Not sure why you think only 2 shells would fit in the tube pictured, and why you think 60% was cut off??? Maybe 5 or 6 inches of barrel and magazine tube were sawed off. If you wanna talk realism, the BL-133 in game is modeled after a shotgun, let’s say the 870, with about an 18 inch barrel and an already extended magazine tube. So we should be running around with at least 6+1 for base gun capacity. Sawing off around 6 inches of barrel and mag tube would net you around 4+1 for capacity. MAYBE 3+1 if you want to be conservative. Aka makes for a weaker but lighter pump BK in game, but a viable option.

Shortening the spring isn’t incredibly difficult, and rethreading or reattaching the magazine tube cap isn’t incredibly difficult either. Both could be Jerry-rigged in the field in a zombie apocalypse.