Dave: Target shooters- who worry about everything ---- tell me that using Magnum caps causes the PRB or bullet to move forward BEFORE the main powder charge begins to burn. This changes the dimension of the "powder chamber" of the barrel, and opens up groups. THEY prefer using standard #11 caps.
With the unmentionable zip-guns, we also know that the powerful 209 shotgun primers are also causing those loads to move forward, giving wide variations then in MV, and group size, using all those modern powders, and components. Someone is now bringing out less powerful 209 sized primers, for use in Mlers, as a result of the complaints.
Similarly in BP Cartridge shooting, we find people reducing the size of the primer hole in the casing, and using pistol primers instead of rifle primers, so that the gas produced by the primer does not move the bullet forward in the casing before the powder in the casing builds up enough gas to begin pushing the bullet down the barrel.
I use standard #11 caps on my percussion DBL shotgun in the field, rain or shine, cold or warm. I have never had a misfire, or hang fire attributed to the percussion cap with that gun.
HOWEVER, I have ground down the face of both hammers so that they strike the top of the nipples evenly all around the nipple. This is something other percussion shooters almost never do. If you don't check to determine how "square" the face of your hammer strikes the top of the nipple, you will bang and collapse an edge, and then you can have trouble with ignition with some caps. The more you bang that top edge crooked, the worse the problems become. You might overcome a small problem with a more sensitive,"Hotter" cap, for awhile, but each hammer drop worsens the problem, IMHO.
My brother bought some of the Magnum caps by mistake, and we compared them with standard caps one evening, after dark, firing off primers, and watching to see how long a flame came out the muzzle of the guns. I have no doubt at all that the Magnum caps are "hotter".