How many  external (cm 2 ) are there for the .338 lapua magnum chambering?
CREATE TABLE "bolt_thrust_estimations_for_various_rifl" (
  "chambering" text,
  "p1_diameter_mm" text,
  "a_external_cm_2" text,
  "p_max_bar" real,
  "f_bolt_kgf" real,
  "f_bolt" text
);
SELECT COUNT("a_external_cm_2") FROM "bolt_thrust_estimations_for_various_rifl" WHERE "chambering"='.338 Lapua Magnum';