Minimum rifle weights in (rounded) lbs would be:

450NE = 9 1/2lbs
500NE = 10 3/4lbs
577NE = 13 1/2lbs
600NE = 15lbs

As a check for an express rifle I tried 400grs at 1900fps. Minimum gun weight came out at 7 3/4lbs.

Seems a reasonable starting point.

Thanks for posting the formula Escard.

JPK