Norsu told me to post here as apparently some other people have also had problem with their models ending up with waaaay too many sections despite faces sharing same material and texture. This apparently happens often when parts from old OFP MLODs are copied into new O2.
And as everyone hopefully already knows, too many sections = baaad for performance.
This is our starting situation. Note number of sections in status bar on top of O2. This is waaaay too much for such middle LOD and as such, ingame performance more than halves when there are full squads worth of soldiers with RK95 onscreen compared to if they were equipped with BIS' M4's.
Now, to fix that we first select all faces that are supposed to be in same section and hit "E" key to get to face properties dialog. Note that "User value" field is empty instead of having number there, it means faces in selection have different values in there, which causes them to be split into separate sections. We type there "0" and hit Apply.
We do same thing for textures and material fields in they contain "..." and make sure that Lighting & Z-bias also have identical values.
Now we can see that amount of sections has dropped to 3, but model still contains 1 section for muzzle flash and 2 sections for weapon. It is a lot better, but we want to still get it down to just 2, muzzle flash and weapon.
Next step is going to "Face" menu and selecting "Sort"
Often after this operation number of sections displayed doesn't want to update right away, so we go into another LOD and then come back to editing same LOD. After this small detour however finally our model is where we want it to be - one section for muzzle flash and one for whole weapon LOD and now ArmA engine can render whole weapon using single Direct3D call which makes modern GPUs very happy.
<span style='color:red'>4 hotlinked pictures over 100kb removed.</span>