Hi I'm just to start my first ar-15 build.  I do have back ground in 1911 building my last gun was an sti hicap limited gun, the only thing I didn't do myself was the hard chroming.

Well there's my skill level, so I don't think there is any part of the ar15 I wouldn't be able to handle, but I have only ever just fired one, that's the most I know about them.

Here's the question

I coyote hunt and shoot ipsc and would like a gun that might not be the best for either, but would be a good gun for both.

I think what I want is an 20" barrel. ok that seems like the easy part but I have found bull barrels and what model1 calls standard stainless barrels, would I want the bull or maybe the stainless because of weight?

The other question I have is twist rate, it looks like I can get either 1/8 or 1/9. I guessing that the 1/8 is for a lighter bullet and the 1/9 is for heavier bullet. what kind of weights can I run with either? Lets say for a min. that the 1/9 runs good with an 55 grn. bullet, what happens when I run a 40grn thru it? does it go from 1moa to 5moa?

I would like the flat top version, do the add on sites work ok? I will first shoot this in the limited class where you must use iron sites.

thanks in advance for the help guy's...

Bob

Actually, I'd recommend a 16" barrel for the shooting you mention.  For most people, the carbine will swing faster and can be just as accurate as a 20" for hunting under 200yds.  

The bull barrels are very heavy and not a good choice for competition unless you're sitting at a bench.  The stainless or regular chrome moly would serve you well.

You have your twist rates backward.  Fast twist is for heavy bullets, slow twist is for light bullets.  The 1:9 is a good compromise for most bullet weights.  The 1:8 is popular in the DCM/Highpower crowd for the heavier match bullets.  1:7 is the current twist for USGI M4 and M16 rifles.  1:12 was the original rate for 55gr USGI ammo.

I shoot 45gr - 77gr through my 1:9 barrels with very little difference in MOA at 100yds.  At longer distances, the heavier bullets perform better with this twist.

If you get a flattop, you have many options for iron sights.  You can use a removable carry handle which mimics the A2 or add one of the aftermarket BUIS from KAC, ARMS, DPMS, LMT, etc.