For these RnGs I'd say the major factors are weight (mags, gun, and ammo) and bullet drop. Both of those factors heavily favor the AR15 in 5.56 over an AK in 7.62x39. My man @bulletpaluza tackled the 600 yd shot with an irons only AK in 2018.
Speaking as someone who did the first RBGC RnG in 2018, I'd never shot over 100 yards prior to the competition. I was able to make the 600 yard hit with a 16" 5.56 AR and a Primary Arms 1-6x using the BDC reticle and a Grippod. It took a few rounds (I realize now that it was related to the...
I couldn't last year, it was pretty wet/muddy so no dust puffs. I'd say it's a maybe depending on conditions.
Edit:This was with a Vortex 1-6x Strike Eagle, better glass may make the difference.
Definite advantage to starting in the am, I wish they had a way to start folks sooner/get through faster (maybe 2 of each stage?). You should have told them that you'll be hopping on one leg the whole way and thus need to start ASAP in the am =D.