1/What hopper should I use on the HK Army SABR?
Hoppers are fairly universal, so you can use a standard gravity fed hopper, or you can use an upgraded electronic hopper, which will feed more consistently, whichever works best for your playing style!
2/What paintballs should I use in the HKL Army SABR?
The HK Army SABR can use any .68 caliber paintballs, however we would recommend using only Recreational grade, or Mid grade paintballs, just to ensure that you are not unnecessarily breaking overly brittle paint in the marker.
3/Can the HK Army SABR be upgraded?
At this time, about the only HK Army SABR upgrades are going to be the barrel, using Autococker threads, it is very easy to find a replacement barrel for it.