I've only been messing with AR's for five years.
Didn't know about the Mk12 when I put an 18" together.
If I seen a pic of one I probably assumed it was just an M16.

I had a 14.7" and wanted one with a longer barrel.
16" was on the shorter side and 20" was longer than I wanted so an 18" Wilson Combat .223 Wylde was what I picked.
Tried a couple different scopes, a 1-8 and 4-16.
Didn't like those and in the end wound up with a NF 2.5-10x42
It was more than I planned to spend but it checked all the boxes that I was looking for.

Later on I read up on the Mk12 and seen one of the scopes was a NF 2.5-10x24
I picked the NF due to the 42mm objective, among other things.
It's overcast a lot here and I find a bigger objective to be useful enough for a few more ounces.
Good scope and seems to fit this rifle well.