The amount of extra material needed because of the structural weakness of sintered/deposited materials as opposed to machined materials would probably make such a gun fairly bulky, unless you are willing to take the risk of the breech portion exploding when firing the gun.
You could offset that to some extent by using less powder in the charges or you could use the gun for what they're used most for anyway (threat, rather than use).
Some companies are very far now in the production of single units or small series of parts for prototype or one-off purposes, every year the field moves ahead steadily and it is very impressive what can be done, but there is no substitute (yet) for a single properly treated chunk of steel machined to the right dimensions.
Parts made like this are rapidly replacing machined parts in places where strength is not such a big issue, you'll find them in just about anything that has a mechanical component to it. But they are not yet able to compete with machined materials for the most demanding applications.
Especially tensile strength is rather low, and pieces tend to be brittle.
