The .303 is a good round and since it is your grandfather's gun, it should stay in your collection. Reloading for it is best option for ammo.
However, it would be unwise to not have a "popular" caliber rifle for scenarios of a survival nature. A bolt action is fine for hunting but IMHO refusing to use an auto-loading rifle for a defensive scenario is not a great idea.
In the past 100 years of combat, volume has beaten accuracy in almost every case. Reflect on every major conflict and pick which rifle you would want to be issued. The majority of times, it will me the rifle with the higher
ROF. The only exceptions to the rule would be if one had a more powerful round or better reliability than the one with a higher ROF.
My picks (and most other people):
WWI: Enfield rifle
Vietnam: M-14 (or the AK-47)
Most US wars beyond that rely less on small arms squad on squad conflict but even still...
Desert Storm: M-16A2 or AKM
Afganistan War: M-4 Carbine or AKM
Iraq War: M-4 Carbine or AKM
Basically, a bolt action rifle would not be the first choice of any general infantryman since WWI.
Sic Semper Tyrannis