I have built several builds, from pistols to rifles, SPR's to run and gun. I haven't built a lightweight build though. I have a few questions.
1. What weight is considered lightweight?
2. What parts do you save weight on?
3. What parts do you avoid going lightweight?
4. Are there any manufacturers to avoid?
5. Are there any manufacturers to buy from?

DSArms sells titaneum Take down pins, castle nuts and grip screws.

http://www.ar15.com/forums/t_3_118/638524_Lightweight_Parts_List_with_WEIGHTS__AR_15_.html

http://www.ar15.com/forums/t_3_118/625051_ULTRA_light_weight_picture_thread____the_under_6_crowd_.html

I have a lightweight build, at least I consider it a lightweight build like you I don't know exactly what constitutes a true light weight build or even if there are rules to it, but mine weighs in at about 5 3/4 pounds and is easier to control than a heavier weight one, I used vertexops.com they have tons of lightweight stuff, either reduced weight or titanium, obviously your going to be paying extra for the titanium but I think it was 100% worth it. I didn't skip out on bolt carrier and pistol grips for weight as I wasn't looking for a sub 5 pounder but I did get a lightened lower from 2A Armament and a lightened upper from DSA. Vertex deals a lot of V7 parts and from what ive seen they are top notch especially finish but fit is dead on.one easy way to deal with losing weight is a barrel mine from faxon is amazing just remember it gets hot quick so good rail covers are a must if you go that direction hope this helps, all in all for everything it ended up around $1300 for everything im sure someone could do one for cheaper but its not cheap to go light sometimes

I have used v7 weapons parts on my lightwe builds. The fit and finish are top notch. You will be surprised how much it drops weight on your rifle. The down side is the cost. It is worth it though.

I researched a lightweight build for about a year and looked at many different parts and components.  I ended up with a 10.5" SBR, but made several compromises that did not achieve the lightest I could have.  Primarily, I went with a more standard receiver extension to use a collapsible stock.  I could have saved about 5 ounces if I went with a fixed stock.  I went with Battle Arms Development LW receivers.  Today, they are not the lightest available.  V7 has a lithium alloy that saves about an ounce from the BAD receivers.  I went with a BAD UltraMatch Light Sabre fluted 10.5" barrel.  You will have a difficult time finding a lighter barrel.  What I like about it is that even after a shooting a lot of rounds in a session and getting the barrel hot (not mag dump hot) the POI stays relatively consistent.  Groups open up maybe 1/2 MOA.  But my SBR is for CQB, so I'm not as concerned about tight groups at distances beyond 50 yards.  I went with SLR's Helix rail, Griffin Armament's minimalist brake mount for their Recce5 suppressor.  Stock is the MFT minimalist stock.  I really like that stock, and would have picked it even if I didn't want to go lightweight.  The buffer system is Taccom's lightweight buffer, and I'm using Faxon's lightweight gunner BCG.  I have tried a friend's titanium BCG that could save 1/2 ounce, but cost $200 more than the Faxon BCG.  I'm using mostly V7 parts in the lower and upper.  This gun weighs 4lbs 9 oz. without magazine, optic and suppressor.  It is lighter than my goal of 4 pounds 12 ounces.  Using the SLR adjustable gas block, I have it tuned well and it shoots extremely well, flat and very low recoil.  The big downside is that it is extremely expensive.  It took many many months of buying the parts, looking for sales and sometimes making compromises just so I could afford it.  And its a two stamp gun on top of all that.

The experience of planning and then building this gun was great, especially with the end result being so good.  But I'm broke

One lesson I learned, is that there are a lot of conflicting information on the web and from vendors.  One vendor told me my plan was going to be a disaster, and almost made me change it.  I'm a knucklehead and stayed with the plan and I'm glad I did.  If you understand physics, and how the AR platform really works, you can overcome the myths and wrong information you are sure to encounter.  Good luck, it's a challenge, but a fun one.

I would consider anything under 6 lbs to be lightweight with optics. You can literally save weight on every part of the rifle. I wouldnt say to avoid anything besides polymer. The only thing polymer/carbon fiber I would buy would be the adjustable stock ( which most if not all are made out of polymer). Don't do a polymer receiver or trigger groups. I suppose carbon fiber is okay for a handguard, but no where else.

Titanium parts are good and so are the lithium aluminum parts V7 sells.

Yup. 6 lbs is my arbitrary benchmark for a "light weight".

The one polymer lower that is ARFCOM approved:

http://www.gwacsarmory.com/lower-receiver-ar15/



Also available in Sabre Defense guise (Cav Arms manufacture) very inexpensively from http://www.trinityordnance.com/

Yes I know. CavArms. But lets be real. How many of us would like to put that thing on our rifle?  Its ugly and non adjustable. a small percentage of us have them.

Well, when you can pick up a lower and stock for $60, it makes for a rather inexpensive lightweight build.  Not everyone actually needs an adjustable stock.

Not to mention you can technically adjust it.... once!  I think some one here did this very well in a lightweight thread. Josh something....

Just finished a light weight build, used as many  V7 titanium parts as I could as well as a few aluminum ones.  I absolutely love my rifle, but it ended up costing me a pretty penny when all said and done.

Vertex ops is a great resource for lightweight parts. Also, i found if the company (such as V7) does not have parts in stock, if you email them and ask they may send you to different sites where their parts are commonly found.

 
Just called and asked about them and they had no idea what I was talking about.

http://www.gwacsarmory.com/lower-receiver-ar15/

Look for the blems at $85.  I have a spare lpk so I bought one.

6lbs is the number for 16" but most shoot for closer to 4-5 ish.

Check out v7 weapon works they have a website. Also vertex ops has a tone of lw stuff. Titanium isn't lighter than aluminum but you probably already no that. Titanium is a good replacement for any part that was originally steel. Aluminum is good for anything that is already aluminum. Don't risk using aluminum gas blocks, barrel nuts are fine though. Also titanium doesn't stake well it actual chips. If you use a lw bcg I'd go with an adjustable one or an adjustable gas block.

Your going to want to do your own research from here. As you could get the best of the best and pay a lot or you could go cheap and just cut some weight here and there.

Oh also the easiest way to cut weight is going to be with the barrel and the optic but I'm not a fan of lw barrels on a rifle it's just a personal choice though. Lastly the guy that ownsbv7 use to work for Noveske.

I did a budget LW build.
Faxon 14.5 pencil barrel
Aero no FA upper receiver
MI 12.625 light weight mlok rail
Magpul K2 grip and SLK stock
Most other parts were typical AR parts.
With BUIS it was just a hair under 6lbs.
With a Burris FF3 and their mount it is 6lbs 4 oz.
Very happy with it and didn't break the bank.

I need to stop by there anyway and pick up a rifle handguard cap.  I'll ask the owner if he's not selling them anymore.  Last gun show a few weeks back, he still had 'em.  I bought my last one a couple of months ago for $60.

You asked for a Sabre Defense polymer lower?

We have several (pallets) You happened to get a new guy who had no clue who cav arms was. Sorry about that. Call back and we will get you taken care of. If someone gives you the run around ask for Richard. The guys on the floor know what's in the glass cases, but not really what's in storage.
Once again, sorry about that.

Richard

1. anything under 5.5 lbs complete
2. EVERYTHING. if there is an AR15 part, there is a lightweight version somewhere. Biggest savings comes in barrels, receivers, rails and stocks.
3. I avoid polymer receivers, just my person preference
4. I tired to only buy from reputable companies when building, so nothing to add here.
5. for LW parts I go with 2A for receivers, Faxon barrels, and everything by V7

good luck. research and money will get you in the LW game!

Best and cheapest areas to save weight
Barrel
Handguard
BCG
Stock (a carbine stock is very lightweight, fancy ones are often heavy)
Receivers

I've got one just over 5 pounds, not sure if it's your thing though.

KISS

Sorry I missed you.  Thanks for the rifle handguard cap & the rollpins.

Easiest way to do lightweight and reliable is to pick up a Colt 6720, add a Fiberlite N/1 stock, and light BUIS/optic of choice. Anything more is expensive for a few ounces of weight savings and better spent on ammo and mags.

You get a barrel! You get a barrel! You get a barrel! *Points everywhere*

its just like any other aspect of the AR game. People who chase the ultimate accuracy, or people that chase the ultimate retro replica, or the rarest Lower or anything else. LW is just like that

Just because its not for you, it might be for someone else

This is my lightweight build overview video where I go over all of the parts and components (parts list):



Here are some pics:

In searching those Cav Arms lowers they happen to have some Blems for $85.

Blems

I bought one because I have a lower parts kit laying around.

 Tankless: Excellent idea, a truly lightweight AR is a joy to hold and shoot.  My example is in 6.8 SPC and ended up costing approx. $1000 to build, using some recycled parts I had.  Weight now is 4 lbs, 10 oz with iron sights and no mag, or 5 lbs, 7 oz with Leupold 2-7x28 scope.  I refer to this as a modern sporting rifle, optimized for hunting purposes, in contrast to some of my earlier builds which feature military configurations.

The essential parts that need attention are the barrel, the handguard, the lower receiver, the bolt carrier and the buttstock/buffer tube/buffer combination.  Adjustable gas is required to keep the lightweight reciprocating components from beating themselves to destruction.  Stick to brand names with a reputation for quality.  The ordinary forged upper receiver is already light enough, especially if you buy one without a forward assist.  Here are my suggestions for the essential parts, assuming you intend to shoot .223 or 5.56 ammo:

Barrel - 16" pencil type, weight should be under 20 oz.  Faxon and others make these.  Either get unthreaded, or use a thread protector instead of a muzzle device
Handguard - I used Bravo Company's KMR10 with a titanium nut.  Its just long enough to cover a titanium low pro gas block.  Total weight with nut is about 7.5 oz and is about a good a FF handguard as money can buy.
Lower receiver - I used a Mag Tactical receiver, other brands are available, but pricy.  Look for something in the 6 oz range, that's about 2.5 oz less than mil spec.
Bolt carrier - Numerous options here, skeletonized steel, titanium, even aluminum are used.  Mine is a Rubber City titanium unit with an adjustable gas key.  It weighs 5.75 oz.
Buttstock assembly - A key component, I recommend you get one of the new unitary designs where the buttstock and buffer tube are one carbon fiber part.  Mine is from Smoke Composites and weighs about 4 oz.  Inside is a 1 oz buffer and special reduced strength spring from Taccom.
Sights - Irons are light enough, some red dots are light, I used one of Leupold's Ultralight line which weighs about 8.5 oz, paired with an Aero Precision ultralight mount which weighs about 3.5 oz.
Grips - check out Magpul's K2 line and others to shave off a few more grams.

That's about it.  You can add a few other titanium parts like takedown pins.  Just remember, a few grams here and there add up to ounces saved.  Good luck - CW

i am about to finish a semi light weight build. i had light weight in mind when i was selecting parts, but i wasnt pushing it to the extreme.

complete build with MMBUS Pro is 98.54oz or 6.15 pound. i think it is light enough with no compromise

MFT Battlelink Minimalist Stock Mil Spec 6.14oz
MFT Engage AR15/M16 Pistol Grip  2.11oz
Mega Billet Lower - GTR-3H (Ambi) 8.85oz
Mega AR15 Megalithic Rifle Length Upper             20.14oz
PWS Enhanced Buffer kit 4.23oz
Battle Arms Development Enhanced Bolt Catch      0.72oz
Strike Industries Enhanced Magazine Catch 0.42oz
Fortis Hammer AR15 Charging Handle 1.5oz
Hiperfire Hipertouch ECLipse Adjustable Trigger 3.10oz
JP Enterprises GEN 2 SCS                                       5.25oz
SLR Adjustable Gas Block - Titanium         1.62oz
Lantac Dragon 556 Muzzle Brake         3.26oz
Lantac E-BCG                                                        11.65oz
Battle Arm 16" barrel                                               24.6oz