This information was extracted from RimfireCentral:
MR definitely means Remington made, but Remington did ship barrels with the North Haven roll mark to the Remington factory in Ilion when they closed down the North Haven plant in 2010. Likely that gun got one of these barrels. The gun should also have an REP proof mark on the right side of the barrel, just forward of the receiver, another sure sign of a Remington made gun. Marlin made guns will have a JM proof mark on the left side.
Take it from there. Contact Gravel Agencies
How many different makes/brands of ammo have you tried?
CCI Mini Mags are generally the best; Remington the most questionable.