Longer is quieter. I have a bolt action .22 rifle with a 24 inch barrel and it isn't nearly as loud as my buckmark pistol with a 5" barrel using the same loads. Shooting subsonic loads out of the rifle really isn't even obnoxious without hearing protection (wouldn't recommend a steady habit of it), while its still very loud with the pistol.
Fix it right the first time...
Use Baling Wire !