From Guns.com:

In its present form, the measure it would protect large caliber rifles and shotguns from being reclassified as “destructive devices” under the National Firearms Act. In several states, rifles chambered in .50 caliber BMG have been banned and Bishop’s legislation would prevent such a curtailment on the federal level.

Next, the bill would remove ATF’s ability to reclassify ammunition as being armor piercing. This could derail past rule changes such as seen with the 5.45mm 7N6 loading as well as the attempted reclassification of “green tip” ammunition.