Hello Elaine,
Most if not all ballpoint, rollerball and Gel ink pen
Kits pull apart to access the refill.
They need to be somewhat tight because they are generally twist pens as apposed to desk or click pens.
When you twist to bring the nib into play, a loose barrel would just unscrew and the nib would not deploy.
For more information on pen kits I can recommend 'Timberbits' in Australia. It is the company I use and have done, for quite sometime.
It is a one man company in Sydney called David Lim. His kits are of high quality and not expensive.(does take a little time to arrive though)
His website is full of useful information including diagrams of all of his pens.
I hope you enjoy getting into pen making I do because the variety of pens to make is almost infinite.
Here's a few of the one's I make.
Sierra with Eucalyptus Burr.
Sierra with a Pickguard insert in Purpleheart.
A Sierra in African Blackwood with an accent in Box ( Celtic knot).
A Sierra made with 3 woods African Blackwood/Box/Padauk segmented in 4mm slices.
And finally A high end Fountain pen in Gold Titanium with Rhodium accents again segmented, same woods in a helix pattern.
John. B