You can get some really nice upgrades in Resident Evil 3 Remake if you manage to find some gems. The blue gem can be used to obtain the tactical stock of the shotgun, adding additional precision and stability to the weapon.
To get the blue gem you will need to get the bolt cutters first, and you can do that by simply playing through the game. Carlos will ask you to put the substation back into service and you will need to find a fire hose. to put out a fire in an alleyway so you can get to that location.
To do this, you can access the security room just beyond. There you will find the bolt cutters. When you get them, come back in your path, down the main street until you come to a sign for Lone Wolf Cigarettes. Next to it, on the main street, is a set of double doors which are chain locked.
Use the bolt cutters to access the shop, and just inside the door you'll find a fancy box that will give the blue gem. You can also find high quality health spray and gunpowder in the shop.
Continue down the road, back to the subway where you got the supplies Carlos left you at the very start of the game. Head to the strange clock in the corner, interact with it, then use the blue gem. This will give you the tactical stock, which you can place on your shotgun.
There are also red and green gems, and we can help you find them as well.
