It does seem to be true that guns will shoot different ammo differently, some better than others. As for grouping, if I want to drive a tack I use my thumb.
If I want to shoot deer, than you don't need a tack driver. I am much more of a hunter than a shooter so if my gun is grouping a couple inches at 100 yards that is plenty good for me. But I hunt where I only get 100 yard or less shots. If I was in the midwest or west where the shots could got 200+ yards, then I might want to tighten up the groups.