It's not a primer (also called an igniter) for a cannon or an artillery shell, it is the primer for an artillery projectile's propellant-powder casing. Basically the same as the primer in a bullet's metallic casing, but much larger. Yours has a deep firing-pin imprint at the center of its base, so it is a "used" one. Without any maker's-markings, yours is difficult to specifically ID or time-date. Based on your finding .45-70 rifle bullet casings nearby, and based on the time of earliest use of metallic-cartridge artillery projectiles in the US, your primer dates from about 1880 at the earliest and perhaps as late as World War One army artillery practice firings. Of course, Spanish-American War (1898) practice is also a possibility. The old .45-70 Springfield Rifle was still being used for training recruits during both of those wars.