I can do pull-ups in the 6-8 range, but I can't flexed-arm hang with my chin over the bar for more than a few seconds. Have I unwittingly been using momentum to get my chin to the bar without a full range of motion? Or might my anatomy make the flexed-arm hang difficult, with my top range of motion being slightly lower than my chin?
When you are at the bottom of a pullup, and your arms are straight, your biceps start the lift. Your back comes into play when it's time to pull your elbows back, which gradually becomes more relevant as you get higher up.
Yeah, it sounds like your biceps are doing most of the work during your pullups. You gain a lot of momentum so that your back doesn't really need to contribute a whole lot. It's just icing on the cake, so to speak.
Start doing your regular pullups, but try doing them very slowly. That way, you have no momentum, and your back will have to do more work in order to transition from the start phase of the lift, into the top portion.
Key point here is to always be in control of the movement.
Sets of 3-5 reps is absolutely fine, as each repetition will have way more work.
I.e. climb up to get your chin over the bar, and lower yourself slowly. Very slowly. This works all the same muscles, but in reverse intensity, so your back is the primary mover from the beginning.
Key point here, again, is to always be in control of the weight.
Sets of anything between 5 and 12 is cool.
This is where it gets fun and creative. Here's one I like. For each rep, do this;
go 1/4 of the way up
go 1/2 of the way up
back down to 1/4
up to 3/4
down to 1/2
all the way up
This will get real heavy, real fast, but it's the variety that gives our muscles and nervous system the huge kick in the balls it needs.