i've shot 38's and the mag's through the same gun before. there is a little bit of difference. i don't think it's all that bad though imo. i was shooting a sixshooter and my friend had put 5 38's and 1 357 in it and let me shoot it. like i said there is a bit of difference, and you will be able to tell. but it isn't that bad. you don't have much to worry about that's for sure. unless the bullet weight and load are extremely different then it would probably be more noticable.