Young Buck is back in prison ... The former G-Unit rapper has officially begun his 18-month sentence for gun possession.
Sources connected to the rapper tell us ... moments ago, Buck checked into Yazoo City Medium Security Federal Prison in Southern Mississippi.
As we previously reported, federal agents recovered multiple guns in Buck's home during a 2010 raid. At the time, he was prohibited from possessing any sort of firearm because he was already a convicted felon.