Kind of common with 3D printed stuff if the filling is set too low.
I have built several 3D printed blasters and they all require a lot of surface treatment, sanding, more filling, more sanding and so on... (with one exception which is from a photo-hardening resin printer).
You might want to consider insert something into the barrel, PVC pipe or metal tube to help line up and reinforce the body, otherwise it will crack always at the join when subject to stress (handling / trooping). I rebuilt mine like 2 to 3 times.