The small parts that stop a build: pins, springs and detents
Every stalled build we hear about comes down to one of about six small parts. None of them is expensive. All of them are easier to buy with the frame than to chase later.
Three pins
Trigger pin, trigger housing pin and locking block pin. They are usually in the lower parts kit, but they are also the parts that vanish into carpet. A steel pin kit costs little and saves a week.
Slide lock and its spring
The slide lock is the flat tab on the left of the frame that lets the slide come off. Its spring is a small flat wire that sits under it in a channel. If the slide lock falls out during assembly, the spring is either missing or installed upside down.
Slide stop lever spring
Different part, similar problem. This is the sprung lever the magazine follower pushes up. If your build never locks back on an empty magazine, check this spring before blaming the magazine.
Trigger spring
The small coil between the trigger bar and the housing. It is fiddly to hook and easy to launch. Buy a spare with the build.
Magazine catch and spring
The catch lives in the frame and its spring runs up the side. A worn spring lets magazines drop under recoil.
Channel liner and striker parts
These live in the slide, not the frame, and come with a complete slide assembly. If you are building a slide from parts, add the spacer sleeve and spring cups to the list; they are the two most commonly forgotten.
For AR builds
Takedown and pivot pin detents and their springs, the buffer retainer and its spring. The buffer retainer spring is the one that pings across the room, so order two.
This is general information, not legal advice. Laws on unfinished frames and receivers vary by state and city and change often. Check the current law where you live before ordering.
