You can certainly use a non-flanged carto in a F16 tank. It will fit just fine and, provided the inner o-ring on the tank is in good shape, it will not leak. I do it all the time. Just don't forget that you no longer have a flange that stops the carto from sliding up or down. For example, if the drip tip is stuck on and you pull hard to get it off, the carto will come along for the ride. Can be messy if the tank is full!