Re: My first slug gun - considering a 870 or 220
The question is not whether one can load slugs in shotgun shells. The question is whether one can create loads that replicate the performance of the Sabot loads which cost $3 each.
That, is a very different question and yes, it seems to be impossible to find the matching bullets and sabots for use in the 20ga shells and suitable for the rifled barrels. I investigated this topic for several months and could not find anyone doing this for rifle barreled shotguns.
To the OP, if you do not yet own a ML, I suggest you check it out. If you avoid the pitfalls (wrong bullets and powder / primers) you will find it a capable system. Cheaper than the alternatives and inherently accurate, since YOU can fine tune your load until it shoots great. With the slug guns, one is very limited by available factory ammo and the way the gun is chambered relative to the shells you are actually able to buy. I actually don't know if anyone is making a 3" slug load for 20ga anymore, yet all guns seem to be chambered for 3" shells anyway. This has an altogether different effect in a rifled barreled weapon vs a smoothbore.