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

### Instruction:
What is the  p1 diameter (mm) when .300 lapua magnum is the chambering?

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

### Response:
SELECT P1 diameter (mm) FROM table WHERE Chambering = .300 Lapua Magnum