On mechanical hackamores; they have the same effect as a curb, but if your horse doesn't like, let's say a straight mouthpiece in his mouth, he can prefer the hackamore. And the hackamore is a harsh bit(less), even the ''nice'' ones are harsh. BUT it serves it's purpose. A curb is also a very harsh bit, but when it's used in the right way, it can be a good guide and help for even a green horse.
As an emergency break, if the horse ''always'' is ridden without contact otherwise and listen to seat and leg, the hackamore is ok. Same if it's used to get the head down, if you use your seat and leg and manage to get the rest of the horse with you. Otherwise you'll just end up with a horse that's still rushing forward with 90% of his weight on his frontlegs, but at the same time biting his chest.. And some horses do need a bit of help to realise that the nose should be down, doesn't mean they'll collect just because the nose is down, but it's easier to get the rest of the horse with you if the horse isn't staring at the skies and biting against you. Well.. that's my point of curbs, including hackamores, in dressage-ish work. I thik your use of it is ok tho, but I had prefered a completely loose rein since no matter how light you think you're holding, it's much heavier on the horses nose than your hand.
Is it really a curb you've been trying? It has a chin strap/curb chain? :) Just curious. But I can imagine that reaction if you put a curb with even the easy contact you say you have, on a sensetive horse and without the snaffle (or riding cavesson) there to use as a main bit. Crow reacted just like that the one time I used only his spanish curb for a photoshoot, even tho he's used to having the curb working in his mouth - but with a muserola (riing caveson-thingie) in combination.
When it comes to snaffles, it's really about what your horse prefers. Except the twisted wire, chain, bike chains, chainsaw-chains mouthpieces that I saw when I visited america (what on earth are you people doing to your horses?! :O!! ) and the other less nice that you know about. Even if it's just a little different, let's say it has some small ridge or anything on it, it might be much harsher sinse that rige presses down and rubs sensetive parts of the mouth. The mouthpiece is for the horse to decide. The sides of the bit, is for the rider to chose ;)
But I can give you a little info: Straight bar snaffle
; this ones is usually liked by horses that doesn't want the bit to move around too much. My horse has this. It works a lot on the tounge, and some horses find that scary. It's a gentle bit tho, no upredictable effects that you don't count with when you see it and so on. Not allowed in dressage shows, I think. It can have lose rings or D-rings or so, the loose rings move more and gives a little more freedom, other than that I don't really see the differense.
A jointed snaffle
(not sure if that's the right name), work more on the bars and a little less on the tounge. I say - never use it with a tight noseband, since it may have a nutcracker effect and hit the palate when it does. Some horses prefer this, it doesn't move around as much as a french link. If you pull the reins, it'll squeese the lower jaw of the horses mouth, or the cheeks (if the horse has its head straight up in the air). French link/double jointed
also has a nutcracker effect, despite common belief, but doesn't hit the palate as easily as the bit above. It moves a lot and some horses gets distracted or annoyed by this.
Then there's snaffles with a low port
, that lets the tounge be alone, and I can imagine that lots of horses that dislikes preassure on the tounge and doesn't want stuff to move around, will love that.
And there's myler bits
, and copies of them, that doesn't have a nutcracker effect but still move each side individually. They move quite a bit, but gives the tounge free and give you a little more opportunity to work the horses mouth.
bits is something I dislike. It's much like a chain that twists around the lower jaw, and those balls on it rubs over the bars in the horses mouth everytime you pick up the rein.
I'm not sure of all names so I linked to pictures so you know what I meant :)
The thing with snaffles is that they work directly. If you pull the rein, the same amount of preassure comes straight in the same angle to the horses mouth. While on curbs, the preassure comes down on the neck, and much stronger on the nose/in the mouth, and under the chin. No option is bad or mean, but they are best suited at diferent situations.
Ask if you can try your friends snaffle first. If the horses arn't used to bits, they may chew on them a lot in the start.. but see how they respond to it and give them a little times to learn it.
If you want to use bitless, but still have a snaffle effect, the best option is a riding cavesson
or a muserola
. A sidepull works too but isn't as clear to the horse due to the placement of the reins.
This is a home made, bitless thing
I use/used for Crow. It's a hackamore for the curb effect, and a riding cavesson/sidepull for the snaffle effect :) The hackamore reins are always loose unless I needed them specificly. Pretty much the same as using a curb with a snaffle, or a curb with a riding cavesson :)