I know from being a machinist, aluminum is a very soft materials. However with proper metallurgy, one might be able to get an aluminum pin to work. My concern would be failure due to bending and crushing of the point. I know T-6061 is pretty durable aluminum and used in many hard metal applications. I know that there is harder, but not sure what it would be.
One would befit from Titanium though. I think a Ti bolt handle assembly, firing pin and barrel lug, trigger assemblies, coupled with a carbon fiber wrapped barrel and stock, you would have about a 5# rifle with scope... maybe, but that thing would need a massive muzzle break. That would be freaking awesome. If they can use it in a pistol frame, I don't know why it couldn't be used in rifle applications. That would be about a $10,000.00 rifle though. You start getting into ceramics and Beryllium, you start dropping weight. You could make a 3# rifle ($10,000 to $15,000 rifle).
Ain't science grand?