The strain is caused by the explosion of the powder in the round. There really isn't anyway around it. If you are going to accelerate a projectile to speed, then you are going to get equal but opposite force on the device(per Newton's law of physics). You might be able to print a low speed, low weight projectile item like a b.b. gun.
You would still want metal for the barrel (of course) and trigger assembly. Pretty much anything taht routes hot gases or contains explosions has to be fabbed from steel. The lower is only a mount point for some comopnents and a guide for the magazine.
Also, is it conceivable that new gun mechanisms could be invented, optimized for plastic guns? Would the strain be less if you shot plastic bullets?
I don't know anything about gun physics (as is probably obvious).