AR-15 Upper Mods?

[TooTaxed]

I know you can help me, folks! I plan to order a new flattop upper for my pre-ban Colt SP1 CAR-15 5.56 NATO, as it has a 1:12 (or 1:14 ?) twist barrel, and I want to shoot long range. I'm considering the Rock River Varmint and National Match uppers. To help me order, I have the following questions about AR-15 upper configurations:

(1.) Exactly what do I gain in velocity or accuracy in going from a 16" bbl to a 18" bbl, and up to a 20" bbl? I'm told it's negligible... (I plan to order a flattop and mount a scope.)

(2.) What range of bullet weights is best for a 1:8 twist? A 1:9 twist? What weight is optimum for each?

(3.) What are the pros and cons for a standard configuration AR front sight? (The National Match upper standard.) A Weaver gas block? (Varmint standard.)

(4.) What purpose does a high rise flattop option serve?

(5.) I've been told that the Rock River uppers are small pin same as my pre-ban lower, and I won't need an adapter pin. True or false? (It's odd that this isn't clarified in their catalog...which contains both adapter pins.)

Thanks!

 

[rbernie]

1) Dunno - never chrono'ed anything other than a 20" barrel and a 24" barrel. I'd guess around 40fps/inch, but that's just a guess.
2) As a very generic rule, I hear that 1:8 likes 70gr+ best, and 1:9 likes 62-70gr best. My only personal experiences have been with 1:12 barrels, and they maxed out at 62gr.
3) A low-profile gas block gets rid of the front sight. If you're always gonna scope the rifle, why have the front sight tower out there to catch on things? Besides, if you get a gas block with a Picatinny rail, you can always mount a BUIS front sight. OTOH, if you want/expect to use iron sights under anything more than a backup basis, you'll probably want the traditional front sight/gas block.
4) A true flat-top with low-mount rings will position your head too low on the stock, making your cheek and neck really hurt. Most folks with flat-tops seem to wind up putting risers on the rail to elevate the optical platform; a hi-rise upper simply accomplishes the same thing without requiring that you add any additional parts.
5) Beats the snot outta me.