Looong Time Ago...
I ordered 2 stripped lowers from QPC/Bushmaster via a local FFL shop, a couple of how-to manuals (back in 1992 while the Clinton AWB was looming and there was no World Wide Web) and proceeded to order all the other parts from QPC/Bushmaster (these days I reckon the original folks are Windham Weaponry).
Timely shipping, correct parts, & timely shipping on the spare hammer and trigger parts I experimented with.
It was very much a learning experience assembling and tweaking for both guns, and I am very satisfied that I went to the "do-it-yourself" mindset, in addition to the go/no-go/field gauges I also ordered.
Thinking (wrongly) that Bushmaster knew I was a PFC (Private effing Civilian) with no NFA stamps, I ordered a 14" M4 barrel (1 in 9" twist) WITH an AK-74 type muzzle brake and it came with a few peened-over washers "securing" it to the barrel.
Well, that didn't pass muster and I had the brake welded to the barrel before assembly. (That brake on a 14" barrel was so loud that it sounded like a .308 going off, but NO muzzle rise. It raised eyes at the range due to the decibels using standard PMC 55 gr. ammo.) No longer have it, but wish I did for the compact tele-stock and the barrel length. Vey good handling carbine.
It had the A1 sights and shot decently at 100 yards. It weighed about 8 lbs. loaded with a 30 round mag and a RediMag accessory to hold another loaded mag.
Also ordered a 20" fluted 1 in 9" twist HBAR for the other gun. This one shot MOA at 100 yards at rest with the A2 sights, but it was a heavy rifle (for an AR) at about 10 lbs loaded, with small spare parts in the hideaway grip and a cleaning kit in the fixed buttstock. The accuracy was VERY good but my eyes are no longer good, and my son now owns a very accurate rifle. Way too heavy for me at my age (62) and I'm no longer going to pretend to go to war against tyrannical enemies (unless they come to the door).
That's what 1911 .45's are for...