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I have also tried the Surefire 60rd mags, 2 of them, and they are not reliable in my 3 AR's or the SCAR.
Wish they worked.
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Do you still have the Surefire 60rd mags? If so I'd give them a try in the AUG just for the sake of thoroughness before buying a $100 drum.
If they are still unreliable, I'd contact SF, I think they may have a Gen 2 mag? They'd likely return or replace them if they failed in 3 AR's, an AUG and a SCAR.
That being said, the real question is, what do you plan to use the drum/NATO stock for? More for "serious defensive use," or more for fun / nice to have / theoretical breakdown of society leading to a state of Hobbesian Anarchy?
For the latter, I totally get the appeal- I recently snagged a 24" barrel to build a HBAR LSW for plinking / forming a theoretical SHTF strongpoint at the gates of my gated community. I'll likely end up getting the NATO stock and 90rd snail drum just to complete this fantasy build. Masturbatory consumerism at its finest
But for actual "serious use," it's really hard to beat the AUG 42rd mags, or Magpul 40rd mags for the NATO stock. The 42rd mags still offer 40% more ammo then a 30rd, fits easily into any AK magazine pouch, and are clear so you can keep track of rounds.
The drum does offer 18 more rounds, but the penalty is that it weighs nearly 2x as much. The D60 is 20oz Unloaded, 44.4oz loaded; a 42rd Loaded AUG mag weighs 23oz, and a loaded Magpul 40 weighs 22.6oz. And the mag is solid black, so you can't keep track of how many rounds have been fired. For the weight, at least in an AR or SCAR, I'd rather have 2x40rd mags clamped together over the D60.
If you're going to go with the bulk and weight of a drum, I think it should hold 75-100rds.
TL;DR: Try to get the Surefire 60rds to work, and if they don't, stick with either 40 round mags or the 90rd drum over the D60.