DM and MS knew each or one of them had a gun. They texted about it, and ammo for the gun(s). Neither of them disputed Tim was shot by a gun.
These two two were not naive in terms of gun. Happily posed with gun and wrote lyrics. About guns. Guns were not a forbidden subject. They lived the drugs, guns, party, anti LE, lifestyle. You aren't serious that one didn't know the other had a gun and was prepared to use it? Why buy ammo if gun was just a symbol of being tough? Why load gun before test drive? Fear of truck owner perhaps?