"Gun Disassembly 2. Volume 1" hits Top-7 in the Mac App Store.
Several user reviews:

Extremely Detailed
Not a game (though it's fun for gun-nerds, lol), but it's a amazing way to learn! Lots of detail, runs flawlessly, and has well noted instructions! I'm convinced you could become a gun expert just by playing these disassembly 'games' a lot! Wanna know how to disassemble that glock better? Get this app. You won't be disappointed.

Impressive
It's not a game, don't expect that. But for showing the inner workings of these three firearms it's nothing short of amazing. It's smooth, the app functions well and there is a great amount (almost too much) detail.

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