I used a Glock model 27 at the Nationals in 2004. I had only two G-27 mags and two G-22 mags. No refs dinged me for having the longer magazines. You can only load six. I couldn't lay my hands on my rule book this morning,so this is only personal experience.
If you do not have a current rule book, get one. If that soesn't address your question, email or call the NRA LE activites division for more info.