Protectrons feel a bit like bullet sponges at time. Shooting them in the head does break the glass thing and often blows the head off but it doesn’t seem to cause serious damage. Do protectrons have a weak spot?
For most robots in Fallout that have legs, the legs are usually the weak spot. If you have a rifle, shooting the foot section of a protectron, which is quite large and easy to hit, will often kill it in one shot. Blowing the leg off a protectron will sometimes cause it to crawl but most times it will seriously damage them and often kill them in a single shot.