what brands make m16's with metal receivers?

I want to get one but I don't one made out of plastic. The only model gun I ever saw up close was plastic with metal inserts.

what brands make m16's with metal receivers?

Check this excellent thread by MadMike, it lists all available models of M-series rifles:
https://mp40modelguns.forumotion.net/t1467p15-mgc-m4-photo-gallery

You can find more information about specific models browsing through our forum list of reviews:
https://mp40modelguns.forumotion.net/t2134-list-of-reviews

is it possible to get a marushin with an aluminum receiver any more? I'm pretty sure mine is zinc.

You are right, Marushin uses zinc alloy, not aluminum. I was wrong then.
I heard that HobbyFix used aluminum for their rare and expensive dummy models.

I don't think you will notice a big differenece between the two alloys with a modelgun. Zinc is a little lighter, but overall Marushin`s M16 feels okay.

the pot metal barrle and bolt carrier screw up the weight of the marushin. I'm trying to make a steel barrel with a real front sight, hand guard cap and flash suppressor, but i can find a 14x1 metric die to thread the end that screws into the receiver.