No, but he needn't have sent people with guns at first. The first step in enforcing any law is just asking people to comply with it. The next step is putting fines on them. Only the last step involves the threat or at the very end, the use of force.
Given the common attitudes of those who purchase and keep AR-15s, perhaps the chance of them complying with any future legal ban on assault weapons is small, but in that situation, by not complying they would be committing a criminal act.
> if necessary.
Yes, if necessary. A lot of issues we treat as law enforcement problems can be resolved without bringing violence into the picture, but that's only possible if the systems are set up that way that encourages non-violent intervention first. Unfortunately, our current law enforcement systems treat many situations as having a high probability of requiring violence.