Stick to Your Guns (1941)

"BAR-20 BOYS AGAINST AN ARMY OF KILLERS...but nothing can stop Hopalong and his pals when justice calls!"

Buck Peters arranges for Hoppy, California, Johnny and other cowboys to go to the aid of friends whose cattle are being rustled. Hoppy and California locate the rustlers' hideout and join the gang by posing as outlaws themselves, but must find a way to let the rest of the posse know where they are.