You've gotten a lot of good advice so far so I guess I'll repeat a few things and then give you product advice specifically..since Im willing to bet that you will ultimately still buy whatever you have your heart set on.
1. Yes it would make life a lot easier to just buy a pre-built gun. What you're missing in the overall equation is the fact that when you assemble, you become the QC and the one thats ultimately reliable for making it run correctly. You dont know what you dont know.
2. You will at minimum need some tools.. armorer's wrench/castle nut wrench, torque wrench, a punch set, mini brass/rubber hammer, I highly recommend a vice..and a mag block at minimum. Throw in some levels, and an inch pound torque wrench as well if you plan on mounting any optics. Probably a good idea to get a mini propane torch too. I also wouldnt be assembling a single thing without a reaction rod...and some go/no go gauges..but I know guys who have done it with less.
3. You'll need some degree of manual/mechanical skills. I have friends who cant get the roll pin started in a gas block..and would be stumped by that alone. Others who mangled FCGs/Trigger installs by man-handling and forcing shit that didnt fit right because it wasnt being installed correctly (think friction fit). You also need to be able to recognize this situation..which will only come from experience in building, or having someone more experienced show you.
Also, you'll lack a second, well known, running rifle to quickly grab parts from to test a potentially out of spec BCG, or similar. This is critical. Once again, you dont know what you dont know.
now..beyond that..I think you have overpriced your budget for this first one, and mismatched a lot of parts based on fragmented (and brand fanboy) advice you've collected between ar15.com, reddit, and who knows where fuck else.
So here's my analysis on your parts, and keep in mind the advice is worth exactly what you paid for it, but this comes from my trials and errors by now of building first hand (or assisting others) with close to 2 dozen rifles. Im sure for many thats not much either, but its where Im sharing advice from either way.
Lower – LMT Mars-L (the only part I currently have)
Lower Parts Kit – LMT
Your lower is not compatible with all off the shelf stripped uppers and fully assembled ones as well. here's a good list I found for you to review:
I would probably sell that to some LMT fanboy, and buy a standard milspec lower. Aero enhanced is my recommendation to everyone to start with but I believe there's other similar options, for more money. The enhanced are better than the standard milspec ones with a built in trigger guard (an easy part to break when you install a trigger guard onto a milspec standard AR Lower) and a threaded pin instead of a milspec install with a roll pin for the bolt catch..which normally requires a thin punch and is overall a waste of time. Worth it for those items alone. You can even buy a ballistic advantage enhanced lower now for like $60 and it is identical to the aero in every way, minus the rollmark.
Either way dont waste extra money on a lower, its only a vessel, and as long as the parts fit in it, it will function fine.
Lower parts kit - buy whatever you want, these parts are the cheapest components and more or less meaningless. It might be easier to get one where the baggies are individually marked for each spring and detent and pin. I believe CMMG still does that? Plenty of others do as well. Dont overpay for this.
At minimum, get yourself a document that shows true to size comparisons of each detent, spring, etc. that will help in assembly.
Trigger – Hiperfire Hitouch Reflex or Geissele Hi-Speed National Match (DMR springs); The Hiperfire doesn’t have the reputation for reliability like Geissele but I have no experience with two-stage triggers.
Dont buy gaming/competition or super lightweight triggers if you intend to do home defense, CQB, SHTF or similar. Get a "combat" style trigger. You dont want a twitchy trigger or something super light weight. A good 2 stage will let you "preload" and then have a clean break for the pull. These cover that requirement.
Geissele SSA (not ssa-e) or the SSA's cheaper version- the G2S. You can try their flat bow or similar versions as well, thats 100% strictly shooter preference, regardless of what anyone tells you. Hold out for deals on these. They can be had as cheap as $140 for the G2S or $160ish for The SSA.
Buffer Kit – no idea; does this even matter?
Yes. Matters a lot. Your rifle wont run reliably if you fuck this up. There was years of people testing weights, and springs, and other combos, then VLTOR came out with the A5 kit and basically ended the conversation. buy the RE A5 Receiver extension, and the buffer spring and weight kit. They can sometimes be found as 1 package, sometimes you gotta buy them separately. This will save you a TON of headaches. Dont get into any spring upgrades or weight..you DONT need them yet. The majority of rifles will run perfectly fine with the standard vltor a5 weight and spring.
Upper – BCM Mk2 or Vltor MUR; the BCM seems like it’s the new hotness, but it appears that BCM doesn’t sell these to the general public anymore. Is it worth waiting and hoping or just go for it and buy a Vltor when I can find it?
As mentioned previously, if you stick with LMT, you will need an LMT compatible upper. If you decide to get a standard lower instead (milspec or enhanced) pick up a similar option them. If you're set on BCM (based on arfcom or reddit advice), just buy an assembled upper (barrel and handguard/rail included) and skip the next steps. This will save you a lot of trouble shooting, time, and money.
Barrel (16 inch) – FN HF, Noveske N4, or Criterion Core. Leaning towards Noveske because the profile and it weighs less than the FN and the Criterion is at least a 6 month wait (I believe that all the barrels are made by FN). Really looking for feedback here as I expect this is where accuracy is made or broken. Is 1.5 MOA a reasonable expectation for accuracy with decent factory ammo or progressively reloaded ammo? If necessary, I have the time to wait while Compass Lake cuts me a SS barrel.
You're all over the place on barrels, to be honest with you, so time to reel you in.. You have no clue what you want or need (yet) to look to compass lake to custom cut you anything.
FN and Criterion can both be found online at various places. Sometimes you get a backorder, sometimes its ready to buy. Another good option for the money and what you're looking to do is
https://www.lothar-walther.com/. They were in the sub $300 price range last time I had them quote me an off the shelf barrel.
You can reasonably expect below 1.5 MOA accuracy with any decent barrel. If you want 1.5 MOA..buy some budget grade ballistic advantage hanson barrel.
As far as accuracy being made or broken..thats likely going to depend on ammo and you more than the barrel. Can you even shoot Sub MOA with an AR? standing? Prone? Bipod or sandbags? ETC. Any money saved from your original budget should be spent on ammo and training to shoot sub moa in the first place.
Criterion Cores can be had for $250 with discounts from many shops..you just have to wait to snag one in stock. Aside from that, I'd look hard at Lothar Walther. All the other options you mention are likely going to be above $400, unless you buy a PSA FN made barrel, and even then, quality and qc from PSA can be hit or miss. I dont think you need a $400 (or more) barrel at this point. Thats honest advice and you REALLY should consider that above all else. Barrels are consumables and you still can buy multiple ones or replace it. If you're gonna blow your wad on one, buy something Bartlein made from Frank Greene..if you read all the contributions he makes to the forum regularly you'll see why.
Gas Tube – no idea if this even matters - not really, just get the right profile based no your barrel, from just about anyone.
Gas Block – Vseven titanium; If I go with pinned (and not clamped) will I need to have someone modify my barrel?
If you're gonna spend this much on a gas block, go with a superlative arms adjustable one. That makes even more sense if you plan on ever going suppressed. I like these so much I put them on every rifle now. It helps with tuning gas to get a smoother rifle and avoid premature wear on components. If you use a set-screw style gas block instead of a clamp on, you'll need to dimple the barrel. its pretty easy to do if you're comfortable with a drill. If not, pay someone to do so.
Bolt Carrier Group – Bootleg Adjustable Bolt Carrier or Rubber City Armory Standard Mass 5.56 x 45(223) M16 BCG Complete w/ Adjustable Gas Key. I have no idea if one is better or worse than the other. I am looking at these because my hope is that eventually I will be able to use a suppressor but wanted a non-adjustable gas block for reliability
The world is your oyster on these..I wouldnt get anything adjustable at this point, just get something that has properly staked gas screws and good quality components. I hear microbest makes the bulk of them out there, including for popular brands like SOLGW. A Microbest branded one is like $100..A chrome one is $150 ish. You can find the Centurion C4 or the SOLGW one for below $150 and those will be quality BCGs you wont have to second guess, with brands that stand behind their warranty.
Grip – Magpul MOE K2
Dont get hung up on this. you will likely try a few until you get the right one. However, an MOE k2 is angled a little more for shorter stuff/PDWs, so I would start with an MOE instead.
Stock – Magpul STR Carbine Stock Mil-Spec
I think these are like flavors of ice cream..everyone has their own favorites. My personal one to run on a combat style rifle and recommend to everyone is VLTOR IMOD. Get the standard spec. Clubfoot is mostly for kitted up use..think plate carrier and whatever else on your chest.
Charging Handle – PRI M-84 Gasbuster with combat latch
Pointless to get a gasbuster unless you're already running a suppressor. Get a Radian if you want "gucci" or some flavor of Aero radian.. The Aero Ambi with extra large hooks are nice. Especially for gloved hands. Otherwise just get a milspec one to save yourself some money.
Muzzle Device – Hellfire linear flash suppressor
You probably dont need that. Muzzle device will change if you run suppressors, in the mean time..a VG6 or a precision armament M4-72 if you're looking for "flatter shooting" or recoil reduction. No muzzle devices help with noise other than suppressors.
good data in some of these on muzzle devices:
Want less recoil and vertical movement? We hands-on test some of the best muzzle brakes for recoil reduction, side blast, sound levels, and reticle movement.
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Welcome to the third installment of our 5.56 muzzle brake testing series. This time around we’ve put 42 new muzzle devices to the test, measuring the recoil reduction performance of each. Recoil Testing The procedure for this test was the same as the previous brake tests. You can find the first...
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Good luck, continue to ask plenty of questions.
