I have no experience with 16 gauge slugs, but my experience is extensive with 12 & 20 gauge slugs. Most of the time, in most of the smoothbore slug guns I've messed with over the years, I've had my best luck and accuracy with Winchester Super X foster slugs. Of the 4 smoothbore slug guns me and the kids currently have, with Bushnell Banner 1.5X4.5 scopes, all are quite capable of SUB 4 inch groups at 100 yards with the Winchester slugs mentioned. Sighted in 2 inches high at 50 yards, the 12 gauges are an inch high at 75 yards and dead on at 100 yards, the 20 gauge sighted in in the same manner is dead on at 75 yards, and an inch low at 100 yards. Shooting a sub 4 inch group at 100 yards makes them quite capable of taking deer/hog sized animals out to 100 yards with no holdover required.
Obviously, as you mention, what you'll need to do is try several brands off of sandbags at a range, and see your results to draw the proper conclusion. You mention you shortened the barrel, so I assume you've eliminated any choke the barrel had?? That is a plus for slug shooting. Other than that, the real key is a RIGIDLY mounted set of iron sights/scope/red dot, whatever, just as long as it's rigidly and solidly mounted. The gun is only as good as the sights it has and if the sights are rigidly mounted, I'll say what I've always said, and that is that most smoothbore slug guns are capable of better accuracy than the shooter is capable of holding the gun steady. Good luck, let us know how you make out, I'd be interested in seeing some results with a 16 gauge, as I have never played with one.
Garry