You have a start of a nice build. Here's a couple things you need and to change.
Change the DSA Buffer to find a Mil-Spec Extension Tube to a 7075. If a buffer extension tube doesn't say 7075 for it's alloy description, then it's only 6061 alloy and not a true mil-spec inside and outside w/proper machined threading. Nothing wrong with it, but you're going this far with a nice build---do it right the 1st time and you won't be sorry.
14.5 barrel. You WILL NEED a 2" flash hider/compensator or suppression mount system. It WILL NEED to be pinned and welded to be "minimum" 16 inches for legality. UNLESS, you apply for a "stamp" to have weapon used as SBR(short barrel rifle) OR Suppressor usage. Apply and receive stamp before build to be legal if going for SBR, and another stamp if also going with Suppressor. $200 for each stamp.
This is also if you decide you're going as a rifle build and not a pistol build, in which case, I believe the barrel/flash hider can't be over 12" total to be considered a pistol.
Also AP Barrel, while it does specify 4150 CMV, it doesn't say if it's chrome-lined, but most are at this hardness---might want to call AP to ask. Never mind as I see barrel is QPQ treated.
You need a Carbine Handguard(round style and not triangle type).
You'll also need FSB(front sight base), FSB Barrel Pins, Front Handguard Mount Plate, Front Sight Elevation Tower and Spring, and a Stainless Carbine Gas Tube and Gas Tube Pin, if it doesn't have one. For FSB, you'll need to (completed upper assembly--except for FSB) take to gun smith to have barrel drilled for FSB pins for proper alignment. Pins are tapered and require special drill bit.
Need a Rear Flip Up Sight or Carry Handle Sight System that is QD(quick detach).
Another way to get around the FSB is to go with a 9"(stock length) to 13" Free Float Handguard Kit of your choice and you'll need a "low profile" Gas Block. I prefer a steel gas block over aluminum in it's strength and also being the barrel is steel, they both expand/contract the same. You go this way, the FF Handguard will include it's own Barrel Nut, so you won't need to install the Delta Ring Assembly or front handguard attachment ring.
Get you some Aeroshell 33 grease for the barrel nut assembly---no matter what way you go. Or use some synthetic wheel bearing grease. Very little grease is used for barrel nut. Learn to install and torque the barrel nut correctly according to which handguard system you go with.
Get a vise, torque wrench, AR15 punch set and brass hammer, masking or electrical tape(you'll understand after you build just one), AR15 multi-task wrench(es), upper/lower receiver build blocks. Watch youtube videos to build and learn, learn, learn.
I think I gotcha all you needed, with a couple changes for the better. Magpul is good stuff, but they don't have FF handguards. You'll need to go elsewhere like Yankee Hill, JoeBob Outfitter, PSA, Brownells, MidWayUSA, Amazon, lots of others. Just watch for Chinese stuff as some is ok, while much is crap.
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Last edited by Triumphman; 05-23-2016 at 08:46 AM.