You might want to check out the thread about Jason Armstrong who did 2406 pullups in 12 hours…breaking the world record (at age 16).
A lot of guys are slagging the kid on the site asking dumb questions like, “How much does he squat.” The answer is, it doesn’t matter, he trained to do chin-ups.
The reason I mention this is that his father, a former Marine Major, once held the world record as well, and developed a program for increasing one’s reps. It is called the “Armstrong Pull-Up Plan” and can be found on almost EVERY Marine Officer Candidate website. You can just google it and find it.
If too lazy to do that, then, here is a link:
Just scroll down to the Physical Training portion and click on the link (there is an alternate program there as well, just so you can change up after 4 weeks and do something different).
I am not a jarhead (I am in the Army), but, if there is one thing those jabronis do well, it is pullups.