FOC - Front Of Centre - Balanced Carbon Arrows.
FOC - Front of Center:
A projectile's flight is most stable when most of the projectile's mass is positioned Front (or Forward) of Centre [FOC]. As such, an arrow should be heavier in the front than in the back.
If you've ever played a friendly game of darts, you've surely noticed that the dart is designed so that it's heavy in the front, and light in the back.
If the dart were weighted the opposite way, with the tail being heavier than the tip, it would literally spin around and hit the target tail-first. Obviously the ballistics of a dart and an arrow are a bit different, but the underlying concept is similar.
Where's the "perfect" balance point?
This is another hotly debated issue among archery enthusiasts. If your FOC is really really out of whack, it's an issue. But don't spend too much time splitting hairs about whether your FOC should be 9% or 10%. If your FOC is reasonable (7-15%), your arrows will function as they should.
The ballistic physics for FOC include some rather elastic variables that make finding an "mathematically optimal" FOC very difficult to declare and prove. So don't pull out your scientific calculator on the FOC issue. It's not necessary. Just choose a reasonable value and move on.
What puts your FOC Out?
The only real danger of slipping off the FOC precipice is if you use really heavy fletching and super-lightweight target nibbs, or if you choose small light fletching and a jumbo tip weight (or a heavy brass insert). For common arrows with basic vanes or feathers, aluminum inserts, and 85-125 grain tips, chances are your FOC will come out just fine.
With all that said, it is generally believed that an arrow with a high FOC will fly well, but with premature loss of trajectory (nose-diving). While an arrow with a very low FOC will hold its trajectory better, but it will fly erratically. So again, another trade-off for you to consider.
Calculation of FOC
So computing FOC is pretty basic. As an example 30" long arrow has balance point that is 3" forward of the arrow's actual centre (15"). So its FOC is 3/30 or 10% of the total arrow length forward of the arrow's physical center.
Example 2: If a 28" arrow balanced 2" forward of its physical centre, you would compute the FOC as 2/28, or 7.1%. Easy!
So when you order your custom arrows, keep FOC in mind. If you choose heavy 5" vanes and an anorexic 50 grain tip, you'll likely have an FOC that is too low. On the other hand, if you choose 3" lightweight feathers and a jumbo 175 grain tip, you'll likely have too much FOC. So try to avoid opposite extremes at the ends of the arrow, and choose an arrow setup that will give you an FOC balance of roughly 7-15%.