Other than to use as a "Spare Tank", I can't get into them, so I don't have any, similar to those 30 round fun sticks for Glocks and so-forth (I have one, which I use to keep spare rounds in). If I were to use it in the prone, I think I'd need a 5" neck extension, and just getting back 1/2" from fixing crushed discs cost damn near 30k so I won't be exploring that option! I do like the D60 drum mag, but haven't felt compelled to buy one just to have. 60 rounds would get boring quick, although it would work for a predator/varmint hunting all day hike, so there's that to try. When I started, I thought I needed a combat load for every rifle, so I have pmags coming out my ears. For an all day session, I'll toss a dozen or so into a satchel to bring along with my long guns which means there's mags I've never touched since taking them out of the packaging. That covers the 30 rounders, then I got into a 20 round buying spree when I discovered they didn't interfere at all with prone shooting of a bipod or a bag. All told, I don't think I have to buy any more mags other than that drum someday maybe.

The 40 rounders might be able to adjust the balance of certain guns... Say you have a SBR with a UBR on it and want to make it less butt heavy, it might help shift some of the weight towards the center of the rifle just by moving weight toward the muzzle...