My approach is that there is a tension between three things:
1. The "combat simulator" built into the rules. I run this according to the spirit of the rules, so that players' investments in classes and feats pays off as expected. Otherwise my players feel cheated.
2. The simulation of the world. This is important because it makes the world feel real and believable (and because as DM, I get many of my plot ideas by "simulating" consequences).
3. The story. The campaign should ideally tell a story. Sometimes this means involving what I think of as "the Rule of Cool (But It's Only Cool the First Time)."
The "peasant railgun", unfortunately, fails all three tests. It isn't really part of the intended combat rules. It doesn't make sense when simulating the world. And it probably doesn't fit into the campaign's narrative because it's too weird.
On the other hand, if a player proposes something really cool that fits into the logic of the world, and that also fits into the story, then I'll look for ways to make it happen.
Let's say the PCs find 200 peasant archers, and set them up on a high hill, and have them all rain down arrows on a single target. That seems like it ought to work, plus it's a great story about bringing the villagers together to save the day. So in this case, I'll happily handwave a bunch of rules, and declare "rain of arrows" to be a stupidly powerful AoE.
But different tables like different things, so this isn't one-size-fits-all advice!