I've got EXACTLY the same rifle... a .50 caliber CVA Gamster Hawken with a 28 inch barrel. The rifle has double-set triggers, but no patch-box or brass "furniture", the trigger guard is painted black, it has an adjustable (for windage & elevation) rear sight and is made in Spain.
My rifle is an excellent shooter... I use it as my "target" rifle because it is very accurate.
Using 47 grains of Swiss FFFg, a Hornady swagged, round ball with a .016" denim cut patch liberally lubed with a mixture of bee's wax and Crisco Oil and using standard CCI percussion caps, the rifle is capable of shooting the middle out of bright orange target paster at 50 yards and has done so many times off the bench-rest.
At 25 yards, the rifle is capable of almost "one-hole" three shot groups with an occasional "stray" hole a tad out of the center of the paster which I use as "bullseyes".
The rifle shoots to the SAME point of aim/impact with 47 grains of Goex FFFg as well as 47 grains of Swiss FFFg at 25 yards
, but I haven't tested it with Goex at 50 yards yet.
The "cap" to which you are referring must be an after-market item since my rifle doesn't have anything like that on the nipple.
There are two (2) things I'd recommend you change on that rifle... and one thing you don't do when cleaning the rifle after shooting it.
1) Remove and exchange the drum's clean-out slotted screw for a similar screw with a hex-head or Allen-wrench screw. That screw gets unbelievably tight when the rifle is fired and the "crud" coats the screw's threads... and it is very easy to mess up the slot in the screw and then, have to use a Easy-Out to remove the screw. I don't recall if that is a 5mm or a 6mm screw, but definitely replace it ASAP.
Be aware that the nipple screws down into the same area into which the drum screw screws into, so the drum screw must NOT
be too long or it will interfere with the proper seating of the nipple. You may have to shorten the new drum screw to avoid that problem.
2) I'd recommend is that you replace the front "bead" sight with a squared-off target front sight with a flat top on the blade. This type of sight makes it MUCH easier to more accurately hold the same site-picture time after time and, thus, considerably improves accuracy.
3) I recommend you do NOT remove the nipple when cleaning the rifle. The fairly soft metal that the nipple screws into is easily "wallowed-out" and the threads messed up. Rather than remove the nipple to clean it, simply run a bent-open paper clip down through the nipple after thoroughly cleaning the rifle. I've never had a mis-fire using this method, but I did have to re-tap the part I screw the nipple into and install the next larger nipple in the firing mechanism because the previous owner had cross-threaded the nipple and stripped a good many of the threads which eventually allowed the nipple to blow out when the rifle was fired and, fortunately, missed my forehead and imbedded itself in the outer wall of an old barn (which was immediately behind me) instead of my head.
The #11 percussion cap should fit tightly on the rifle's nipple, you don't need that "cover" except that it would tend to keep your percussion cap dry. It sounds like a good thing for a hunting rifle which will be used out in all kinds of weather.
If your inherited rifle is anything like mine, you've got yourself a mighty fine little rifle for target shooting, plinking or some very serious deer hunting.
Incidentally, my deer hunting load is 70 grains of Swiss FFFg which shoots almost as accurately as the 47 grain "target" load. I limit the range at which I'll take a shot at a deer to 80 yards or less... preferably less.
Strength & Honor...