Yes, 6.8mm Remington SPC has a bullet diameter of 0.277" while this VG6 Precision Lambda PRS30 Muzzle Brake is bored for .30 cal, making it compatible as long as you have 5/8x24 threads.
Caliber wise, yes, but fitment of this brake will depend on your thread pitch. Assuming we're talking about an AK, most AK's don't have 5/8x24 thread pitch. Check out the VG6 Precision Episilon AK, threaded M14x1 LH specifically for AK's.
https://www.opticsplanet.com/vg6-precision-episilon-ak-bbss-muzzle-device.html
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The VG6 Precision Lambda PRS30 Muzzle Brake will work on a 300 Blackout chambered AR15 as long as the muzzle is threaded for 5/8x24.
The VG6 Precision Lambda PRS30 Muzzle Brake will fit 5/8x24 threads and is compatible with .30 Cal up to .300 Win Mag.
Not at this time, the Lambda is designed as a stand-alone PRS muzzle device. VG6 Precision is currently finalizing the design of a blast forwarding device.
This muzzle device as it is would not be safe to use with a .458 SOCOM rifle. It would need to be bored out in order to use it with that caliber.
If the barrel is 5/8x24 threaded, yes it should absolutely work.
There is not. The thread pitch also isn't going to be compatible for most 5.56/.223 caliber rifles - you would need 1/2x24 thread pitch. You would also be losing a lot of the effectiveness of a muzzle brake by having an oversized caliber brake. I would recommend the VG6 Gamma or Epsilon in conjunction with the VG6 CAGE for a similar effect.
https://www.opticsplanet.com/aero-precision-vg6-precision-epsilon-556.html
https://www.opticsplanet.com/aero-precision-vg6-precision-gamma-556.html
https://www.opticsplanet.com/aero-precision-vg6-precision-cage.html
If the barrel is 5/8x24 threaded, yes it should absolutely work.