I'm not a lawyer but a couple quick google searches revealed this.
1) Form 4473 which you will be required to complete asks if you have ever been convicted of a felony or any other crime for which the judge could have imprisoned you for more than one year. The important part here is could have, it doesn't matter if you only got a fine.
2) In PA an M2 could get you up to 2 years in jail.
Put these 2 together and it looks like you could not legally purchase a gun in PA, I don't know about expunging though, but it might be worth checking out.