Because the aiming down ironsights and scopes is tied to the character animations. It's not a problem to lower the scope via the dummy null in max and make it look like it's attached to the weapon, but then you won't be able to aim properly. You'll be looking at a bad angle making it unusable. GRIN has released no information on how to line up your own sights. If it's something simple, I think they would have told us by now, but I'm pretty sure it has to do with re-creating lots and lots of char animation files. They haven't provided any examples and it would take a lot of trial and error to guess what the solution is. So, the next simplest solution is to put a spacer like Snowfella did in his M4 mod.
Thank you! But that's not Mod - friendly ...