Roll my D&D character's ability scores
CJam, 43 41 40 bytes

{6a4*:mr$0Zt:+}6*]$2m&lt;"FRXWCDM"r#4e&lt;3e\p


Thanks to @Sp3000 for golfing off 1 byte.

Try it online in the CJam interpreter.

How it works

{             }6*                       e# Repeat 6 times:
 6a4*                                   e#   Push [6 6 6 6].
     :mr                                e#   Replace each six with a pseudo-randomly
                                        e#   generated integer in [0 ... 5].
        $                               e#   Sort the results.
         0Zt                            e#   Replace the lowest result by 3.
            :+                          e#   Add all four integers
                ]                       e# Wrap the 6 generated integers in an array.
                 $                      e# Sort.
                  2m&lt;                   e# Rotate 2 units to the left to assign the
                                        e# second highest result to Constitution.
                     "FRXWCDM"          e# Push that string.
                              r#        e# Find the index of the input.
                                        e# The string doesn't contain B, P or S, so
                                        e# those push -1.
                                4e&lt;     e# Truncate the index at 4. This way, C, D
                                        e# M all push 4.
                                   3e\  e# Swap the integer at that index with the
                                        e# one at index 3, i.e., the highest one.
                                      p e# Print.