Do you mean en eagle eye lens that screws into the foresight? If so they are not permitted in ISSF competitions,but are allowed in county/NSRA competitions I believe. My advise on filters would be seek advise and try them out before investing in any,you could waste a lot of money for little reward until you know what will work for you
Steyr LG110 Hunter,AA410 in Gary Cane stock,HC, Steyr LP50,Morini 164ei,Morini CM84e,Anschutz 1417 thumbhole,Rimfire Magic 10/22,Anschutz 1913,Rieder and Lenz Z2,Keppeler 6mmbr