Badger is my go-to. Just got a couple recon unimounts and a set of SASR rings for a .50 So they make the rings in both steel and alloy, 4-4, 4-6 and 6-6. You may have to search or call, for some reason not all their items will come up when you look under one section or another --you have to play around with it. A call will fix it quick though. They told me when I called that the alloy caps are absolutely fine for use on the steel rings, even the SN'd rings. They said military does it all the time and I actually have one of their alloy Marine rail scope caps here about to be sent back (not needed) which came in the military box from Brownells.
I THINK the unimounts are all alloy. I can't see how you'd need a 4-6 steel unimount anyway and it would be oh-so-heavy. Their 4-6 SASR rings are awesome for the .50BMG though, better IMO than their 6-6 .50 rings and they'll work with clip ons. They're steel, but they'll also accept any of the alloy accessory caps too, some of which already have NSN's but don't show it on their site, like this rail mount.