If the gun used is the qf 12 pdr 12 cwt, what is the shell weight?
CREATE TABLE "table1_16439764_1" (
  "gun" text,
  "m_v_ft_s" real,
  "shell_lb" text,
  "time_to_ft_m_at_25_seconds" text,
  "time_to_ft_m_at_40_seconds" text,
  "time_to_ft_m_at_55_seconds" text,
  "max_height_ft" real
);
SELECT "shell_lb" FROM "table1_16439764_1" WHERE "gun"='QF 12 pdr 12 cwt';