I'm not sure about these beams, but at least GRBs from collapsing stars come from a relativistic shock wave with gamma of ~1000, IIRC. Such high gamma gives a v

I'm not sure about these beams, but at least GRBs from collapsing stars come from a relativistic shock wave with gamma of ~1000, IIRC. Such high gamma gives a very collimated beam of radiation, of order 1/gamma, due to relativistic beaming. So taking 1/1000 radians as a ballpark value, 1mrad is about the divergence of a typical laser. (Laser pointers have slightly higher divergence due to the short cavity, but things like He-Ne lasers are typically 1mrad.)