In 1999 Castle State Arsenal re-branded and re-vamped the GPR-76 concept, in an attempt to pitch it to the Cordian Expeditionary Corps. While the 'Corps settled on the MBR-13 instead, many nations chose to upgrade their GPRs to BRT-99s, or purchase the smaller, intermediate caliber ART-99. Functionally identical to it's older brother, the BRT-99 can even accept GPR-76 barrels and gas systems parts, while the ART-99 is a scaled down version designed for intermediate cartridges. Both retain the GPR's distinctive quick-change barrel and flip-up front sight post.
The BRT-99 was offered in calibers from .280 British (7x43mm NICO) to 7x55mm Cordian and 7.62x51mm NATO, while the ART-99 has been chambered in any number of intermediates, including 5.56x45mm NATO, 7.62x45mm Czech, 7x40mm SPAC, 6.8 SPC and 6.5 Grendel