I want to start gathering parts for a Mk 12 Mod 1 style precision rifle.
I would definately place field / range accuracy over technical accuracy in this build, so there's a some leeway in parts selection.
This means that I don't need a KAC rail, I can use another vendor's rail system.
Barrel - Where to get a good, accurate barrel ? 1 MOA isn't good enough, my mid 90's Colt HBAR will do that with PMC 55 grain ammo... I want better.
Rail - Recommenations ?
Flash hider - Is the Ops Inc. more accurate than say a Smith Vortex ? Need some help here. The rifle won't get a suppresor anythime soon,
I have a Spikes lower with an A2 stock, so that will be the basis with a gissele trigger system
BUIS- I like the ARMS 40L that I have on another rifle and will probably use a set.

I'd like to get evarything ordered now so that I;ll have it in hand for Christmas

I'd start here, Great info. & feedback. Its a pic thread, but most SPR related info. can be found from the postings.
http://www.ar15.com/forums/topic.html?b=3&f=118&t=294303

Have a great one

so far for mine i have:
noveske spr 18 in barrel
noveske dust cover ()
noveske lower, w/ magpul grip
magpul prs buttstock (not pictured)
kac spr rail -non float- (not pictured and still in route)
few parts i am still working on, good luck with your build and im me with any questions

Noveske SPR barrel with Vltor VIS or Switchblock if you want to run a can.  I haven't seen a more accurate barrel than Noveske,  and I was skeptical until I tried one.

The Daniel Defense M4 12inch rail would look right and be cheaper then the KAC rail.  Not to mention it is good quality.  The LaRue 12 inch rail is another option, but you would be looking less like a MK12.

White Oak makes a good SPR barrel that is one of the cheaper options out there.  It is also a good shooter.  SubMOA with good ammo should be easy.

As for flash hiders, if your not going to use a can, no point in spending the money on the Ops Inc mount unless you are going for the look.

If you do end up buying BUIS, consider the KAC 2-600m micro.  It would be more adjustable allowing you to use the irons on your precision rifle out to longer distances.

Locating a barrel is going to be the hard part, most everything else you can make substitutions for and still look the part.

If you want accurate, have a barrel profiler (like Compass Lake) cut you a SPR profile barrel out of a Krieger (or other "benchrest grade") stainless steel blank. My 16" Krieger on my RECCEish forgery can do under 1/2 MOA with factory loaded match ammo (Fed Gold Match)

If you are really doing a mk 12 (semi) clone, I think that is the one area of the appearance that you just got to have.  In my mind, a precision rig should have a brake/comp over a FH anyway.

Otherwise, if you just want a good, accurate rifle, that works too- just don't bother with the mk12 nomenclature, its just going to get in the way.

I used a Douglas Stainless SPR barrel from Compass Lake Engineering. That's the brand the military Mark 12 Mod 1 uses. Reasonably priced at $315 shipped

Update!
I ordered a Kreiger SPR profile barrel & bolt from Compass Lake Engineering and a BCG & charging handle from Spikes.
I looked at Ops Inc.'s website and didn't see the muzzle brake separate from the suppressor. Can I get the brake only ?
Spikes didn't offer an upper receiver in standard configuration ( no M4 cuts ) or I would have ordered it from them. Who's another mfg that makes a good standard flat top upper ?

Yes you can get them without getting the can. Look at ADCO or Rainer Arms. They run about $180. Or call Ops Inc and ask them. Their webpage is in need of being updated.