If you use weapons made for v1.46 in Resistance (v1.75 and later) the muzzle flash will be there. You must download a new version which supports Resistance. If you cant find a versin for it, you can modify the cpp so that the muzzle flash wouldn't be there. At least what I've heard of. But if the weapon isn't too "old" you should be able to find a new version which supports Resistance.