What determines the minimum chainsaw bar length for a particular saw. I can easily understand running out of power for long bars. My question is specific to a Husky 55 Rancher. The manufacturer states suitable bar lengths are 13" - 20". What would prevent a 12" bar from working just fine? I want a short bar for this saw to use while pruning my apple orchard. I find a lot of 12" bars available on evilBay, not many 13" bars at all. I can't imagine there are "rules" for minimum length as long as you match pitch and gage with the proper chain length but I don't know for sure if this would present a problem. Your thoughts and opinions?