chesty: check pm's

FOC is Front of Center. Like a dart, crossbow arrows fly best if their balance point is in front of the center point. Usually this is measured in % FOC. For example, if an arrow is 21" tip to tail, the center point is 10.5". If the arrow's balance point with broadhead is 3" in front of that, it is said to be 3/10.5 or 29% FOC. This is the point the centering force from the vanes is working against. The shorter the vane and the longer the distance from the vane, the more this force centers the arrow for stable flight.

There is also an effect caused by arrow spin. It is said, but I have no test data to prove it, that the faster an arrow spins, the less likely out of eccentric forces are to pull it off line. I think this is basically the principal which which a childs top works with. When you spin them fast enough, they spin in place. As they slow down, they move around. I use a Blazer vane with right helical. I'm not sure if left or right helical is the trick, but I do see helical deflects more air and thus should give more spin. Does it work better than offset or straight, I don't know. Offset or straight might be mandated if the barrel is narrow on your crossbow and your vanes are long. That might cause a touch condition which surely would be bad.

So first, I spin test arrows to get the best spin balance and also to check straightness. I make them straight by putting inserts in the shafts and then puttingpractice points and checking straightness If not straight, I turn it 1/8th of a turn and recheck. I can always find a straight point. This makes the broadhead straight.

Last, I use one different colored nock so I know which one to put in the barrel for consistency. It may not matter, but with flat nocks, this may help a shooter consistently load arrows. If one is bad, it would be culled, checked for straightness,checked for spin balanceand rebuilt.

Right now, I don't balance inserts/shafts/broadheads to a single weight, but I could see the value in it if I wanted to shoot over 50 yards. Out to 50, these tricks get me ondeer. Now, how far would I shoot a deer, well. . .I need to prove this all works at 20, 30 and finally 40, before I consider going farther.

I can tell you that at 467 grains and 300 fps, I can shoot through a layered foam broadhead target. That's some penetration! A couple of grains heavier and I will need a second for back up!

FOC is Foreard of center or Front of center and it is very important to good arrow flight.
The FOC % of your arrow indicates how much weight you have forward of center,opinions differ on the proper amount.
If your arrow balances in the center,you dont have enough weight FOC.
If you add a lighted nock to your arrow,the weight at the back will change your FOC and you may need a heavier BH or a brass insert with you regular head.
There are so many variables.

The original poster
Lucky Shot
That is very different POI (point of impact) between BH's and FP's There is most likely some other problem.
I use G5 Montec's while the POI is different from FP's it's not that much.
I rarely shoot FP's anyway,the Montec's are resharpenable so I shoot with them most of the time.
You could just sight in with you broadhead of choice,make sure it is shooting good at all ranges and go with it and not worry about the FP's for now.

Good luck and let me know how it turns out for you

With all due respect, I believe you are mistaken. 3" diff = 15% FOC.

First off, try a short 4 bladed broadhead like Slick Ticks. I found that out the hard way after missing 2 bucks with Thunderheads. Large heads make short crossbow arrows plane. FOC is very important in crossbow arrows. The extra weight up front, coupled with less weight in back, (I use 2" Blazers) actually 'pulls' the arrow to the same point more consistently:a.k.a accuracy. I use Gold Tip Lazer II's with 110 gr brass inserts and 2" Blazers. When coupled with 126 grn slicks it gives me nearlt 25% FOC and a very accurate arrow. They shoot essentially the same as my field point, I don't even bother to practivce with broadheads.
Mecahnicals are ok if they don't open in flight, which the sudden surge of 175 lb draw an easiely do. I've tried NAP Spitfires, they fly OK but not as close to same PIO as my slicks.

Awshucks, can you please tell me how to calculate FOC. I seem to be having a brainfart.