Man, I thought I was all alone in my air-pressure world. Just about all of my air guns (6 SuperMaxx 1500s, 4 SuperMaxx 3000s, 3-4 AirTech 3000s) all died because of OTHER users who didn't follow my instructions for responsible use.
One of my 1500's (internals) has been in THREE different configurations (integrated to a SM 3000, integrated into a Sawtooth, now going back into a new-version 1500 shell), and has never leaked or cracked. My current air-powered primary (old-version 1500) has AT2000 internals for power with little difference in performance.
Most spackling compounds (like DAP brand) are for filling in slight defects in drywall and wood before priming and painting. They are made to be easy to sand and feather out, so generally they're pretty weak as far as durability for something that's going to get abused as much as a Nerf blaster.
Boltsn!per's first FAR might have had something different, but the industry standard is that spackle is good for filling holes in your walls, but I would NOT use it for gaps and body work on blasters.
I'd use either Bondo brand resin body filler, or 2-part epoxy paste, or even the pliable epoxy "clay" in two part formulation; those will give you durability matching or exceeding that of the plastic. Only thing, it's a total biotch to sand and smooth out (unless you have the shape pretty much down while it's still setting).
Heck, I've even used hot glue in bulk to fill some gaps. I try to smooth it out with a wet finger before it sets up.