I spent a few minutes looking through my Port Plastics Tech Manual. I hope this helps:

ABS - Acrylonitrile Butadiene Styrene

PVC - Polyvinyl Chloride

As you can see from the names, they are completely different plastics. The data sheets I have available on them make it look like they are very similar in performance with a few exceptions. ABS is limited in it's chemical resistance, doesn't handle petroleum oils well, is hygroscopic (absorbes moisture), can have a huge range of color and is very inexpensive. PVC has excellent chemical resistance, is more flame retardant than ABS, but is a little more spendy. Our Nerf guns, like most other children's toys, are mostly ABS, I believe.

On a practical level, none of this really matters. The only thing I could think of would be if you were trying to weld a piece of ABS to a piece of PVC using a solvent. Because of the different chemical structures involved, I'm not sure you would get a good weld, if at all. I'm not a chemist and I've never tried joining the two, so I can't really say. This is probably of no consequence either, since hot-glue and tape seem to be the method of choice for joining parts.

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