.50 Cal Hunting Rifle doing WAY more damage than other guns?

Solution 1:

Because it's a bullet, not a spread.

Your shotgun does it's damage divided evenly among some (currently unknown) number of pellets. If every pellet hits, it'll add up to 70 damage (as far as exploding ammo goes, I'm not sure if we know if it applies to every shot or every pellet, but either way, we'll get into it). But if you miss with any pellets, your damage suffers. Additionally, the DR system in F4 is weighted against shotguns in general. Each pellet is DR'd separately, resulting in far less damage getting through.

Lets pretend your enemies have an average DR of 70. That means if your shotgun was loaded with slug rounds (which you can't do in the baseline game or any mods that I know of), you would deal ~35 damage per hit, because a 1:1 DR to Damage ratio reduces the damage by 50%. Now lets pretend your shotgun fires 5 pellets, for 14 damage each. The DR is now up to a ~5:1 ratio, which will be much less than 40% damage (~6 per pellet or ~ 30 damage if every one hits). Even if explosive ammo applies to each pellet, you're still around 29 damage, so a 5:2 ratio, still around ~61% reduction (~11 per pellet, ~ 55 if they all hit). You're also playing on hard, so each attack does 75% of the damage it should have when you're attacking enemies, dragging your damage down further. If it's more pellets than my pulled-from-thin-air-5, you're at an even greater disadvantage.

Whereas this sniper rifle, with it's 90 damage, somewhere around 45% of the damage is blocked, leaving you with ~50 damage, which is going to average MUCH higher than your partial hits with your shotgun.