Defendant's sentence for pleading guilty to two counts of possession of firearms by a felon, which included a four-level enhancement based on a finding that one of the firearms "had an altered or obliterated serial number," is affirmed, where: 1) a serial number does not have to be illegible to be "altered" for purposes of the sentencing enhancement; and 2) a serial number that is made less legible, as here, is made different and therefore is altered for purposes of the enhancement.