It has been some time since I read the regs, but IIRC the federal government prohibits a firearm purchase by any one convicted of a crime "punishable by imprisonment for a term exceeding 1 year". The definition given somewhere at 18 U.S.C. 921of "a crime punishable by imprisonment for a term exceeding 1 year" includes any misdemeanor punishable by imprisonment for a term exceeding 2 years. Hence, a second degree misdemeanor (punishable by a maximum term of two years) does not fall within federal disqualifying criteria while, as you have already observed, a first degree misdemeanor in Pennsylvania means you will no longer clear a background check. Pennsylvania is one of only a few states that have misdemeanors punishable by more than two years.