I genuinely like your posts and you are spot on 99% of the time but on this one, hard pass. Most of this stuff is so commoditized a blind squirrel could build one. There is no voodoo magic involved here and like you, own factory guns but also have plenty of DIY home brews with zero issues through multiple barrel changes from usage. I do agree that one should use reputable parts from reputable vendors however.
Bash parts together, absolutely. Anyone can do that and most do. It will look like an AR-15-type object when finished, but will it run?
You see it over the past decade + where the most focus is on what kind of cool furniture or receivers the person selected, or what trick finish is on the BCG, or something other than the barrel, BCG, gas block, and gas tube fit to the carrier key. Anytime I see someone’s build list and you have to really look hard to find the barrel and bolt in there, you know what I’m talking about.
I’ve had to fix so many of those, I lost count. Main issues I’ve seen and continue to see:
Aluminum gas blocks on CLGS or MLGS, (though thankfully this is less and less of a thing now)
Nitrided extractors snapping (too brittle)
Improperly-installed gas blocks
Gas tube binding/clipping
Wrong chamber jamming M193 into the lands
All 3 carrier bores too large causing mass gas leakage into the fire control pocket and short-stroke malfs
Way too large gas ports (this is more common than not)
Barrel extensions that don’t even come close to TDP dimensions...causing bolt lug shearing/grinding when trying to go into battery or unlocking
YFS carrier key fasteners snapping/coming loose-more gas leakage/short stroke
Nitrided/exotic hard finish carrier keys eating gas tube flanges and causing short-stroke
Pistol grip screw threads not drilled and tapped all the way
Receiver faces out of square from anodizing causing off-axis barrels and uneven bolt lug lock-up
Extensions not torqued on fully
“Mil-Spec” RETs too narrow or too large
Zinc-plated detents garbage
Soft hammers and triggers
Jacked-up disconnectors with slanted hook surfaces
Sharp and almost-proud ejectors that shred brass for sport
Extractor lips not relief-cut per TDP (which like to hold onto brass and not let it go)
Firing pins out of spec/not hard enough/pitted firing pin tips result
Garbage cam pins undersized that allow slop and galling of the cam pin body against the cam helix in the carrier
Breeches not cone-ramped (with barrel mfg stamps still visible)
6061 charge handles that snap
Muzzle threads all over the map for concentricity and squareness
Dudes clamping in the upper to install or remove muzzle devices with cheater bars (often with their “gunsmith’s” help)
I could go on...
A DIY builder can combine some or all of these problem parts into the most cool-looking Imitation-15, get the receivers and handguard Cerakoted, then post up pics of them all over the net talking about flawless reliability, but when they show up to courses, they immediately rip-fart the bed on spastic bowel cyclic rate.
There are a lot of worthless parts out there that should be scrapped, not assembled into a Vismod. Most DIY-builders just don’t have the ability to identify these parts because it’s relatively new to them. Even experienced builders aren’t aware of a lot of the critical specs inside the carrier, barrel extension or materials for extractors and other parts, and they have no way of testing them other than assembling and rolling the dice with shooting.
It’s why the old advice of maybe assembling your lower and slapping on a reputable-built upper still holds true. Just be selective on where you get the LPK from and RET. Drop in one of the many after-market higher quality triggers or an ALG FCG to avoid the common problems with factory semi auto FCGs.
There are several assembly methods that the more-informed builder can use to build a reliable upper with a quality barrel, gas block, gas tube fit, BCG, upper receiver, barrel nut, and handguard. Most builders are not aware of these methods, and are already validated in the visual product of their limited labor. How many times have we heard how easy it is to assemble an AR-15?
Now to assemble a reliable, accurate AR-15, that is a much different story.
