I don’t think that Obama would use the US military or current police structures . I think that Obama will create a new agency with people loyal only to him to take part in the gun confiscations. If he uses the US military or current police structures, he risks mass desertions and/or a coup attempt.
If it were to happen it would clearly trigger America's first true civil war. The military officer corps swears an oath to the U.S. Constitution - not the "King" - and the employment of any force outside that framework simply would not be tolerated. As a retired officer, I have no doubt of that. I also have close friends and family in police and federal law enforcement that feel just as strongly. Add to that hundreds of thousands of armed civilians.
After the first few mass actions were attempted, any praetorian guardsman would have the life expectancy shorter than a rookie WWI pilot.
And he risks a civil war if he creates a new 'branch' of armed troops in the USA.