I currently dont have a bar, but luckily I have a tree branch about 5 inches in diameter, I can jump to and hang easily off. Should I be expecting to do more or less reps on an actual bar. I should also probably add I'm already doing pull ups around 10 per set but I am not extremely active and trying to figure out what I should be expecting when I enter the airforce.
You'll do less; 5" diameter will be pretty hard to grip. Also, branches generally aren't totally flat so it will be a little off-balance. I'd consider picking up a set of gymnast rings to hang from the tree branch.
With some rings, you get a few advantages:
- They're even in height.
- You can do pullups.
- You can do ring dips.
- You can do a host of other gymnastic exercises like levers, crosses, and shoot-thrus.
Plus, they're portable and relatively cheap. For several years of my life (living in a developing nation) I got by with some gymnast rings and a kettlebell.
If there is a difference in grip it might change the amount of reps you can do. Other than that, from my personal experience I can do the same amount of reps on virtually any kind of horizontally oriented object I come accros. It doesn't even need to be round, it could be a brick wall or the (squared) metal frame on a bridge.
This probably due to the fact that during pull-ups, even on a bar, I have my thumbs over the top of the bar rather than underneath (see image). So I basically hang with my fingers. You can have this type of grip on most objects. If you're not used to doing pull-ups like this I could see this being a reason to not being able to do the same reps as on a regular bar.
There's no clear answer. You almost certainly will be able to grip a normal bar more easily (and therefore for more reps) than you can grip a 5-inch tree branch. You probably cannot finish the pull-up as completely on the tree branch (due to its relative thickness) as you can on a bar. That said, I applaud your pragmatism and motivation to make-do with what's available to you.