Adnan, may I ask what you will be using this particular needle for? The reason I ask is that neither AWG (American Wire Gauge) or SWG (Standard Wire Gauge) provide sizing designations for crop feeding tubes, even though they are referenced by gauge sizing.
To clarify, an 8 gauge feeding tube/crop needle has an internal opening/diameter of 4mm or 0.15748 inches, while an 8 gauge hypodermic type needle has an internal opening/diameter of 3.429mm or 0.135 inches.....there are also different diameters for sewing needles and knitting needles, so I can understand your confusion.
Since you asked about crop needles, I am going to assume you want something to feed baby birds with, so you probably need to search for suppliers of veterinary or animal breeders supplies. I doubt if you would be able to find them at a local pharmacy or chemistry shop , so I found you a couple of internet suppliers:
As the page at the above site mentions, crop needling requires a special technique, so you will want to get properly trained before attempting this. Many breeders/hand feeders use a rubber/vinyl flexible tube instead of a metal needle for feeding.
Usually the length increases as the size (not gauge) increases.....this is because smaller needles for smaller birds, trading up as the bird grows.
If the sites/vendors you are looking at offer needles of the same length, you need to expand your search of vendors/products.....maybe search by specie as you would not use a feeding tube/needle for a macaw on a finch chick.....
