You probably should have that .303 ammo on hand long before the SHTF. That really goes for any caliber, as ammo will fly off the shelves of every retailer at the slightest sniff of unrest. If said unrest actually occurs, there probably won't be any restocking going on for quite a while.
Well, if you team up with some guys to go make a move on some enemy somewhere, and you are down to 5 rounds or so, your buddies are going to want to donate a few rounds to you so you can maybe save their backsides when it gets hairy. If you’re carrying a stupid old 303, what are the odds, you think, that one of the others might have a few of those 303s in their pocket?
I’d say you’re SOL and your buddies aren’t gonna be real happy about you carrying an old relic that will turn into a club in about 5 pulls of the trigger.
Leave the relic on the wall and get something chambered for a NATO round. Stock up on NATO ammo. THen when you do team up with some buddies, YOU can be the one that donates a few rounds to the poor schlub that runs low and YOU can be the one that tells the yay-hoo with the 303 to give the antique back to gramps and go find something that doesn’t use extinct bullets, ramrods, or flint.
BTW, most of us are going to be too old to actually go hunting the enemy(if we aren’t already there) if and when it comes down to that. The oldsters with weapon caches will not likely be more than secret resupply bases for the kids that are doing the fighting. You won’t even be useful for that if all you got is a hodg-podge of old junk that nobody’s(meaning the kids that are doing the fighting) ever heard of.