poor shots/CO2, weight, parts are hard to find, fixed barrel
The P-12 and (pre-Sheridan) PMI1-DF have been field workhorses in the form of rental guns for quite a few years for a reason - durability. These shooters are designed to take good amounts of abuse with minimal maintenance required. The stock bolt is somewhat stiff unitl broken in, but with some working and oil, the gun can literally be recocked with 2 fingers. The stock 12gram changer knob takes 9 turns or so to remove - far too many when under fire. A 3 turn speed changer can be easily installed, and still found from several airsmiths. The fixed, unported brass barrel emits a hefty bark, and smaller paint will roll out the end due to no ball detentes. The somewhat complicated valve system gives roughly 22 good shots per 12gram. Performance can be improved by several custom shops. To ease in FPS adjustment, a rear velocity adjuster is also a good investment. The gun is easy to clean from a paint break using a T-stick squeege. The P-12 has a little more heft than many pumps, but the extra weight helps to acheive a steady shot without the use of a stock. With a quick polish, the unported brass barrel makes accurate shots, although the ball will take a noticeable downward arc beyond 50ft or so. This is easy to adjust for, and can surprise opponents. The loader is a 40rd ammo box with a brass elbow. The 1" feed nipple makes fitting a larger hopper a bit of a trick.
I purchased mine new in March of 1992, and it will be the last one to disappear from the arsenal. These guns can be dragged through mud and snow, stepped on, dropped, and still shoot like a champ. Due to its age, the P-12 is somewhat limited for upgrades, but with some perseverence, it can still keep up with the higher-end pumps. Many players have customized their own over the years, and it is hard to find 2 that look exactly alike. In stock form, the weight, fixed barrel, heavily threaded CO2 knob, and semi-fixed fps keep many players away from in in leu of more modern pumps. They don't know what they're missing out on. With some TLC, a little oil, a few sought-out upgrades, and casual maintenance, the P-12 will still be a favorite long after many other pump models have gone by the wayside.