A larger bullet needs less speed to confer the same amount of stopping power.
If anything, the fact that he was using a longer barrel made him more accurate. (And, by the way, a billet in a rifle will exit the muzzle faster than the same bullet exiting a shorter barreled handgun.)
Again, the muzzle velocity thing just seems silly to me, but let's have a conversation about it. How do you propose to regulate it? All guns? Have you read anything that indicates what the max muzzle velocity should be? (Honest question, not a trap.)