Noblade is correct about the bullet fitting the throat. Probably the easiest way to see what your throat Dia. is is to use one of your unsized bullets, drop it into the chamber, then take a steel or brass rod, tap the rod several times to drive the bullet into the throat, then knock it out , the front driving band should have been sized to fit in the throat, then mike it and you have what you should be sizing to.Say it mikes .3095, go to .310. Some will suggest using a egg sinker since it is soft lead and you might be able to use a wooden dowll .
OR seat the GC by hand, and either pan or dip lube some bullets, now see if these bullets will fit in UNSIZED cases, if so try them this way with your 4759 loads.
BUT first of all give your BBl a very good cleaning to remove any copper fouling that might be left over from shooting jacketed bullets