Hitting a Cantalope at, 500 yds is probably very doable with, the 7mm mag using, the "right" bullet and load "work up".
Do you have, a Concentricity fixture / dial indicator, "set up" ?
Personally, I'd try some, Berger 168's and IMR 7828 SSC and get, the concentricity / TIR of the Bullet down to .004 / .005 Max. TIR.
Don't go crazy buying, needless "Chit" but, I value setting my Bullets, to a LOW TIR reading, very high, on my list.
And, I LIKE, Forster Dies, to get "this", Job, done !
My 2 slightly Modified, Factory "Hunting", Tikka's, both shoot, SUB, 3/4 MOA, reliably and I'm getting 6" dia groups at 700 Yards
( mild winds of, course ! ) with my .270 wsm, so IT,.. can be, done with reasonable and careful reloading.
Does your Remington have, a "good" 1.5 to 2 pound, crisp, Trigger pull ?