Optimal Barrel Length for XCR 6.5 Grendel pistol?

[MeatAxe]

Have been thinking about pulling the trigger on a Rob Arms XCR pistol in 6.5 G. Starting with a pistol so I can legally convert it to a rifle and back again to a pistol, if I choose, without jumping through a lot of batf hoops.

Just wondering what the optimal length would be for a self defense pistol / truck gun in 6.5 Grendel?

Thanks!