Although I don't own an ARP9 one of my mates who I skirmish with regularly does and uses a B&T tracer unit and BOCCA Suppressor, however had to get a small 14mm ccw thread extender due to the rail guard not allowing the "thicker" suppressors to fit properly. If you are happy with an extension then any will work (assuming 14mm CCW) however other than that you will need to get quite a slim suppressor to fit.
Although I don't own an ARP9 one of my mates who I skirmish with regularly does and uses a B&T tracer unit and BOCCA Suppressor, however had to get a small 14mm ccw thread extender due to the rail guard not allowing the "thicker" suppressors to fit properly. If you are happy with an extension then any will work (assuming 14mm CCW) however other than that you will need to get quite a slim suppressor to fit.