We have discussed this here several times. No, it is not safe- the body of the shell, shot load and shot cup have to squeeze down in the forcing cone, jacking pressures WAY above what the gun was designed to handle. Can it be done? Yes. How many times before the gun displays its upset with the abuse by coming apart at speed?
Ah- THAT is the question! I would not do this except in the gravest emergency- I have only one round of birdshot and a large carnivore has me in a tree, and it is eating the tree.