Нет такого в природе - "отечественный грендель". Опытный ответ на не состоявшийся 6мм SAW есть --> 6Х49, над которым работы ведутся с 80х. Патрон 6Х49 на модифицированной гильзе промежуточного патрона конструкции Елизарова и Сёмина.

Разрабатывался как основной универсальный, под АК/СВД/ПК. Насколько известно прошёл испытания в СВД и ПК (UMG). ПК в 6Х49 отстреливался начиная с 1993-го года, когда не гренделями не спс и не пахло.

Пошла родимая;

Presumably inspired by the U.S. Army SAW research, the Soviets started developing the 6 mm ammunition primarily intended for infantry machine guns and sniping rifles in about 1980. The first type revealed was the 6 x 53 mm, which launched an 5.2 g bullet with a muzzle velocity of 1150 mps.

About ten years later the 6 x 49 mm cartridge was introduced. It has a rimless case with approximately the same rim diameter (11.4 mm) as a classic 7.62 x 39 mm round. The 6 x 49 mm round has muzzle velocity equal to the 6 x 53 mm, it is also extremely hot having higher chamber pressures than the 6 x 53 mm. While there is a risk for case separation, the case has a circular groove formed near the extractor groove to allow case stretching without risk of separation. For reliable operation, adequate headspace is critical in self-loading firearms. On the other hand, high pressure ammunition will require tight headspacing to be safe. Controlled case stretching as used with certain aircraft cannons allows use of workable headspacing, yet the gun is also safe to fire.
http://izhevsk.club.guns.ru/eng/pkm.html

Если это "тот самый" модифицированный АК в 6Х49 то интересно как 60 патронов запихали в 4-ре ряда...