I didn’t have enough time to load rounds into the shotgun.
Put more emotion in your performance! Are you really a professional theatrical actor?
You know that 込む is "to be crowded." This, however, is when you make something crowded. How do you make something crowded? You have to put in more and more into the thing you want crowded until it is crowded. To do this act (which you are doing), you mail (める) things in over and over again. This will crowd the post office if I include all this mail in this mail box! Ha ha! What a rebel!
The reading comes from 込む. No reading too crowded to include here.