I have Geissele triggers in most of my ARs, going from a standard mil spec trigger to a 2-stage will as Shoots said a little practice to get use to, but once you do be careful you will want them on everything.
I would agree that the temp had the most to deal with that, but if you want consistency use the same device, and what Steel Head said many ballistic calculators can help compensate for the changes from one event to the next.
I know this is an older thread, but worth reinvigorating. I like the paintball idea, cheap and a good splatter. Others mentioned golf balls on the T, I did paintballs on a T with a white background. I also have some bowling pins. You can set them up just to knock them down, I also tie them to a...
I think there are a lot of good options floating around. I started out with a aac 7.62 can as a do it all can. Several cans later, I still think it was a good start that I could move from gun to gun, while I waited for money and stamps.