Will 28 degree framing nails fit into a 31 degree framing nailer?

Even though there are people that will surely argue to the contrary, probably not.

It's not only the difference in angle, it's also frequently the collation type and other nail specifications. 28 degree nails are mostly collated (held together) with wire. 31 and 33 degree nails are typically collated with paper tape.

When the angles don't match, they usually don't feed well. How happy are you going to be if you shoot a bunch of blanks because the nails didn't advance? How happy will the person you are framing for be when the project you made falls apart because half the time you thought you drove a nail in you didn't?

Even though some times you can get away with it, using a nail that is not approved for the tool can cause things to go real wrong. I have seen tools loaded with unapproved nails try to drive two at the same time. This can easily ruin your tool, ruin your work and ruin your day. In the long run, you will accelerate wear to key areas (expensive parts) of the tool.

I wouldn't risk it. Putting the wrong nails in your framing tool is like putting premium unleaded gas into a diesel. There is nothing wrong with the gas, it's just not supposed to go in a diesel.

One last thing... always wear your safety glasses. :) is better than .)