the one or two times I tried it, I would guess 1/3 to 1/2 the injection seeped out before I got it on the smoker. With my experience I would advise you to 1) Inject from one side of the butt, and at as vertical an angle as possible 2) add spices, rub or something other than JUST liquid in order to give lasting flavor to the meat (as some of the injected juices will be lost) and 3) foil early - 30 mins or so - into the process to in order to maximize the flavoring effect of any injection. Then unfoil as needed to allow the bark carmelization that you desire.