I'm going somewhat disagree with tiny here. While the main part is the mouthpiece - I completely agree, some horses are very sensitive to what kind of rings you use. My qh absolutely hates O-rings because of the movement, but happy with the stable eggbutt, on opposite my paint while doesn't mind any type of rings clearly prefers O-rings that keep her entertained (and she manages to lean and lock on D or eggbutt).
If you switch to D you won't ruin your horse, but personally if he's happy in O-rings, why to spend money on new bit? BTW, lots of eventers here go in O-rings.
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