One trick I've used is to mount a decorative (finished oak) support bar under heavy loads mounted on drywall (I know you are dealing with plaster). These I mount to studs if possible, or used big toggle bolts if it is common drywall. It should work with plaster and lathe walls, but I haven't tried it. This method helps carry the weight, engineers call it a load bearing cross beam.
Over a door, you may have the ability to use the actual door frame, which is incredibly strong, to carry the weight of the display if the layout and engineering works out for you. In other words, maybe rest the bottom of the gunrack on the top of the doorframe ledge or shim it with a piece of wood (painted to match the wall) to elevate it a little. This way the majority of the weight load is carried by the door frame and not on whatever bolt/drywall fastener/screw arrangement you have to attach the rack to the wall.
I don't know how your pulse rifle attaches to the mount, but you may want to put an additional way of securing it to the rack itself. I live in earthquake country and my own pulse rifle weighs a ton, being all metal. It is on a metal grid rack that has metal arms that support the gun. However, in a side to side shake (like from an earthquake) it could be dislodged and fall. I recommend that you consider going to the hardware store and picking up some lengths of metal cabling (small diameter, sold by the foot) similar to the type used for bike gear shift and brake controls. You can measure what you need and cut the cables as needed. The cable can also be purchased coated in a vinyl sheath to protect the finish of your PR. They sell little do-it-yourself metal ends which I think are called "ferrules." Loop the cable into the ferrule, hit (crimp) with a hammer and you have a custom made "leash" with looped ends. You can choose to use little locks (for added security) or simple snap hooks or any other non-locked method to secure the cable ends. The cables are small and inobtrusive. An alternative would be to take some paracord/550 cord and tie it in a couple of spots.
I bolted my rack to the wall studs using lag bolts and large washers to disperse the load a little. Here's how it looks (cable retainers not shown). If you need pics of the cable leashes, I can post some later.
