GEORGE WALLACE says: Muntjac are deer and are therefore protected by the Deer Act which prohibits the use of any smooth-bore gun, except in very specific circumstances.
If you find a deer caught in a fence, injured by dogs or hit by a car, you could use a shotgun to end its suffering but otherwise you should use a proper deer rifle.
This means, in England and Wales at least a .240 calibre and in Scotland, where no calibre is mentioned, a bullet of at least 100 grains which effectively also means .240 minimum.
There is no set close season for muntjac because they had not been invented in 1963 when the basic legislation was enacted. Since then there have been various suggestions for their humane control, but current thinking is bucks may be shot at any time but does should be shot only when visibly and heavily pregnant.
This is because they are mated again almost immediately after giving birth and will have a dependant fawn until only a month or so before the next one is born.
Got a question? Contact: firstname.lastname@example.org or Sporting Gun, PO Box 157, Stamford, Lincolnshire PE9 9FU