.223 80 gn SMK/Varget COL

That is normal for the 223/5.56 to have a short chamber. Yours is actually way more than what I have to work with on my Min spec chambers. Yes the 77gr is what most use as a max weight in the 223. Those shooting in competition normally have their chamber cut to shoot what they want, so they can shoot the heavier ones.

I would check SEVERAL cases and bullets to get an average measurement. Mag length is your limit without some mods. Unless you're running 1:9 rifling or faster the heavies wont stabilise good. I usually always find a 68 or 69gn load that can stabilise in most .223s and deliver plenty of twack and shoot flat. If you really want to push an 80gn I'd look for a compressed load of a faster powder but your ceiling is still going to be 2600-2800fps.
What is "long range"? again I'll point to the 68-69s but depending just how long you think long is, velocity is king. Smaller pills going faster.