Can you shoot a slug with an 870 that has modified choke on it?

No, you should not shoot slugs with a modified choke on an 870. Only certain chokes are designed to safely fire slugs, and using an incorrect choke can damage your gun or cause injury. The modified choke is not designed to withstand the pressure and velocity of a slug, which can be much higher than that of birdshot or other ammunition. Slugs should be fired through a specifically designed choke labeled "slug" or "rifled." Please refer to the manufacturer's recommendations and specifications for your 870 shotgun to determine the appropriate choke to use with different types of ammunition.