From The Truth About Guns:

That title is now held by the National Shooting Sports Foundation (NSSF), which NBC News reports spent over $5.4 million on federal lobbying last year, more than any other year in its history, and significantly more than the NRA. The NSSF, with a 60-year history of promoting hunting and recreational shooting, has emerged as the country’s largest firearms trade association amid the NRA’s struggles with declining membership, revenue and internal scandals, including a corruption ruling against its leader Wayne LaPierre.