Sounds like you are shooting them too far away. Get closer and shoot them in the neck. Between the muzzle blast and the powder burns, your cape should be tick-free.
You could aslo try throwing them in the bed of your pickup next time. It is a whole lot easier to wash out your bed then it is your pack. It's also more comfortable driving without all that stuff on your back. Instead of a plastic bag, you can spend a few bucks more and get a bedliner. If you ever hunt in Cali, you won't be able to use a plastic bag anyway as they have been banned in a good portion of the state.
As long as you keep the windows rolled up good and tight, you should be ok.
If you do get a tick on you, twist it counter-clockwise. They come right out.