A motorized hopper with at least a 15 bps feed rate is the most important. The second most important upgrade is a compressed air tank. These two items will bring the MR3 to almost peak performance. Also, a longer barrel would be nice for increased distance, but the stock barrel is already very accurate.
Very fast fire rate
Anti chop eyes work perfect
Easy to clean
Stock becomes slightly loose while playing.
Grip does not cover electronics well enough.
This is a very good marker. The trigger is very smooth. The stock barrel that comes with it is decent for only being 10 inches long. With it's speed and accuracy, it is deadly in any woods game.
The MR3 is built like a tank. It can take all the abuse you can give it, except in one area, the grip. The grip does not seal the electronics behind it well enough. I can see water or snow easily working its way onto the circuit board, even exploded paintballs. This and the fact that the marker is fairly heavy, (about 4 lbs by itself) are the only serious imperfections I have noticed with the marker. The screw that extends the stock also seems to come loose, but this is an easy fix with some thread tape or lock-tite.
It is a very easy marker to clean, on or off the field. Just remove a pin and three screws, run a squeegy through it, and you are good to go. The electronic eyes work pefect. They are also easy to clean, even without removing the bolt.
You will chop balls in full auto without a fast feeding loader, so don't even try it without one. The eyes only work in semi-auto, which is the mode I recomend.
Shooting compressed air with the MR3 is awesome. This is the only way to go. The shots are clean and precise. Also, every shot has a nice snap sound to it that really scares your oppenents. I would not try to hide for too long while shooting this marker, its lowd and easy to spot. Its an excellent marker for running and gunning.
I strongly recommend this marker to any serious woods baller out there. Out of the box the MR3 shoots perfect. This first time I used it I was outnumbered three to one and eliminated all three players with ease, and they are good players. True story. It shoots so fast you can keep any ones head down with it. It is also very loud. I like that because when you shoot at someone, they run away in fear. It's very intimidating.