I'm in search of a bolt carrier for an psuedo-SPR that I'm building.  I've found a lot of different options out there and I'm wondering if any of them make a difference that can be noticed in actual shooting.  I've got a lot of experience in long range shooting, but little in building extremely accurate rifles.  Any help would be greatly appreciated.

Now that's an answer that's right to the point!

But why would anyone buy a JP Enterprise bolt carrier or something else exotic when a standard mil spec does the same thing?

tighter tolerances
lighter
heavier
prettier
tailored
bling

Quoted: Quoted: But why would anyone buy a JP Enterprise bolt carrier or something else exotic when a standard mil spec does the same thing? Because no one wants to admit that most  standard ARs will shoot 1 MOA out of the box with good ammo.  Its easier to blame the equipment than the shooter. Dont worry, I'm sure some dealer with shelf full of these things  will chime in about how they are better.

WHAT DO YOU MEAN BY THAT?

Any advantages or disadvantages to lighter or heavier bolt carriers?

As for tighter tolerances, if a maker really wanted to make them up that tight it seems that they'd have to produce the carrier to fit one particular upper.

Quoted: Any advantages or disadvantages to lighter or heavier bolt carriers?

Yes it changes the timing of the rifle.  On an over-gased (carbine length gas systems typically) gun it's the modus operandi to add mass to the carrier and buffer.  In the race gun crowd it's well known that even with a compensator a heaver recoiling assembly gives 2 jerks to the recoil, one when the buffer bounces off the rear of the buffer tube and one when the bolt slams home.  Momentum is momentum, the heavier something is the harder it is to start and stop.

So theoretically a lighter bolt would assist in keeping the rifle on target as it wouldn't bounce as hard at the most rearward position and wouldn't slam as hard when it hit the barrel extension.  How would this effect realibity?

If your talking about the reliability of the super light carriers made out of aluminum they are replaceable items, some have better luck than others but the most I've heard is 9k rounds.   The light weight steel and stainless steel carriers are mechanically as reliable as a GI carrier.  Now when it comes to feeding and what not it depends on the interaction of the gas system, the mass in the carrier and buffer, the buffer spring and the mag spring, there is a lot of interaction going on here.  An over-gassed lightweight recoiling assembly can out run the mag spring and cause a jam.  The mag spring can't keep up with the cyclical rate in this extreme case (I've had this happen with a carbine when it got too hot with an H buffer and an AR carrier).  But if the gas system matches the recoiling mass it's not a problem.  On the other end of the spectrum if your recoiling mass is to great for the amount of gas you can short stroke the rifle.  The trick is to get everything to work together, as they say there is more than 1 way to skin a cat.