Well done for getting off the mark John.
Just to add what Dave said about squeezing in the mechanism a tad more, and give you a bit of a heads up - the mechanism has a little groove at the top of the brass section, which the manufactures reckon is where you should push it in to.
It's total nonsense, and the amount you push it in to depends entirely on how long your barrel is, (which is really what Dave has already said) So use the line as a guide only and aim to have 100mm from the end of the kit nib to the end of the mechanism on most slimlines or slimline variants, (which all seem to use the same mechanisms) in order for the refill a) to come out far enough, and b) to retract far enough.
I'm not saying you should measure it, in fact its quicker with a bit of practice to do it by trial and error, but I promise you that's how it works out. So you can see there's no such thing as a barrel that's too long or too short. (unless it's extreme or course)