The Classic Army G36C is considered to the shortest model, while the G36K is slightly longer, or is medium lengthed, while the Classic Army G36 is the longest. These are all HK G36 Replicas
All three models unless upgraded still use the mini or small 9.6 V batteries.
Fortunately, we have 2 9.6 V batteries that are compatible with these models. The Sanyo 9.6 Volt 600 mAH and the 9.6 Volt Kan 1050.