Iowabucks44

Not all primers are made the same length or even the same diameter. They will not all fit the primer pocket the same, which is not usually a problem. If the primer stays in the primer pocket and the hammer or the firing pin can reach them to set them off - all is good.

Silver (aluminum battery cups) primers - probably are some form of Winchester primer.

I have a MK and i use all different types of primers in it without any problems. I just make sure that the primer i use for hunting is the same primer I tuned the gun with.

But, really personally in the MK-LK or actually any hammer (plunger) gun I really do prefer a #11 mag cap.