looking for possibilities

yes, I loaned my friend a burris 4x short mag, used weaver low bases and a weaver mount, the optics were fine, but the rifle still won't feed after 2 trips to remington.

took gun to a local gun shop and could not get it to bore site with many base and ring combos
tried to shim but can't get close to being able to adjust down more than one click.
Now what should i try?

The "possibility" is that your rifle is incorrectly drilled an tapped.
Adjusting "more than one click" is hardly an issue with today's scopes.
If you center up the scope's internal adjustments, slap the CORRECT weaver base and rings on it, where does it shoot (not boresight ... shoot) at 25 yards ?
Is the scope quality or some Chinese junk ?
Never owned a 750 but various 4s, 740s, 760s and 780s (deer slugs) have passed through my hands and been set up with Weaver bases and rings carrying Leupold 1-4s or 2-7s and there was never a problem with getting them zeroed.
Never had a gunshop mount a scope yet. Buy the correct base and rings direct from Brownell's, it's not rocket science.

i may try mounting it my self. scope is a Leupold vx3 3.5x10x50. I myself have had 2 other 7400's and there was not an issue so we'll see what come's of it on my own.