I have been through this a few times with my vet and last time he went online and checked the veterinary equivalent of the British National Formulary (what we use in human medicine) with me there so I could make sure!
This prescribing index says that rabbits can receive between 0.2mg and 0.6mg per kg of body weight in 24 hours - this is the recommended
range so what a vet prescribes could be anything within this range. However, exotic vets may give a higher dose than this - as it is thought that rabbits can tolerate a lot more than was originally thought due to the way they metabolise it.
The dose you give will be dependent on the
strength of the solution you are giving. So the dog metacam which is usually 1.5mg/ml given at the upper dose of 0.6mg per kg would equate to 0.4ml per kg.
It really is up to your vet based on their clinical findings, what it is being given for and because it is prescription only - only your vet can tell you what to give.
(When Nino has acute bloat I give him the full dose with permission of my vet - He is a 1.5kg bunny so he has 0.6ml. However, when it is very bad I have permission to go up to 1.0ml - this is for an acute pain where he is very ill. His daily maintenance dose for his arthritis is right down the other end of the scale - so it does depend on what it is being prescribed for. I personally feel that anything less than 0.6mg/kg for acute pain is not enough and that the range on the database needs updating a little in line with new findings on metacam tolerance of rabbits)
p.s. the syringe in the dog metacam box is much thinner than a normal 1ml syringe so if you are doing it in mls/per kg body weight then you need to use a 1ml syringe. The kg syringe in the box is purely for giving buns a certain dose x the dog dose, which I personally find far too inaccurate and never do it this way.