CREATE TABLE table_77028 ("Nation" TEXT, "Model" TEXT, "Length" TEXT, "Barrel length" TEXT, "Weight" TEXT)

What is Length, when Barrel Length is 750mm / 29.5 in?
SELECT "Length" FROM table_77028 WHERE "Barrel length" = '750mm / 29.5 in'
SELECT "Length" FROM "table_77028" WHERE "Barrel length" = '750mm / 29.5 in'