On the average, you should see a increase of 50-60fps per inch in handgun barrels of 2-8 inches. The increase in velocity starts to fall off with longer barrels.

But this is just an average and your M28’s could be the exception to the rule. For example, I have a 6” model 66-1 that consistently outperforms my 8 3/8” model 27-2 by a bit over 50 feet per second using the same box of 158gr ammo despite having a shorter barrel. Only a chronograph can tell you for sure how your guns are working.