1.5" reaches down to the corner of a 3ml vial, making extraction easier. 21 gauge punctures the BAC seal less. But I am just starting to use this toy for reasons of merely having a cool new tool that you do not have: https://www.amazon.com/dp/B0F319TL9L and https://www.amazon.com/dp/B0DTV1QRDP . So I need to stop bitching about the price of BAC vials as these add to the cost of an oil change. I found that punctured 3ml cartriges still leak air on purging even without a needle in them. So, I started using smaller vent and fill needles which help but do not entirely eliminate the leakage/weakening of the rubber membrane I use for protection when injecting my girlfriend's big brown beaver: https://www.amazon.com/dp/B0GMXDWRLN . You may copy me but I will still continue to have cooler tools than you because I just can't help myself.