Mudbug..
I'm not sure if these are helpful with GRS's but I know they work pretty good for optics. I wont be using it with GRS's but I will be using it with a red dot that's in the mail. Thought it could be helpful for you.
$30 on amazon, prime eligible, arrived in less than 36hrs.
If planning on getting one, I'd steer away from anything cheaper than this one, where as they're diameters might not fit tightly in chamber causing the zero to be way off. This one looked the biggest, plus it has a flange like a shell does so it will be pressed against into place nice & flush with the bolt & barrel. Others don't have that flange so there might be a possibility of movement or it totally sliding down the barrel if its too small.
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