You didn't mention which grade Nikon scope in your post. If it's the Buckmaster 1 or 2 or Pro staff model that is most likely your problem. They are known to lose their zero big time, if it's a Monarch you should be good they are great scopes. The first two I mentioned I wouldn't use as an anchor they are way too unreliable.
Did you clean your barrel after you sighted your rifle in? If so that could be your problem.
Most of us have been taught the importance of keeping our guns clean. After every outing, we are supposed to make sure there's not a spec of dirt, powder, or copper anywhere, inside or out. This will ensure both the accuracy and longevity of our beloved hunting rifles. I don’t want to stop that...
www.outdoorlife.com
.
If you didn't follow these steps.
Clean your barrel with a good Lead and copper remover like Tetra. Use a (bronze) bore brush not a copper brush. Clean it toughly until your patches come out without zero green on them, then add a few drops of oil to a patch and pass it through you barrel a few times.
After it's cleaned shoot 5 to 10 bullets through it before you start sighting it in.
It could be a combo of a fouled barrel along with a rushed or jerky trigger pull.
Get a clear sight picture and slowly squeeze, squeeze until you shot surprises you that way you're not flinching before the shot and jerking the gun off target.
My guess is you have developed a flinch which is messing you up, it happens to the best of us.
Do the squeeze, squeeze deal until the shot surprises you and see what that does to your consistency.
I bet it has nothing to do with the weapon unless it's a pro staff or buckmaster scope or you dropped or banged it really hard. If it has the pro staff or buckmaster scope I'd send it back to Nikon and have it tested. If it comes back a bum scope you really need to upgrade to the monarch they are great scopes.
Either that or buy a
Vortex Viper HS even their cheapest scopes are great scopes.
And dump the 50 MM bell that right there could be your problem because you have to raise your head up off the comb because of the tall ring mounts. That is a huge no, no. Dump the 50mm bell. the biggest I would go on a hunting rifle is 44mm so you can use lower rings so you can maintain a solid wood to wood/ cheek to wood mount with a clear sight picture.
Good luck.
Dan