Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
How many  external (cm 2 ) are there for the .338 lapua magnum chambering?

### Inputs:
table_name: table, columns: ['Chambering', 'P1 diameter (mm)', 'A external (cm 2 )', 'P max ( bar )', 'F bolt ( kgf )', 'F bolt']

### Response:
SELECT COUNT A external (cm 2 ) FROM table WHERE Chambering = .338 Lapua Magnum