My uncle ran a manual gun on a 320 for years and swore the battery ones push grease too fast and blow out seals if you're not careful. I read on a forum once that 3 pumps per fitting is about right for most pins anyway, so you're not really saving time with the fancy gun, just saving your arm. Wiping the fitting first is the part most guys skip and then they wonder why their pins wear out. Keep the 12 dollar gun, boots matter more than looking cool at the yard.

That shim trick is smart and I would take it a step further. If the frame is warped you can stack a couple thin nylon washers behind the magnet too, they won't rust or crack like plastic can over time. The key is using something non metallic so it doesn't mess with the field, and keeping it thin enough that the gap stays in spec. I've done this on a few older wood frames where the door sagged and the bracket had no more adjustment left. Inspector never blinked because the gap still reads right on the meter and the contact closes clean. Honestly beats drilling new holes in a finished frame any day.