As above, you gotta make sure that barrel nut is tight, and the action/stock screws are Snug/unmoving.
Also, check the crown with a magnifying glass...a dinged up crown can cause all sorts of issues...
Marlin's Microgroove barrels are know for better-than-average precision, so take a look at CCI Minimags & Federal Automatch,
they're known to be more reliable...AVOID Remington Golden Bullet at all costs...they truly suck great big gobs of greasy grimey gopher guts.
The thing about Microgroove barrels is the lands & grooves, while more numerous, are Smaller...so the fatter or jacketed bullets work best.
Once you find the right ammo, and check the crown for issues, it should lay 'em in for quarter-sized or smaller groups at 50 yards,
and pop cans at 100 should be no problem at all.
Also, if you have a scope on it, (hopefully Not a TASCO), watch for Scope Creep...those 3/8" rail mounts are known for sliding backwards...