The VF-17's gunpod has 7 barrels. This is clearly shown in Kawamori's lineart.
I'm pretty certain, I've previously seen reference to it being 30mm caliber, but checking through the usual source books just now, I was unable to find mention of the caliber.
When the VF-17's gunpod is shown being fired in the anime, it is clearly a conventional shell firing gattling type gunpod, not a beam weapon.
As mentioned, it does have the ability to eject the standard ammuntion magazine and fit a single shot high power beam gun adaptor in place of the standard magazine.
My theory, is that this adaptor is purely a battery and the actual beam weapon is built into the gunpod. I think the two red prongs at the top and bottom of the muzzle may be the actual beam emitters.
So basically, I think the gunpod is designed as a standard shell firing gattling type gunpod with perhaps a higher rate of fire than the VF-11's gun pod, hence 7 barrels versus 6. However, the VF-17's gunpod also has a high power beam gun built in, but this can only be fired when the standard shell magazine is removed and the beam gun battery is fitted.
Graham