BARRELS- Don't get too hung up on barrel choice. For awhile, I waffled between the long life between of the chrome lined 4150 CMV and accuracy of the of the unlined stainless steel barrels. When I finally pulled the trigger on my PSA carbine kit, I finally settled on the stainless steel barrel for two reasons-
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1) I wanted to see if the durability of the stainless steel barrel would stand up to a heavy firing schedule and-
2) more importantly, the PSA carbine kit with the stainless steel barrel became available before the FN 4150 CMV barrel did and was at a price I could afford.
Regardless of the barrels selected, short of simple abuse, it will take a lot of rounds before a good barrel truly wears out. For a guy like me, the CMV offers little advantage in barrel life over the stainless steel and the unlined offers little advantage in accuracy over the lined barrel when using bulk ammo. There may be a half minute difference when using good ammo.
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Shot with the precision AR from a bench at 300 yards using bulk ammo. 2.5 - 3 MOA
So what it comes down to, for everyone agonizing over which barrel to choose for your general purpose carbine, choose a quality barrel, whether it's chrome lined CMV or stainless steel.
LEGOS- There are a lot of folks out there that will tell you assembling an AR is as simple as Legos. Compared to building a FAL from a kit, assembling an AR is rather simple.
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But, if you don't get the right parts, or don't understand an assembly procedure and it goes sideways on you, it can get complicated quick. No matter how easy anyone may think assembling an AR is, if you don't take time to pay attention to the details, the devil will be smiling over your shoulder.
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CONFIGURATION- Get a good quality 16" barrel and a free float tube. Heavy profile barrels aren't needed for accuracy. It's the quality of the chamber, bore and crown that counts. A 16 inch barrel gives good reach while being short enough to be truly handy. The only thing a shooter needs to do to change a carbine from CQB to a precision rig is change the sights. An SBR would be handier for house clearing and a 20" bull barrel musket will serve better picking off P-dogs way out yonder, but a shooter with a 16" carbine can do either task adequately. There is a reason why a 16" AR carbine is just about the single best selling rifle in America today.
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USE THE RIGHT TOOL- There are folks who will use Vise Grips for installing some pins. Don't do it unless you don't mind the rifle looking like it was gnawed on by the odd angry beaver. You might get away with it once or twice, but sooner or later, you'll slip and mar the finish.
Be careful installing trigger guards. It's easy to break the little ears off. Brownells sells a block that will support the opposite ear when tapping the trigger guard pin into place. Another way is to use a c-clamp. It will support the off side ear while gently pressing the pin into place.
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Use a tappet wrench to install muzzle devices. Regular wrenches are wider and when installing a muzzle device with a crush washer, the crush washer will sometimes grow in diameter as it compresses. It will trap a regular width wrench and cause marring of your parts. You can see the difference in width of the two wrenches in the photo below. I used the regular width wrench to install this Battlecomp and it boogered things up a bit. I made sure to get a set of tappet wrenches shortly after.
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A magnet can be a big help installing small pins.
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Assembling the receiver inside a large clear plastic bag can save you many hours of searching for tiny springs and pins if you should slip while trying to install them.
Use a long brass punch and masking tape when pinning the bolt release into place
If you find yourself getting tired/frustrated/hungry/thirsty, take a break