Add these 5 lines to GTAIV\common\data\default.ide (make a backup first):
In the "objs" section, after the line that starts with  testTerrain  and before the first  end

ov_sil_pistol,   ov_attach,  100,  0
ov_sil_smg,      ov_attach,  100,  0
ov_sil_ar,       ov_attach,  100,  0
ov_sil_sniper,   ov_attach,  100,  0
ov_scope_sniper, ov_attach,  100,  0

(Keep Windows line endings: edit the file in Notepad, not in a web editor.)
