Saints Row IV

The easiest weapon to upgrade is the Loud Locust, a hidden weapon at a Let's Pretend in the Southwest corner of the map. It only has one attribute to upgrade: Recharge Rate.

I've embedded a video (not mine) for how to find the gun. Once you've found it, you simply need to upgrade the Recharge Rate 5 times at a gun shop.

The cheapest I have found is the baseball bat it costs 42,600 to fully upgrade. Save before entering friendly fire, get the achievement and load your save if you don't care about the bat being upgraded.

Maximum Stopping Power - 20G (Original Achievement)

Difficulty: 2/10

To get this Achievement, you simply need to fully Upgrade 1 weapon.

If you have roughly $50,000 you'll have no problem.

First head to the nearest Friendly Fire (Gun Store), then open the shop menu and select Upgrade. I suggest upgrading the Baseball Bat as it is the cheapest besides out of the way hidden weapons. Once all of the slots have been bought in each area, the Achievement will unlock.

To upgrade a weapon go to any Friendly Fire and open the 'Buy/Upgrade Weapons' menu.

You have two choices for this, you can either find the secret 'Tiny Pistol' weapon at the South 'Lets Pretend' clothing store and upgrade it since it has only one slot to upgrade (Recharge Rate) or you can buy the Baseball Bat for $100 which costs $42,600 to full upgrade and is probably the cheapest method.

If you wish you can save just before doing this, upgrade your weapon, quit and then reload to keep your money for other things.

Here's a video by TheReviewDemon showing you where the secret 'Tiny Pistol' is.