| Are the needles you're talking about for insulin pens? The smallest syringe needle I've heard of for insulin is 8mm.
The 5mm BD insulin pen needles are meant for "no-pinch-ups" on injections. The idea is to just jab and inject. I have, on a few occassions (less than 5 times), injected a "bubble" from the smaller pen needles, but I've liked them for my arm injections, when it's impossible to pinch up on my skin.
Novofine 30/31 needles are 6mm and just as nice. BD's 31 gauge needles are 8mm, just like syringes.
Oddly enough, I've met people who claim the "best" are 29 gauge 1/2" (12mm) needles. They weren't joking, either. Having been dx'ed just in March as a type 1, I guess I don't agree with their long-developed opinion for larger-is-better idealogy of syringe needles, but I'm still trying to learn! | 
