Magnesium AR lower

[polymorpheous]

I'm assuming you meant magnesium as manganese is an entirely different element. Autocorrect?

Sorry to nitpick, but might as well be accurate. A manganese lower wouldn't be very lightweight, it's more than double the density of aluminum, let alone magnesium.
Magnesium-vs-Manganese

Hell, even if you made everything out of magnesium that is normally aluminum your weight savings wouldn't be that substantial if most of the weight is coming from the steel barrel anyway.

Autocorrect in my brain.
I'm a welder.
My brain said manganese though I meant magnesium.
Thanks for the kick in the butt.