C'mon Bruh, don't sh*t on them so much...that's the SOP
If you don't lower your gun then you'd be entering the room something like this
View attachment 21470
which means your gun would enter the room before you. And there is fair chance that someone hiding near the door can grab your long ahh rifle by his left hand, deflect is upward and then use a short ranged weapon like pistol to engage you. So it's Standard Operating Procedure to lower guns if they're not something compact like SMGs.
Lowering the gun also has the additional benefit that you won't be "flagging" your buddies; meaning at no point your barrel would be aimed that them. This reduces chances of friendly fire