If you have a buttload of cash and can find one, (highly unlikely in the US and it will be select fire swing down receiver only) you could snag an HK32. HK's only production rifle chambered for the M43 cartridge. The mags for these are reportedly astronomically expensive.
The PTR's are GTG and well made. The PDW's are exceptionally nice. The mag wells have a steel insert that accommodates the AK mag dimensions. Early models were really partial to the Bulgarian slab side mags and there were some metallurgical trunnion issues. The later versions and/or PDW's seem to take all AK mags. In addition, Promag makes an AK drum that fits in these guns without any alterations and the fact they do take AK mags is definitely a plus.
I highly prefer the x39 round over the 5.56 anyway thusly feel the roller lock platform in this caliber is the best of both worlds, IMHO.