Ok then. So, that a decent basis to start from.
Before I got into my opinion, I'll give you my background. Grew up hunting since I was 11. My dad only knew old school and DIY hunting from PA, so that's what I learned. Y board Tree stands from ply wood, we'd sight our rifles in every season using a black and decker vise with a blanket in it so as not to scuff the rifles. I remember when my dad bought a lead sled for an upgrade. The last 7 years or so, I've gotten into long range shooting (out to 1200 yards), and PRS competitions. My skills grew as I pushed myself, and I've learned alot about gear, about myself as a shooter, and this has changed how I think regarding shooting, and how I approach hunting.
I say all that because I'm just some guy on the internet and I have an opinion.
That little .308 I posted I haven't sighted-in in 5 years. I throw it in my truck, have dropped it from trees, and have had it knocked around by airline workers really good (so much so the level I had mounted on the scope tube was shifted considerably) and it's never lost zero. Only time it gets cleaned is if it rains really bad when I've hunted with it. I have 100% confidence that if I took it out right now, I could hit 1/2in (1/2 MOA) circle at 100 yards first shot. At 500 yards, I'd be confident out to a 2 MOA target in most conditions. The reason for the MOA difference at 500 vs. 100 is environmental and wind calls. I still feel I suck at calling wind, but I definitely suck less than I did a few years ago. I've seen wind humble some great shooters (Guys who shoot 1000 rounds a month and shoot sexy sleek 6mm hot rods). And, 500 yards is definitely a long of distance for wind to affect where your bullet impacts.
I built that .308 with quality parts. Not the most expensive, but quality. I've been at matches and had numerous gear failures, and thus I prefer buying quality gear that Likely won't fail.
For your situation, most rifles these days shoot 1 MOA, even the cheap ones, so I'll set the rifle aside for now and assume it is 1 MOA capable.
The first thing I'd check/change are the rings. The least expensive rings I'll use are Seekins/Vortex PMR rings. I have punished two scopes mounted in those rings, and they've never failed me. Hawkins, MPA, Sphur, etc are other solid ones but are more money. The link between your scope and rifle must be rock solid.
Next is the scope. Nikon scopes did not have a great reputation for mechanical reliability. There's a reason Nikon didn't sell many and stopped making scopes. Look up a tall target test, buy the applicable target test it (Mil or MOA) for your scope. Assuming the scope fails, buy a quality scope you can dial out to the range you want to shoot.
For a cross over scope that can be used for targets and hunting, I highly recommend the Bushnell LRHS 2 sold by GAP. For the money ($1000), it's one of the best buys in that range, IMO.
For practicing and sighting in, sight in as you naturally shoot. Lead sleds and other contraptions sound nice in theory, but by definition they make you interact with the rifle unnaturally vs. any other time you will shoot it. You need to sight in the rifle and make sure you understand your natural point of aim. Your POI could be different simply because your eye, cheek, and fingers are all interacting differently with the rifle while you are sighting in vs. when you are out shooting. All my rifles are sighted in with a bipod, and rear bag only. Either full prone, or modified prone on a bench. When shooting from field positions, I try very hard to make sure the rifle feels exactly the same every time. Cheek is smae position, trigger hand/finger are same position, etc.... Dry practice fundamentals in your garage or bedroom too.
If you do those things, and you're still having issues, there's tons of other rabbit trails and holes you can sink money into.
The other option is, your setup could very well have a 400 yard limit, and that's ok. Every rifle can't do everything. If you want to get into longer range shooting, might be better to just start from scratch with a new rifle, scope, everything and practice, ALOT.
Edit to add, as an example of why you don't need a lead sled to shoot a good group. This group is 5 shots at 100 yards with nothing but a bipod and rear bag. This was the one and only time I sighted in my PRS rifle.