Is that why every hospital in the world, who can, uses them?
Xrays are accurate if the viewer is even partly trained, anyone who says they can not tell should either take another at a different angle or not be doing it. I have seen xrays with stones in them; I am not trained but could see them clearly. We have had many differnet xrays of different pigs and different issues and seen all of them clearly.
We had a vet decide to take 3 seperate xrays at differant angles as they wanted to make sure it was the right diagnosis. No gas needed, as gas does not affect an xray, there is nothing (except metal) that would affect an xray that would hide stones in a GP.