I got this weird issue with my model when i extract in unity some polys are missing here are screenshoots.
What should i do?
3dsmax
Unity
Just try to flip these polygons.
I think its problem with unity i imported the same model in 3dsmax and it was OK.
Do you have the trigger area as separate polygons? It might not be importing the whole model, depending how your importing it, you might be saving it when your in sub object selection mode, and only grabbing the main gun element. But do check and make sure your polygons are not flipped the wrong direction as well 
Well i have problems on some ares where i used bevel and reciver part was made like separetly and than joined on the rifle i think that its the problem but cant fix it.
I think i found a solution I needed to convert whole model to mesh it worked but doubled my poly count .
Shouldn’t double your poly count >…> make sure you collapse your modifier stack before importing, will throw off the counts sometimes. usually from mapping.