AR 15 Build.

[nepawolf]

If you're looking for a marksman of varmint gun, go with the 20. You could compromise and use an 18, but the longer the barrel is the better the velocity of the round and the better the accuracy. Remember, it's not just the size of the barrel, it's how it's coated that can make or break it. A nice crown wouldn't hurt either though.

Barrel length doesn't really have any bearing on accuracy. You might lose 50fps using an 18" barrel versus a 20" barrel. The 18" guns are nice because they are a bit more maneuverable and still use the softer recoiling, rifle length gas system.

Non chrome lined barrels shoot about .5 moa better than comparable chrome lined barrels. Outside coatings have no real bearing.

You can buy a White Oak Armament stainless barrel for under $300 and those will shoot .5 moa with handloads. They come with an 11 degree target crown (threaded) and can be had with M4 ramps. It's the best bargain in accurate AR barrels.

I was referring to a moly or ceramic barrel lining, not an outside liner. The only way an outside coating could really effect accuracy is with rapid firing if it doesn't properly dissipate heat.