The low REG setting was just something i stumbled over one day.
Regarding speeds i have tried many from the high 800 to 1100 FPS, my Maverick have also shot slugs " well " at other speeds, but there the POI fluctuated wildly, but around 960 FPS i can actually shoot several OK groups in a row.
For a while i thought my fluctuating POI was just wind playing with me, CUZ i could get 2-3 shots in but the BAM the next shot was .5 / .75 inch away, but then if i did nothing the POI would eventually shift back in a few shots to what i was dialed in for with the scope.
I was also down the lines of " i aint doing this right" in regard to my aiming, so also entertained the idea of bolting my rifle down to see what it would then do on its own with no shooter input.
My fluctuating POI was generally side to side, only a little up / down, like maybe 1/4 inch, which also confused me a lot, and made me think it was wind, but it was still there on pretty much no wind days or on days where the wind was from the back, and if anything i would have assumed my shots would be pushed down as i do shoot a little uphill.
Also when my shot strings are not super flat i would assume to see changes in the up / down direction, but that dont really seem to happen for me.
Actually i often see the slow shot in a string being the one that land high VS where i aim, yet another source of frustration for me.
I do on shorter distances have a funny tendency of shooting elongated groups in the horizontal plane, where as a vertical elongated group is very rare.
So when a shot of mine go high or low i am very fast to call " flier "
Some times i have shot 5 shot groups 4.5 mm high but 20-25 mm wide, and i am like " dude how do you do that when you aim the same place " and the previous group was pretty normal and so is the next one.
Sadly those long groups its not like they move to one sinde nice and easy, so i might shoot 2 right beside each other but then the 3 shot skip 10 mm, which is then filled in by subsequent shots.
Stuff like that severely mess with my mind, and make me think it is fortunate my hair is never in a length where you can grip it and pull it out.
Fortunately most often i am able to shoot my way out of what ever problem i have behind the rifle.