You can get some very nice upgrades in Resident Evil 3 Remake if you manage to track down some gems. The blue gem can be used to get the Tactical Stock for the shotgun, adding some additional accuracy and stability to the weapon.
To get the blue gem, you will first need to get the bolt cutters, and you can do this by just playing through the game. Carlos will ask you to get the substation up and running again, and you will need to find a fire hose to put out a fire in an alley so you can make it to that location.
Do this so you 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 a fancy box that will yield the blue gem. 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 blue gem. This will get you the Tactical Stock, which you can place on your shotgun.
There are also red and green gems, and we can help you find those as well.