The decrease in group size realized by firing 38's in a 38 vs. a 357 can be attributed to several factors:

The first is the shoulder in the magnum chamber sitting just ahead of the wadcutter bullet. No big deal, you say? Maybe, maybe not.

Accuracy/group size is all about a bullet going down a bore, and emerging in as pristine condition as possible. Contact with that shoulder is going to render that bullet less pristine than it was before it hit it---a little or a lot.

Next we have the added distance the bullet is going to travel in the magnum chamber vs. the 38 chamber---whatever that is---1/8"-1/10"---not much, but it's not about the distance; it's about the increase in velocity gained in that distance. I have no idea how much that is, but it's some--more than otherwise. So what?

So the bullet is going to enter the barrel at a higher speed fired from the magnum chamber than from the 38 chamber. Given that, it's going to skid/slide further in the barrel before the rifling grabs a hold of it---and it's going to emerge from that barrel in accordingly less pristine condition than otherwise.

It's the little things that count.

No doubt we will now hear from some folks who say they shoot 38's from their magnum better than they do from their 38. That comes under the heading of a personal problem---get over it.

Step one is to become a believer. That can be accomplished very easily: Get your grubby, little paws on a machine rest, and you'll see the combined effects of the variables I've described above---without you and your grubby, little paws in the picture. And a machine rest costs less than pretty much any gun in your stash!!

And machine rests don't lie!!

Ralph Tremaine