Be careful with copper on horses who tend to chomp on the bit, its quite soft and horses can make sharp edges on it with their teeth! Casey LOVES her Myler Comfort Snaffle loose ring. I got it for $60 on dover: Myler Loose Ring Comfort Snaffle Bit | Dover Saddlery
And Candy has a double jointed loose ring. CALEVOŽ -> Sprenger - KK Ultra double jointed loose ring snaffle, Aurigan-Aurigan Rings, 18mm thick - EUR-93.90 - Sprenger - For*the*Horse - Bits - DoubleBrokenBits - Sprenger KK-Ultra aus Aurigan mit Auriganringen, 18mm Strke - Sprenger
Casey's bit is single jointed with a barrel (roller) over the joint preventing the bit from full nutcracker motion (when you put pressure on the reins of a single jointed bit it curves up and can poke the horse) but not conforming to the mouth as much as a double jointed so its not quite as soft. I am not a huge fan of single jointed bits because of the nutcracker motion, unless you have a horse with a hard mouth or are in a sport where you don't have contact on the reins really. A roller gives something for the horse to play with which helps soften the horses jaw and accept the bit.
D-rings and eggbutts are both pretty similar, basically the only difference is that d-rings are mainly seen in the hunter ring. Here is a double jointed d ring. The middle joint on this is a lozenge, which is like a french link but its an oval and so there is less to poke the horse in the mouth. http://www.stuebben.com/bdb/img/800/1000113.jpg
And a double jointed eggbutt with a french link in the middle. http://meisterider.com/images/French...ffle%20Bit.JPG
Both the eggbutt and the d-ring help steer the horse by putting pressure on the side of the face, and I would guess the d-ring would be a little better with putting pressure because of its d shape. Theres also a full cheek, which puts the most pressure on the side of the face out of the d and eggbutt for steering. Its popular in starting horses because it helps link the pressure on the side of the face from a halter to the pressure in the mouth and side of the face (with the other leg aids of course). http://images.auctionants.com/140530.jpg
You can tell copper apart from other metals because of its orangy tint. And.. I may have made some bits sound quite evil, but when I say that the french link will poke a horse because of the dog bone shape of the middle joint its a very, very mild little poke. It just defines the french link as a little tiny step up from a lozenge on the bit harshness scale. And I only described the most common snaffle bits, and I left out loose ring but its really time for me to go to bed (hope that makes sense, I'm oh so tired)!