That sucks. Unfortunately even Skywarp had some QC issues despite being third in the series. Some people had out-of-the-box breaks, missing/duplicate faces and cosmetic blemishes. At least I hope you didn't have any tail fin cracks (mine did - 3 for 3 dammit) and that the joints were tighter than the other two.
This guy on TFW2005 also had a missing gun. I wish you luck. Since it's a stand-alone piece, you might get lucky and suppliers have spares or returns they can cannibalise.
OMP is probably waiting on their supplier. Small retailers are in a bind too. Ultimately it's TT's fault - obvious errors shouldn't happen with such an expensive figure. It lets down the whole MP collecting experience.