My mare shakes her head pretty bad around the second barrel, she used to do it around all three barrels but ive found that she likes both my hands on the reins. i havent grabbed my horn around the barrel anymore and that seems to help with the other barrels, but shes still doing it only around the second. shes had her teeth done and has had the chiropractor adjust her and do PEMF, ive tried plenty of different bits on her including a hackamore, any tips?

I've never heard of anything like that. Is she young? Have you tried a tie down? Do you have a more experienced friend who could try her or critique your runs?

Some horses are very sensative about the material of the curb. Might wanna test a few different materials.

I agree with having someone watch you and see what you are doing... sounds like maybe something that you're doing is causing the head shaking.

You said you had a chiro adjust...did they do TMJ and Cranial Plates?  

Can you post a video?

Most of the time, when a horse is doing something like head shaking, it is something the rider is doing to confuse/frustrate/bother the horse.

The difficult part is figuring out what that is. 

I would wager --> Have you ever asked your horse to turn the barrel and completely leave your reins loose? Don't even touch her?

Would she not shake her head then? (use your seat and legs to turn, and leave your hand on the mane and leave her alone)

Without seeing a video, that is what I would tell you to try. And it's one of the HARDEST things to do (myself included), to completely trust your horse 100% that you literally don't touch those reins and allow them the confidence to do what they need to do.

 I just had a friend go through this with her daughters horse. No limp, no signs of anything wrong except the head shaking at the first and sometimes going to 2nd barrel. Vet found old torn hamstring and soft tissue damage in front end.