I have an old Mauser action .30/06 that I built 20 some odd years ago. I can regularly stay in a 10 inch target at 800 yards with that old HUNTING rifle and could more than likely lay down longer shots with that old girl. But I'm getting old and long shots on game I haven't done in years. Just too many things can go wrong between time of trigger break and impact of bullet. RR and Mercy both know my opinions on longer than 300 yard shots in the field. If you want to practice long range with a hunting rifle, that old 700 would be fine to practice with at say 600-700 yards since that was probably built when Remington actually gave a crap about quality. I know you don't want to modify it any since it was your gramps rifle (I have quite a few with the same mentality attached to them) but I would recommend a trigger job or maybe a replacement trigger such as a Timney or a Jewel (keep the old trigger) as that is almost ALWAYS an issue with long range shooting. That way you can have a good trigger without any permanent changes that you couldn't just switch out.

Work with that old 06 for a year or so and if you still feel the bug for longer range you will have had a lot of practice already by getting the basics of long range feel downpat. And a heck of a lot cheaper than paying the cost for that .300RUM.