It all started with Classic Arms having a deal on Fostech Lite lower receivers which are a magnesium alloy and the package says 1/3 the weight of a regular AR lower. Then later they had a sale on Fostech uppers made of the same alloy. I built one into a pistol lower for a .22 upper a year or two ago. That didn't go super smooth as several holes seemed to be undersized and the upper barely fits - the rear takedown pin is super tight. This one went much smoother but the threads for the pistol grip needed to be cleaned out, I think whatever coating they used was in them and made it too tight. I would screw in the bolt a few turns take it out, brush off the threads, and repeat, while also cleaning out the hole best I could with a cleaning brush. The buffer stop pin hole was slightly undersize so I reamed that with a drill bit by hand, took all of 30 seconds but had to be done to fit in the part. The buffer tube threads were also undersized, so I hit them with a wire brush while also having to screw in the buffer a bit, take it out, clean out the threads, and repeat. All other pins and parts went in just fine. Finish on the Fostech lowers seems thin and prone to scratching, but they were sold as blemish and maybe that is it. The upper went together with no issues at all, but the ejection port door geometry is slightly custom due to the alloy they use so it wouldn't fit as well into my upper receiver blocks as I would have liked.
CDNN had Bushmaster pencil barrels for I think $89, so that was another key piece. The BCG is AIM Surplus titanium nitride for $120. I would have gone with the same carrier in nickel boron for $20 cheaper but they were out of stock and that was about the last piece of the puzzle.
My LGS weighed several brands of buffer tubes they had in stock and aero precision was a bit lighter than other brands. They also had a BCM (?) lighweight endplate that had a QD socket, which was a nice bonus. The same LGS also had the Strike industries plastic ejection port door, so another minor weight savings there.
The MFT battelink stock is heavier than anticipated and might get swapped out at some point. Same with the normal pistol grip. It has an H buffer, which I've had good success with. I have a standard carbine buffer somewhere I can try when it surfaces. Since I was going with a lightweight BCG and it will probably be surpressed, I wanted an adjustable gas system, something I've never done. LGS recommended the Superlative Arms, which went on great and adjustment was quick and easy. My options for an adjustable gas block were limited by the .625 barrel. Anyway, the Superlative one seems like a high quality option. And during this build I found out there are apparently two specs for gas block screws and the corresponding barrel dimples. My bbl has one set, the gas block has another with slightly wider spacing. So one screw is set into a dimple, the other is just snugged up really well. Both are blue loctited in place. Fingers crossed. Another fun thing, my cheapie Bushmaster barrel didn't include the index pin. I didn't even know that was a thing, but it was a cheap part and tapped into place easily.
Handguards are Aero Precision 7" Atlas S One which also have a QD socket built in. I really like them so far - nice and light and so narrow they are easy to hold onto. One note on those - the barrel nut weighs almost as much as the handguards themselves. Aero includes the special wrench for their special nut, as well as a tool for the tightening system and a shim in case the barrel didn't clock right. I didn't need the shim. Quality stuff that all went together with no problems.
The sight is a Sig Romeo MSR I got from PSA for I think $99 recently. It's OK so far and has a 2 MOA dot and 20,000 hour battery life. When I got it, I thought it was DOA but in playing with with it realized the dot flickered on between clicks. I clicked the dial rapidly for a bit and it's been fine ever since. At 20K hours, I'm just going to leave it on. If this rifle really shines, I'll get an Aimpoint Micro or something similar.
Anyway, final weight is 5lbs, 6 oz including the sight. If it performs accuracy wise, it will either stay with the muzzle nut and be ready for my incoming Gemtech silencer, or it might get a Sig QD mount for another incoming Sig silencer. TBD.
I've test fired it at 25 yards to get the gas adjusted and it was giving me about 1" groups, but I was shooting fairly quickly and not really trying for best groups at that range. I hope to get it out tomorrow and test it more thoroughly.
edit - if someone can give me some tips on my image - either why I can't just an an attachment, or why the link to the shared Google album isn't working, that'd be great.
Bitter Bastard
