I've never tried a sabot I like yet and I've tried a few with several different guns.I don't trust them. I can get most to group fairly well and then one always "Takes off!" Almost every flyer I've seen I've seen the sabot sleeve wad fly off crooked . When I recover the sleeve wad it usually has one wing broken or deformed.If the sleeve flys poorly, so will the bullet with in the sleeve and in my opinion, the sabot design has 1 too many moving parts to be perfectly consistant. If 1 out of 10 fly off, or even 1 out of 20 is too many for hunting. I need them to work in all conditions, every time.
My honest advise is to forget the sabots and the high price. When I go shopping for slugs for a new gun, I start with the cheapest and work my way up. My Mossberg likes Breneke and they're about $1 per slug. The cheaper the ammo, the more you can practice, the better and further you'll be able to shoot.You have a rifled barrel, it will spin any slug designed for rifled barrels but it will spin one load a little better than the rest.
I'm not sure why but the 12 GA seems to be the popular choice for slug guns. Heck, I even have a 12 GA 3.5" rifled slug barrel! The truth is, a 3.5" slug is only more painful to shoot, not more powerful, doesn't shoot any further any straighter.. 3" is plenty enough (how big of a hole do you need?) but IMHO the 20 GA 3" rifled slug gun is by far the better deer gun. Kicks less, shoots further and flatter and in the right hands will ruin less meat.
BTW I'd also forget about shooting my slug gun much over 100 yards any ways. I practice out to 150 and occasionally stretch it out to 200 yards. I practice those just in case for some reason I may need to put a wounded animal down. In over 20 years I've had to take a long shot 1 time. If you can't get with in 100 yards, it's not your guns fault. Man up and learn to hunt. I take shots I CAN'T miss.
When changing or mounting the scope and rings, base, did you torque every thing in properly? Didn't strip anything out? Did you use loctite? Just one loose screw and sighting in can be affected. With out loctite it could be 100 shots later that your scope starts floating around. If you do not use a light weight thread locker it's just a matter of when, not if , the screws come loose.
As for that single shot that NEF shot gun, I love my 30-06 single shot. I've put limb saver pads on my shotgun and rifle and also added a mercury suppression tube inside the stocks. Take the pad off, measure the stock bolt hole for width and depth, buy one that fits the best. Follow the instructions to install, it's pretty easy. For a little gun, my NEF is pretty hefty also. I was leary off adding more weight with the mecury suppressor but I feel it actually balanced it out better. The gun is all barrel and scope. To me, adding weight to the stock makes it feel lighter and comes to my shoulder better, even though its over 5 Oz's heavier. I found this to be true with both my NEF 30-06 and Mossberg 835 deer /turkey combo.