denied for m1 third and last dui in 2000

[phenix]

I was denied the purchase of long rifle at local sporting goods. Sent challenge and due to a DUI in 2000, I was informed i was prohibited under fed law 18 USC ss (g) (1). I do have a third dui in 2000 which pa title 75 ss 3731 (a) (1) does state that a third conviction is M1. However in 2000 the law used a matrix grading system to determine minimum and maxim. Not an statutory max of 5 years. My docket and other court papers show my min is 6 months and max is 12 months. Under the federal law, do they go by grading or by sentencing court judgement?

this has been mentioned before, anyone find an answer?

I do have electronic copy of everything including matrix on docket