Not being a frequent vaccine giver, I used Williams advice and purchased the 22 gauge. If you are using the needles on small animals like cats, I recommend getting the 25s. The needle is smaller, so it probably depends on the application. I found that for vaccinating outdoor and battle-hardened neighborhood strays, the 22s were almost impossible to insert subcutaneously. My vet confirmed that old tom cats have skin like leather. So, Id recommend at least considering the littler needle or getting a few of each if you are working with both spoiled inside felines and street punk toms. The amount of time it took to inject with the smaller needle was insignificant.
these are very good quality hospital grade syringes with sharp needles. Don't get the 25 ga. They are just too tiny to use routinely. Start with the 22 ga. X 3/4 inch which will cover all situations. Occasionally a syringe needle will pop off the syringe during the injection because they are not locked on....doesn't happen often so no star deducted.