This is a QC checklist for AR-15 lowers. It is primarily slanted to the evaluation of 80% AR-15 lowers, but it may be used on any lowers.
A while back, I posted that I do not recommend TR Enabling 80% lowers to anyone. Most of the items on this checklist came from experiences with TR Enabling lowers, but they could happen with any poorly machined, out of specification lowers.
Bolt Catch Pin Hole
Bolt catch pin holes which are too tight, are miserable to try to insert a roll pin into.
Workaround: Pre-compress your roll pin by crushing it with pliers, then rolling it to get it back into round.
Repair: The only real repair is to take a drill bit of the appropriate size and carefully drill it out. I use an undersized drill bit and work it back and forth like a file, due to the difficulty in procuring a drill bit so long that it will work in the hole.
Examples: Multiple Anderson Defense lowers.
Forward Takedown Pin Detent Hole Alignment
Forward detent pin channel should line up with the center of the pin hole. If it's off center vertically, your forward detent pin may pivot as you slide the pin out. If it's off center horizontally, it may not snap into place or it may snap into place too early, causing the forward pin to protrude.
Examples: I have seen this on some, but not all, TR Enabling 80% lowers. This is a factory issue with the way the 80%'s are machined, and is not attributable to user error.
Grip Screw Properly Tapped
Check to ensure that a standard grip screw may be inserted into the lower. It's ok to be tight, but it shouldn't be so tight that it feels like you're going to break the lower. On an 80%, you should check this before anodizing if possible.
Examples: Multiple TR Enabling lowers.
Repair: This is actually one of the easier repairs. If it's too tight, then take a 1/4-28 tap and use it to correct the threads in the grip screw hole. If you don't know how to use a tap, practice on something else first and get a feel for it. On the TR Enabling lowers, I had to remove a lot of material from the hole. Turning the lower upside down so that the chips fall out the bottom, and not trying to run the tap all the way through in one go, helped greatly with extracting the excess material.
Check to ensure that the magazine catch is able to smoothly, and without resistance, travel to a position where it can hold a magazine, and then travel back out. Shoddily machined lowers may have chips, hanging protrusions, etc which will contact the magazine catch and prevent smooth function of your firearm.
Examples: Multiple TR Enabling lowers.
Repair: Determine the points where it's rubbing by working the magazine catch back and forth and watching for contact between the catch and the lower, then use a file on the points of contact until you have completely corrected the issue.
Workaround: There is no way to work around this other than filing down your magazine catch, unless you're fine with just hammering the magazine catch in and then mounting, say, a single-shot 50 caliber upper to it so that you don't have to use the magwell at all.
Make sure that aluminum chips are not present in the detent hole. You can do this by taking a safety detent and pushing it up through, with no selector installed. It should have no resistance coming to the end of the channel. In an 80% build, the chips are not the fault of the manufacturer, as they would've been put there when you drilled the safety hole.
Repair: Clear out the chips. I used an allen key to push the chips were up and out of the detent hole, then removed them with a pick.
Safety selector detent hole must line up with the notch on the selector. To test this, insert a safety and press the safety detent in place with an allen key or pin. It should smoothly and firmly click into place for both safe and fire, without a lot of slop. If the detent wriggles loosely in the hole, it's possible that horizontal alignment of the hole is off.
Repair: The only true fix would be to tig weld and fill it, then re-drill the hole.
Workaround: As long as the alignment is only too deep or too shallow (not off to the side), you can use a Noveske STS safety or similar. The design of those safeties allows the pins to flex more side-to-side, and as long as the lower isn't grossly out of spec, you should be good.
Examples: I have not seen one single TR Enabling lower which did not have this problem. This is a factory issue with the way the 80%'s are machined, and is not attributable to user error.
Check to ensure that the safety you intend to use can rotate freely in the hole which was drilled for it.
Examples: I have seen several TR Enabling lowers, drilled on a standard jig, where the tabs prevented the safety from rotating.
Repair: Short of TIG welding and re-drilling it, there is no repair for this.
Workaround: You could install a plug in place of the safety selector (thus leaving your rifle with no safety) or use an alternative safety without protrusions which would contact the tabs, modify the safety itself to reduce the size of the point on top, or file down the stops until the selector's tip clears them. There are a lot of different workarounds available here.