Bruce,
Not to duck a reply, but that depends on the gun. I had a M-19 that would qualify for a target gun with .38 Wadcutter loads, but my M-686 only gives mediocre accuracy with ammo put up in .38 cases.
The bore diameters are the same in both calibers. The culprit in most accuracy loss it the additional bullet jump necessary for the bullet to travel an additional one tenth of an inch before entering the bore. It give more opportunity for the bullet to enter the bore canted one way or the other.
Doc Hudson