reball goodness
i personally shoot a UL slg with reballs and i can say this.
the bonus of having the purge button on the back cap means that if there is ever a re cocking issue.... (i normally get one every 3 games or so) pressing the button maybe twice will reset the bolt so all is good again.
personally when i shot an ion i had problems with the balls getting stuck and causing the need for re cocking. which was effort without the bolt out back design
i also shoot an 07 pmr which i find works really well with the reballs too. the only thing you would need to do is replace the detents with the slg style sprung loaded ones as they do not wear out anywhere near as fast as the stock pmr ones.
yes the slg UL board is 09 mill compliant and also the 09 pmr's are too.
personally not had experience with the G3's but we do have people shooting fusion 08's and alias style intimidators also.