[Maksimum_Fire]

Power upgrades at the blacksmith not affecting guns has been an issue for a long time, without much of a good reason.  This affects all kinds of guns- shotguns, snipers, handguns, special firearms like the Jackhammer, and with no regard to multishot or singleshot.  Guns just don't get power upgrades, even if they're mutated.  Weapons mutated into guns do get power upgrades, however.

Notable about this is that you CAN pay for power upgrades at the blacksmith, it just won't do anything.  This is an easy pitfall for new players especially to fall into and should be dealt with sooner, rather than later- either by making upgrade power apply to guns properly, or by replacing the Power upgrade for guns with some other UL boosting attribute.

To test:  Upgrade a gun's power stat.  Its supposed power stat will be listed in the weapon description, but the actual power that it uses for attacks and skills will be displayed next to it in your inventory.  Nitrogen Drop, based solely on weapon power, can also easily demonstrate this issue, as there will be no change in damage after upgrading a gun's power.

[Neus]

This is not a bug. Guns use their power bonus uniquely for basic attacks, and it isn't normally applied to the weapon. The reason weapons mutated into guns behave the way you describe is because they do not use basic gun attacks when mutated. This may change one day, but it is functioning as it should, currently.

[Maksimum_Fire]

Uniquely as in exclusively for basic attacks.  After some thorough testing with the basic handgun, I can confirm it's not really used in a 'unique' way for guns- just that power gets divided between multishot weapons.  I can post screenshots if they're desired, but the rough story of what I tested is:

93 SWA(including 3 base power) handgun deals 31 damage 3 times, with +8 damage per shot- 5 from battle rush, and 3 from heat of battle.

95 SWA(including 3 base power+2 power from Sharp quality enhancement) handgun deals 31 damage 3 times, with +8 damage per shot- 5 from battle rush, and 3 from heat of battle.

103 SWA(including 3 base power+2 power from Sharp quality enhancement+8 from fireblood remains) handgun deals 34 damage 3 times, with +8 damage per shot- 5 from battle rush, and 3 from heat of battle.

103 SWA(including 3 base power+2 power from Sharp quality enhancement+8 from fireblood remains) +6 power upgraded handgun deals 36 damage 3 times, with +8 damage per shot- 5 from battle rush, and 3 from heat of battle.

In a different test, I took a basic, unupgraded, unmodded handgun and used Nitrogen drop at rank 5.  This multiplies the weapon's power by 8 to determine damage.  As expected, a 3 power handgun dealt 24 damage, and after upgrading its power it still dealt 24 damage.

Given not only the fact it had no calculation into base power for Nitrogen Drop but also didn't show any change in SWA for the weapon, but increased basic attack damage nonetheless, I can confirm that it only applies to basic attacks.

This is a clarification on the issue.  I'll move any further discussion on the matter to Balance Fu in the future, but it just feels wacky enough to be worth mentioning.  The upgrade power damage bonus that's applied to basic attacks mirrors the bonus it would get from SWA, too, and this only serves to hinder gun autohits- something that guns aren't exactly known for doing well.

Thanks for your time.  Right now It just looks weird and is a confusing issue for players that even I've only recently plumbed the depths of, so I really think it's something that should be touched up.