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cajunautoxer wrote:This is what built for a buddy of mine recently
Lower
Fat Boy Tactical $50 blemished lowerLPK w/o trigger $40
G2S trigger $165 or you can go with ALG Mil spec $45 and JP yellow spring $12 (best combo for less than $100)
Magpul STR stock, moe grip, stock assembly kit, hand guard (can't use unless you go with carbine barrel), and vertical grip $99 (botach service can be iffy so I used my AMEX)
curved trigger guard $20

Upper
Complete upper $100 on sale $75
Handguard ALG modular 12" $145 (has everything you'll need) Now if your going with a 20" barrel you'll want a 15" hanguard. I like JP myself $225
BCG $80 or coated $130 AIM has coated ones for $100 when it's on stock(i used one)
Comp use Miculek's can't beat it for $45 or ALG has one for $35
Barrel I can't really help you since I've never researched 20" barrels. It also depends on what you want to do with rifle also. I'm into 3 gun matches so a barrel that can shoot 30-40 rounds quickly and maintain descent accuracy is important to me. I've been happy with Nordic barrels myself. If you go with their 18" the gas ports are under sized so that you can use a low mass bcg without having to use a adjustable gas block. But then you need a low mass buffer and spring