The S&W Model 645 is one honkin' big 45 ACP pistol, but darned near indestructible and usually boringly reliable. I like them so much that I now have two! For me, one is for shooting, the other, being in near new condition, may get shot a little, but is mostly for displaying in my Galco Miami Vice era correct shoulder rig.
Looks like yours has the original design magazine floor plate. These gained a reputation for coming off of the mag during fast reloads. For shooting purposes and certainly for defensive uses, look for magazines that use a button that protrudes from the center of the floor plate. These come in three varieties, plain metal floor plate, metal floor plate with a rubber pad glued to the plate, and the final design is the black plastic floor plate. Do not buy ProMag magazines. These are cheap knock offs and have a reputation for unreliability and poor fit in the magazine well of the pistol. You have been warned.
Two weaknesses in the big S&W metal receiver pistols. First is the recoil spring guide rod, which is aluminum, and is crimped to the plunger system that retains the slide stop pin. That crimping can loosen and the guide rod will eventually fail. Finding a new guide rod is very difficult, but there is a forum member who can machine one out of stainless steel. His are not cheap, but I'd say it is a one-time purchase for the life of the pistol.
The other weakness is the trigger play spring. It is stamped, riveted to the trigger and if it weakens, the trigger begins to rattle a little. If it breaks, the broken piece can jam the trigger, rendering the pistol useless as a firearm until the broken piece is removed. I suspect many of the weak and broken springs may be due to damage during cleaning. These springs and rivets are still available, replacing them is a pain.
Before shooting your Model 645, I highly recommend a new, 16 pound recoil spring. Also, I recommend using aerosol gun cleaner or non-chlorinated brake cleaner to flush out the extractor channel and the firing pin tunnel. People often try to drip or spray oil into these areas and the oil gets dirty, gummy, and will interfere with reliable functioning. I experienced this with my first large frame S&W pistol, a pilot version of the 4506 (4506 parts, but the receiver marked as a Model 645).
