If I were having trouble getting cross hairs to move, I would be tempted to start from scratch and bore sight the gun first.
On a personal note, I have never had much luck with 3" slugs in general, sabots or not. IME, they just don't seem as stable or consistent. This may be due to a lot of things, maybe some having to do with the shooter rather than the slug [;)], but there it is anyway.
The SST's would be a good choice, as would the Winchester Partition Gold's which have been my slug of choice for the last 5 years or so. These two slugs will most likely shoot very different from each other out of the same gun. The SST is considerably lighter, yet faster.
Since my son never got the deer hunting bug, I am planning on setting up my old 870 smooth bore just for giggles. I shot this rig for years and killed a lot of deer with it. It is an old, plain wood 870 with a smoothbore slug tube, bolt on side receiver scope mount, and a cheap-o 2.5X Tasco wide-field. It was set up for a type of slug either not made anymore or very uncommon. They are called Activ slugs. Red with an all plastic case.
They rivaled early sabot/rifled tube rigs and I have killed lots of deer out to 90 yards with excellent accuracy and loads of knock down. Plus, it is actually fun to shoot and handles way,way faster than my hyper accurate bolt action. I was thinking of carrying it for hunts inside cover where long range wasn't an issue, but a faster handling gun might come in a little handier.