In fact, quite often, ancient magic requires a stupid amount of mana to work, so they’ll basically be handed a gun, but not given any ammo or given like one bullet that regenerates once a day.
I think that, in reality, it would be more like a ten year old child trying to shoot a .50 caliber Desert Eagle while being completely ignorant of what it takes to handle a gun like that. They might just pull it off, but they'll nearly kill themselves in the process.
Similarly, I knew someone at the church I went to who had a funny story about a shotgun his grandpa had... An absolutely ancient and utterly monstrous 8-gauge breach action. As a child, he begged and begged his grandpa to let him fire it. The old man finally relented, then stood several feet behind him. He asked why he was doing that, and the old man said he needed to catch him.
Lo and behold, he gets sent flying backwards by the recoil and gramps catches him just like he said he would.
The story ends years later when gramps is on his last legs and the boy is a grown man. Gramps tells him to go ahead and get the old monster gun and fire off a few shells. At first he's pretty apprehensive, remembering clearly what happened last time, but Gramps assures him that he's ready now. Sure enough, he was able to handle it this time. Still kicked like a mule, but it was manageable. Gramps then pats him on the back and tells him it's his now.
Of course, the moral of the story is if you try to take on something you don't have the capability to handle, then chances are that you're gonna get hurt. But when you're ready for it? You're ready for it. Which is exactly what's going on here between Hajime and Kouki.
