Doublecross weapon post resizing - how to fix?

I bought the LG51 Targetmaster Doublecross.
Since he didn't come with any weapons (except for Haywire), I acquired Twinferno's dual blasters seperately:
https://i.imgur.com/IjNr1Uwl.jpg

But the diameter of the blaster post is slightly too small to fit into his post holes (the hinge on his beast mode jaws that articulates the lower jaw).
So is there a way to reduce the diameter of the post slightly without breaking it?

Should be able to sand it down?

Yep, gently sand the post an test fit often.