The gun charge was a sham. Yes, lying on form 4473 (firearm transaction record) is a federal crime. It’s also one that has historically never been prosecuted on it’s own; it’s always used as part of another prosecution, such as bank robbery, conspiracy to distribute controlled substances, and so on. You can count the number of times where that’s been prosecuted on it’s own on the fingers of one hand, in part because it really doesn’t come to light unless you’re caught committing another crime while also possessing a firearm that you can’t legally have. The justice dept. was originally going to drop that charge altogether, until pressure was applied from the political right. (Also, maybe don’t write a memoir where you admit to crimes, unless you’re past the statute of limitation on those crimes.)

The tax charges were pretty legit though. Depending on sentencing guidelines–which are quite complicated–the odds are pretty good that he would have ended up with large fines and penalties, in addition to having to pay the back taxes with interest that he owed.