tv:
re twist rates and bullets:
Rifles are called rifles because the barrel has "rifling", which is a set of grooves (or something close to that) engraved in the inside of the barrel. These grooves twist, and their primary purpose is to impart a spin to the departing bullet to make it stable in flight. 1:7 is the ratio of the spin, and is the tightest spin generally available on a 5.56 cal AR-15. 1:9 is the common "compromise" twist rate, and you'll also see some 1:10 and even 1:12 guns. The tighter you make the twist rate, the heavier bullet your barrel can stabilize. The downside is that if the twist rate is too tight and you shoot very lightweight bullets, they could fly apart and fragment in the air from excess twist.
People who use an AR-15 to kill very small game (called "varmint" gun builds, because they're used to shoot small varmints like groundhogs and squirrels, etc) like to use very light bullets (40-55gr weight) at very high velocities, so for them a looser twist makes sense (1:10 or 1:12).
Standard issue US military ball ammo in 5.56 comes in 55gr (which will stabilize and be reliable in just about any twist rate) and 62gr (works fine in 1:7 and 1:9 barrels, but may not stabilize well in 1:10 or 1:12 barrels).
But according to modern ballistics research, the best performing rounds (on humans) in 5.56 tend to be the really heavy ones, like the 69gr, 75gr, 78gr, and yes even crazy 80gr rounds. 75gr Hornady TAP is very popular for a defensive round in the AR-15, for example. None of the heavier ones will stabilize in 1:9 reliably, only in 1:7. Some people have success with the 69 in a 1:9 barrel, but it's kindof a crapshoot depending on the barrel and bullet mfg.
Re the uppers you're looking at:
The POF upper you're describing is what's known as a piston-driven upper. Piston guns are a matter of a lot of contention and debate in the AR-15 community. The original design of the AR was to be gas-driven, whereas some other rifles like the AK-47 are piston driven.
Someone decided you could get the best of both worlds by modifying the AR design to be piston-driven too. Upsides are supposedly less recoil, and that the upper receiver and the bolt stay cleaner (less burnt stuff blowing on them all the time), but they still do require lubrication (don't let anyone tell you not to lube an AR just because it's a piston design, it still needs lube, the lube will just last longer). Downside is it's a relativelty recent and unproven serious design change to a good design that's proven itself well over the course of decades in the field. One of the biggest specific concerns is that the bolt is now driven back with an off-center force, which could cause galling of the upper receiver surface. Also has more small fragile parts than the original design, which may or may not fail early (not enough data).
LWRC also makes piston uppers, and are generally considered to be one of the highest quality piston uppers you can buy. If I were going piston, I'd shell out for LWRC instead of POF, but that's just me. Then again, a lot of people will tell you not to mess with a good thing, and just go with the original gas design, which is what you commonly find on all the major AR-15 variants (standard in Colt, Bushmaster, LMT, Knight's, Armalite, etc, etc).
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