YOUNG BUCK HAS TO SERVE 18 MONTHS IN PRISON FOR GUN POSSESSION!

Young Buck won't be a free man much longer -- TMZ has learned the rapper has agreed to serve 18 months in prison for gun possession, following a massive IRS raid on his Nashville home in 2010.

According to the deal, Buck agreed to plead guilty to possessing firearms and ammunition as a convicted felon.

According to legal docs filed in the case, federal agents recovered the firearms in Buck's house during a 2010 raid -- when feds busted into the house to repossess a bunch of the rapper's belongings to settle a $300,000 tax debt.

According to the docs, feds found a .40 caliber Glock 22 in Buck's home along with ammunition. As a convicted felon, Buck is prohibited from possessing ANY kinds of firearms.

As for the felony rap -- Buck was convicted of assault with a deadly weapon after stabbing some guy at the Vibe Awards in 2004.

The judge signed off on the plea deal already but won't formally sentence the rapper 'til July 13th.

First he gets evicted now this! Atleast he'll have somewhere to stay!

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