Wolf ammo is the US importer/assembler, they get the upper, gas block, and small parts and put them together. The upper, being a piston system is different than an AR, and the BCG is also different. If you wanted to assemble a T91NR, you'd need to modify the NR upper, have it re-anodized and then build a lengthened piston system (the T91 gas system is quite unique you might be able to get it work with the standard length gas system) and then have a barrel made and calibrate the whole thing vis-a-vis the gas port. Much easier to get Irunguns or Prophet River to bring one in and set it on your AR lower.