I am planning a build for a target/varmint AR.  Plans so far include the following:

RRA Lower
RRA A4 flat top upper
Carbon Fiber Forend
Jewell trigger

My big questions are this
If I want to shoot bigger bullets, 75 gr, what barrel length and twist?
Also, is it possible to get a barrel that is completely fluted bore to breach?  Does this cause a problem mounting the gas block?  And is it possible to mount the gas block under the carbon fiber forend, hidden?  If it is feasible, who makes such a barrel?  And I know of one carbon fiber forend mfg, any others?  Anybody make a carbon fiber stock?

Thanks in advance fellas

barrel lenght 20"or more
1-7 to1-8 twist for 75 grainers
fluting yes except gas block space as far as im aware.
Carbon forend- jp enterprizes make one, im sure there are others and i think armalite has them.
hidden gas block, yes
Barrel makers ,all do

gj,
I knew about the barrels with the gas block void, just wasn't sure if anybody makes a barrel without the void in the fluting.  
Is their an appreciable difference in barrel length on say 20" vs. 24" with the 75 gr?

And lastly, what other options are available for the stock?  I've seen the thumbhole, don't really care for that.  I'd love to see a carbon fiber A2 style stock to match the handguards.

Thanks again

CavArms makes a lower receiver/buttstock thing, I think it's carbon fiber?  It's all one piece.
You'll just need to get a low profile gas block to fit inside your handguard.
I haven't seen any barrels fluted under the gas block.
As far as I know, longer barrel = more muzzle velocity(up to a point of course).  The numbers that I've seen indicate roughly 100 fps per inch.  

ETA: CavArms MKII

Text from page:

CAV-15 MKII Receivers are injection molded from glass filled nylon 6, a polymer material very similar to that used by Glock, HK, Steyr and other firearms companies. Polymer offers unsurpassed corrosion resistance, finish durability. and is significantly lighter than aluminum based receivers.  

The receivers are molded in two halves and assembled using a vibration welding process.  Vibration welding forms the two halves into one solid piece.  Every contact surface between the two halves is bonded together with a weld that is even stronger than the parent material.

Quoted: I am planning a build for a target/varmint AR.  Plans so far include the following: RRA Lower RRA A4 flat top upper Carbon Fiber Forend Jewell trigger My big questions are this If I want to shoot bigger bullets, 75 gr, what barrel length and twist? Also, is it possible to get a barrel that is completely fluted bore to breach?  Does this cause a problem mounting the gas block?  And is it possible to mount the gas block under the carbon fiber forend, hidden?  If it is feasible, who makes such a barrel?  And I know of one carbon fiber forend mfg, any others?  Anybody make a carbon fiber stock? Thanks in advance fellas

Quoted: gj, I knew about the barrels with the gas block void, just wasn't sure if anybody makes a barrel without the void in the fluting.   Is their an appreciable difference in barrel length on say 20" vs. 24" with the 75 gr? And lastly, what other options are available for the stock?  I've seen the thumbhole, don't really care for that.  I'd love to see a carbon fiber A2 style stock to match the handguards. Thanks again

In my opinion YES
(The 24" vs. 20")  the 24"= higher velocity,,, =s longer range hits and also better ability to stabilize  75 and above bullet weights ! with 1:7 & 1:8 twists