I’d thought before about using a Desert Eagle mag as an ammo carrier for a single-action revolver. But I’ve never actually tried it.
I’ve used some magazines before that are really difficult to unload by hand. Some also have some sharp corners and tend to tear up the thumb when pushing rounds out. As a revolver loading tool, some of these would be less than ideal.
But for discussion sake, I just now grabbed a Desert Eagle mag and a S&W 581 and tried it out. The DE mags actually unload fairly smoothly. They are marked as 9-round mags, but I’ve always been able to load 10, making for two sets of reloads for a 5-shot revolver. Overall, I’d rate it as not be a terrible way to carry revolver reloads.
Having said that, I carry a speed strip when I throw the J-frame in a pocket. They’re inexpensive, flat, and can load 2 chambers at a time.
If you still want to try it, factory DE mags are usually around $45 ea, or aftermarket mags can be found for $20 or so.