There are gun advocacy groups which are issues-focused and compromise far less than the NRA, which is kind of a joke among younger gun advocates. Gun Owners of America is one who frequently supports legal funds for gun advocacy issues. Occasionally, a state gun rights group escalates a case up through circuit courts to the supreme court and deserves financial support. This past year, the California Rifle & Pistol association fought a case around standard-capacity magazine bans, and the New York State Rifle & Pistol Association fought a case regarding restrictions on carrying and transporting firearms. I believe CRPA is a 501(c)3 organization so donations are tax-deductible. I am sure there are others but I would be fine if the ACLU focused on the 1st amendment and left gun advocacy to other more focused groups (although I would appreciate if those groups also got 501(c)3 certification)