit up to 12,000 grit.  It is superb for smoothing and polishing plastics.   So, after I grind away the two ribs, I'll use my micro mesh to polish the inside so that the follower will move smoothly inside the mag.

Anyway, p-mags are cheap enough I don't mind experimenting with them a bit.

I tried to do an old 20 round Pmag before the metal ones were available. Didn’t work out too well. I’ve done and use several 10 RD Pmags and they are all fine.
The problem with the 20 was I didn’t have anything long enough for my dremmel to reach far enough into the body to get all of the bumps out. Using a file got close but not perfect. Also the front ribs help keep the follower lined up. I kept getting tilts and twerks when loading past 10 and was a no go.
If I need more I’ll tape 2 10s together old school style. That would make a statement to the deer.