British mags are steel, and so are the new HK ones... what was the argument, BTW?
The point about steel casings tearing up your brass deflector doesn't relate to hardness (scratch resistance); it's a point of impact resistance, and brass vs. steel doesn't really matter, unless the elasticities of brass vs. steel vary significantly. I should also note that it's Mo[b]h[/b]'s hardness scale.
Also, its really just the finish that gets eaten; brass or otherwise. The deflector itself is rather large,a nd will not really degrade in time. The fix in any case is a piece of electrical tape.
As for myself, I've used several thousand rounds of Wolf and a little more elbowgrease. In the process, I've saved a couple hundred dollars, had ZERO ammo related problems, and I shoot without guilt.
As to the original point, I believe some countries in WWII were using uncoated steel cases rather extensively, where it was found that steel casings had little effect on the guns, but severely degraded the machinery.