I could not tell if you bought a smooth bore or rifled 12 ga. barrel. If you bought a smooth bore, only options will be rifled slugs. These indeed tend to be very iffy as far as grouping, and will lose energy rapidly as it passes through the air. Arc is relatively severe. The reason being is that the rifled slug is a heavy, lousy flier and does not begin to stablilize (spin) until after it leaves the barrel and may be slightly tilted by the time it does .... and the spin is a result of it cutting its way through the air. Energy that is used up spinning the bullet would other wise be conserved in the retention of forward velocity. (It is a physics thing).

If however you bought a rifled barrel, then you indeed have a 125 +/- set-up. The
2 3/4" or 3" shells are usually in a sabot (plastic sleeve). The lands of the rifled barrel + the friction between the sabot and slug, start the bullet spinning before it leaves the muzzle. Much like a standard center fire rifle. In general this type of set up is much more accurate than a smooth bore/rifled slug set-up.

It is a mistake to use sabot slugs in a smooth bore. These will leave the muzzle not spinning, tending to wobble or tumble wildly, resulting in an unpredictable down range point of impact.

Long time ago, I used a smooth bore, 12 ga. pump with rifled slugs as a deer recovery weapon. I never found any ammo capable of decent groups and would not have taken a shot past 75 or so yards.

On the otherhand, one of my regular hunting buddy's, Bill J., uses a 3" chambered, rifled barrel 12 ga. Mod. 870 and Remington brand sabot ammo (which exactly I do not recall) exclusively. Has for eyars. Though he could use center fire, he prefers this set up. Over the past 15 or so years, he has killed hump-teen deer ... probably 120 .... and has never missed as far as I know, and I have never known of one going farther than 40-50 yards. He has killed several from a shooting house set on one end of his favorite food plot, to the far end about 135 yards away.