If you have already gotten the shotgun in Resident Evil 3 Remake, then you will want to get your hands on the tactical stock upgrade for it. This involves quite a few steps and finding two different safe rooms.
First, you will need to head for the safe room at Moon’s Donuts. You can get there after you find the alley fire that you need to put out to get to the substation. Walk back up the main street, and some zombies will know down the fence on the side of the street. Shot the red barrel to kill them all, then head down the steps and into Moon’s Donuts. Down a small corridor on the left, you will find the Safe Room.
Inside is a fancy box that will open when you examine it, giving you a red jewel.
Now, you will need to play through the game until you have the alley fire put out, and can get access to the safe room just beyond it. In there, you will find the bolt cutters. When you get them, head back the way you came, moving back down the main street until you come to a sign for Lone Wolf Cigarettes. Beside it, on the main street, is a set of double doors that are chained shut.
Use the bolt cutters to get access to the shop, and just inside the door, you will find another fancy box that will yield another jewel. You can also find some health spray and some high-grade gunpowder in the shop.
Continue down the road, back to the subway, where you got the supplies that Carlos left for you at the very start of the game. Head for the strange clock in the corner, interact with it, and then use the two jewels. This will get you a grenade for the red jewel, and the Tactical Stock for the blue jewel.
You can get one more jewel, a green one, across the street at Toy Uncle. You will need the lockpick to get it, and it will you give a Hip Pouch and two more inventory slots when you put it in the strange clock.