Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_20821 ( "Gun" text, "m/v ft/s" real, "Shell (lb)" text, "Time to ft (m) at 25\u00b0 (seconds)" text, "Time to ft (m) at 40\u00b0 (seconds)" text, "Time to ft (m) at 55\u00b0 (seconds)" text, "Max. height (ft)" real )

### Question ###
If the gun used is the qf 12 pdr 12 cwt, what is the shell weight?

### Accurate SQL ###

SELECT "Shell (lb)" FROM table_20821 WHERE "Gun" = 'QF 12 pdr 12 cwt'