I tried your instructions, but the weapon is still invisible. I tried putting it on commandos and Tommies, the weapon is there, but is missing a scope.
i have a british sniper with lee enfield scope rifle moded, you can try this ABP:
model =
{
"..\\..\\..\\common\\NewMotionTree.abp",
--"..\\..\\..\\common\\engineer.abp",
-- Common AEF Gear
--"..\\..\\..\\aef\\aefcommongear.abp",
--"..\\..\\..\\aef\\weapons\\m3_grease_gun\\m3_grease_gun",
"..\\..\\weapons\\boys_at_rifle\\boys_at_rifle",
"..\\..\\weapons\\rifle_lee_enfield\\rifle_lee_enfield",
"..\\..\\..\\aef\\weapons\\rifle_m1_garand\\rifle_m1_garand",
"..\\..\\..\\aef\\weapons\\smg_m1_thompson\\smg_m1_thompson",
"..\\..\\weapons\\smg_mk2_sten\\smg_mk2_sten",
"..\\..\\weapons\\gammon_bomb\\gammon_bomb",
"..\\..\\..\\aef\\weapons\\pistol_m1911_45\\pistol_m1911_45",
-- Main Mesh
"sniper",
}