If you're chiefly concerned about safety, the only important factor is the amount of neck clearance in the chamber. If the neck portion of the chamber is of sufficient diameter to allow the cartridge case neck to expand and release the bullet, such a minor difference in bullet/groove diameter should cause no problem.

After I had the neck of my wildcat .505 chamber opened up to provide sufficient neck clearance for the cartridge case, I routinely shot .510" bullets through my .505 barrel, which measures .495" in the bore and .505" in the groove. At the time, the only .505" bullets obtainable were the Barnes copper tubing bullets. They were not satisfactory. John Buhmiller used to say that a Barnes copper tubing solid made a good soft point.

Just out of curiosity, I once shot a wildebeest with one of the Barnes softs. The bullet hit him in the shoulder, knocked him down, he got up and walked a few steps, then collapsed. We were only able to recover fragments of the bullet, which had come to pieces upon hitting the shoulder.

.505 Gibbs being a proprietary cartridge, Kynoch would not sell the componants. They would sell the 570 grain .510" bullets for the .500 Nitro Express

The elephants and buffalo never complained about it, and the accuracy was unchanged.