I know I'm coming a little late to this one but not dropped in for a while.

From reading all the above the main issue that keeps resonating in my thought is the primers. Whenever I have used an ML pistol at least one primer will come off the nipple sufficiently to need to pistol to be shaken on its side to allow the cap to drop out of the mechanism or occasionally needing to be removed with a pair of pliers.

Out of interest is there a way to improve the ear protection for the horses? I have no idea what you are using as earplugs at the moment but if it is say unshaped foam could you make a dobbin-size moulded plug? You can buy kits to make your own ear plugs that exactly fit your ear so why not try for the horses? You may do this already? I don't know.

If you can improve the hearing protection it may negate a lot of your current problems.