How do i keep a characters arm still when moving (like when holding a tool)?

I am working on a gun system for my game, but so far i cant figure out how to keep the right upper arm from moving. Whenever i jump the arm flies in the air and ruins the guns aim, and when im walking the gun sways back and forth, so it also ruins aim badly.\

So far i tried welding the right upper arm to the HumanoidRootPart, but the arm still moves a little bit, and it just looks weird when jumping.

If you can help out please do! Thanks!

This is what i have so far:

local rs = p.Character.RightUpperArm:WaitForChild("RightShoulder")
rs.C0 = CFrame.new(1.5,.25,-.5)
rs.C1 = CFrame.Angles(math.rad(-80),0,0)
local gun = game.ReplicatedStorage.Pistol:Clone()
gun.Parent = p.Character
gun.CFrame = p.Character.RightHand.CFrame
local weld = Instance.new("Weld", gun)
weld.Part0 = gun
weld.Part1 = p.Character.RightHand
weld.C0 = CFrame.new(0,-.45,-.45)*CFrame.Angles(math.pi*1.5,math.pi,0)  
local weld2 = Instance.new("Weld", gun)
weld2.Part0 = p.Character.RightUpperArm
weld2.Part1 = p.Character.HumanoidRootPart
weld2.C0 = CFrame.new(-1.5,.25,1)*CFrame.Angles(math.rad(-70),0,0)