Not sure if this is the correct spot, but whatever. Just ordered the rest of the parts for my first AR build, been lurking for a while but this is my first post here.

t:  You are off to a fast start.  Too bad about the AR-57 brand lower that's giving you troubles.  Hopefully, you've seen that last of those.  Once you get it all together, you will likely want to start changing a few things.  Here's a short list of possible upgrades you may want to undertake, assuming the lower receiver is OK:

Trigger - the standard GI trigger that comes with your LPK is heavy and gritty.  You can do wonders with it by simply polishing the engagement surfaces and replacing the springs with a JP Rifles spring kit for about $11.  For a lot more money, check out Geissele's Hi Speed Match trigger for precision work or the AR Gold trigger for fast double taps as in 3-gun competition.  Both are easily installed and set up.

Grip - there are many different types to choose from, if you have large hands, check out the Ergo Deluxe model.  For smaller hands, the Ergo Ambi or right-handed model; or the BCM gunfighter grip.  All of these have a more vertical angle for greater comfort.

Handguard - unless you really want a quad rail, check out the new small diameter FF handguards that have a fixed rail on top but are smooth on the sides and bottom.  That enables you to add rails only where you want them.  I like the Midwest Industries SS gen 2 model and the Noveske NSR.

Buttstock - So many to choose from to replace the low end model that came with your BCM kit.  For carbine stocks, I like the Safariland SuperStoc

good luck - CW

Those AR57 lowers were known for that during the panic, they would have replaced it for you.

PSA stuff came in today, all I was waiting on was the tools and the blem upper from them, put the upper together in about 30 minutes. I didnt torque the barrel nut down to a spec, but I got it snug and moved it one gas hole over. AT3 tactical rail is a bit misaligned with the picatinny rail on the upper, but once I get access to a vise ill straighten it out and use the jam nut to lock it in place. Went to the range and shot a box of 20, it fed and ejected fine, but the trigger wouldnt reset. I cried a bit then picked myself up, went home, took apart the lower and realized i had the trigger spring in upside down and it didnt have enough tension to put the trigger back forward during the firing cycle. slapped it back together and it seems to be fine now, but i wont know for sure until i hit the range again sunday and put some more lead down field. Just got back from vacation so it will be a few weeks before i can afford sights or optics, so really no point in shooting it other than to shoot it right now.