I'm wrong.
I think it's command post dependent how much a shot costs as well, I will have to check now, as well as recharge speed timings.
I -know- the wait is longer if your bar is empty, but I'm now going to have to time it now to make sure I'm not imagining it.
DISREGARD MY PREVIOUS COMMENT.
Also yes this is a spam game.
EDIT: I TIMED IT AND I AM WRONG AND AN IDIOT. I sort of see why I thought it was like that, given the recharge time, but no. Effectively, you've recharged 15% or so extra during the time you're fitting the next rifle nade. That 'vulnerable' time should probably be factored in, though because engies are easily surprised when fitting the nade.
Rather than thinking of it in terms of the charge bar, I was thinking in terms of time-til next nade... I think.