Its been done, check this article at;
http://en.wikipedia.org/wiki/5.7x28mm:
"The SB193 cartridge features a 3.6 g (55 grain) Sierra Game King FMJBT (Full Metal Jacket Boat Tail) projectile. The SB193's low muzzle velocity eliminates the distinctive "crack" created by supersonic rounds when used in conjunction with the P90 Gemtech sound suppressor. The SB193 suffers from greatly reduced penetration and range, but benefits from slightly lower recoil. The white colored bullet tip can identify SB193 subsonic ammunition. Its sale to civilians is restricted by FN."
The T/C custom shop has barrels.
Threaded Mode