The easiest way to muzzle load with shot is to purchase an inexpensive 209 muzzle loader adapter from
https://www.gunadapters.com/209-muzz...ding-adapters/
It can convert a modern single shot break open shotgun into a muzzle loading shotgun, and they are said to work well.
Be sure to watch the instructional video on the company website [or youtube] to learn how to properly use it.
I've heard that some of the shot drops down into the chamber designed to hold the modern shell, but that doesn't noticeably affect performance.
Someone also recently posted how they experimented with homemade shot cups to make shot loads with limited effectiveness for a slow twist .50 rifle.
Of course every gun and twist is different which would require experimentation..
But it's important to note that a gun without a choked barrel is like any cylinder bore which isn't conducive to good shot patterns to begin with.
Then the rifling only adds to spreading the shot even more,
Even experiments with a 20 gauge/.62 caliber smoothbore that has an open or cylinder choke have shown that it can require up to 2 ozs. of shot to pattern well at 35 yards with 100 grains of powder.
That means that a heavier than normal shot load is required to produce an effective shot pattern when the barrel doesn't have any choke constriction.
Both posts below were written by the same person and provide slightly different details about shot cup construction.
How to make homemade shot cups for a rifled barrel - see post #23:
http://castboolits.gunloads.com/showthread.php?373113-Firing-lead-shot-from-rifled-barrel&p=4530417#post4530417
And post #14:
http://castboolits.gunloads.com/show...me#post4530400