The launch tube for a Javelin missile features a large black ring on either end of the tube. Pictures show the front ring missing during launch, but the rear ring still in place. That suggests the front ring at least is just some kind of protective cap, but what about the one in the rear? Are there important bits of hardware/electronics in there, or is it just another dust cap? Why so big? Do these also serve as impact protection, so the whole assembly can be tossed around without concern while a soldier is running/ducking for cover?
End caps, and yes - they serve as protection for the missile.
The FGM-148 ‘Javelin’ consists of two major assemblies: the ‘round’ (which is the launch tube, explosive warhead, guidance package, and propulsion system), and the Command Launch Unit (CLU), which has all of the electronics outside of the guidance package, battery, daysight, et cetera. The ‘rings’ you describe are foam endcaps attached to the round that are intended to protect the round from inadvertent damage or any material occluding the launch tube. When preparing to launch, the forward endcap is removed and the CLU is attached and powered up.