Giles. My wife has an SX3 and yes, it doesn't like short carts and will misbehave. She sticks with 70mm and it functions fine for PSG.

It did cycle 21g loads fine too (although they don't make power factor for PSG - my Benelli M2 wouldn't cope with loads that low as it didn't have enough thwap to cycle the action reliably)

I've also just modified the carrier because it was like the devil's own chip fork and she put a hole in her thumb more than once. Welded up the end and it is now damn near impossible to get your thumb caught and it still loads fine (birdshot) - not tried slug or buck yet though so will reserve judgement on complete success until then.