The mag guide is probably misaligned. It requires slight bending to adjust the feed angle.
This should help, (taken from the nylon rifle forum)
Remington 10C & 77 mag adjusting
To remove the combination guide and mag retention release .
First remove the magazine from the guide.then remove the front retention pin on the trigger guard . look behind the mag release button (plastic part) and you will see two small tabs on each side of it holding it in place on the metal housing.Use spreading pliers to simultaneously spread the tabs (being gentle).the tabs being spread and a small amount of pressure applied to the plastic away from the stock should release it from its mount.
The screw is now visible to remove the two remaining pieces.Mag guide and mag release piece. I have had to do to several 10c's that I have purchased.seems to work well once the sweet spot is found where bout feed and mag retention is just right for several different mags. Happy adjusting.