Rimfire? Not an issue. Any other caliber, not really an issue as long as you can lock it all closed until chamber pressures drop.
Gotta keep heat off the plastic though.
I am not sure I follow what you are saying. To me, metal parts would be used to contain the cartridge. The least expensive of such parts would be off the shelf. But those parts heat up from the combustion of the powder and the friction of the bullet. Those parts could be aligned to another firearm platform using 3D printed plastic material. What I have found with a machinegun mechanism there is a lot of heat very quickly. Could you dump several mags? Sure and there is lag time transferring heat to the outside of the metal parts to effect the 3D printed plastic parts.
It is my understanding that the home plastic printing materials have a melting point of around 350 degrees. But those materials get soft and can lose their shape well below the melting temperature. So you dump several mags and set aside the "upper". That lag time gets the 3D printed plastic soft and the "upper" deforms. That deformation could be enough to inhibit proper function when the parts have cooled and the "upper" is put on the machinegun again. If it is your design and printer or something that you have the programing for, just print another one. The printing material is cheap enough. But that gets much more complicated if you don't own those things.
With the Tenko adapter, I put somewhere around 5,200 to 5,300 rounds through the 3D printed version. I had five uppers and I would run a Beta-C or Surefire 100 round mag or four 30 round mags and then immediately change the upper. The heat had to go from the barrel to the AR upper, then from the front lug of the upper to the steel front pin of the adapter, then through the steel pin to the 3D printed adapter. I made it a point to always remove the upper from the Tenko adapter as soon as I finished firing the ammo through the upper to limit the transfer of heat from the hot upper.
I used one of PSA FN double chrome barreled uppers to check how hot the front lug would get with back to back Beta-C mag dumps. The front hand guard cap got so hot that the cheap plastic hand guards PSA had shipped on the upper melted into the hand guard cap. The front sight base was 700 degrees.
Could an upper receiver be printed for a Mac style RR, especially in .22lr caliber, I would think so. But I think that full auto fire would need to be very limited. I think that for a machinegun application, metal would be a much better heat resistant material, at least for those who don't own a 3D printer.