If we’re talking about some of the worst factory ammo available on the market, used in low-end rifles, flipping a coin is assuredly more valid at predicting relative performance than any theoretical comparison or generality. The Stag may well be a bit more expensive and may well be a bit “upgraded” compared to a conventional $450 mil-spec-ish bottom end carbine, but that may not actually mean it’s any more accurate, especially with cheap bulk ammo.
“Dollar for dollar,” a bolt gun should and typically does outshoot an AR, AND a bolt gun is typically more “shootable” than an AR, such most guys will always bet on the bolt gun. But if the particular AR happens to like this particular factory ammo and the 783 doesn’t, the expectation could easily be reversed.
But maybe more importantly, for me at least, even when used for the same purpose, an AR and a bolt gun FEEL very different in use. I used to ride super and hyper bikes, and in top end performance, there’s a considerable difference between a 1000cc and a 1400cc bike... one’s going to be undeniably quicker off of the line, but the other will handle considerably better. It makes comparing the two almost impossible for nondescript, general riding. Given a specific application, say drag racing instead of road coursing/GP, the gap between the two widens. But for messing around on 2 wheels on a Saturday evening, comparing the two isn’t sensible - they’re different, and that’s enough.
I wouldn’t personally buy a 783, certainly not for target shooting, and if my objective were to practice long range, or at least longer range shooting, I would put that money towards ammo, glass, trigger, or barrel for the AR you already have, in that order, instead of wasting money on the low end bolt gun.