Not enough to notice even with rifles until you start comparing barrels significantly different, even less with shotguns. Comparing shotguns with 28" barrels to those with 18" barrels and there will be a small difference. There is more difference between different firearms than a few inches of barrel makes. It isn't unusual to see over 100 fps difference between different firearms with the same length barrel. I've seen rifles with 20" barrels shoot as fast or faster than another with a 24" barrel.
