Sienna Miller is also set to star with Clint Eastwood directing.
The pic is based on the autobiography by Navy SEAL sniper Chris Kyle, the Texas native who went from rodeo cowboy to SEAL chief with the highest number of sniper kills in U.S. military history. Grimes will play Marc Lee, a Navy SEAL who fights alongside Kyle.
Jason Hall penned the script with Eastwood, Robert Lorenz, Andrew Lazar and Cooper producing. Jon Berg is overseeing for the studio.
Besides playing Christian Grey’s brother in “Fifty Shades of Grey,” Grimes just wrapped production on “Shangri-La Suite” opposite Emily Browning. Other past credits include ABC’s “Brothers & Sisters,” Fox’s “Taken 2″ and the indie horror pic “All the Boys Love Mandy Lane.”
He is repped by CAA, Global Creative, and Bloom Hergott Diemer Rosenthal Laviolette Feldman Schenkman & Goodman.