He saw the 30.06 that was dug out of his wall.
Also the cops that dug out that bullet were never called to identify anything.
In his 1979 letter, Walker described it as "a bullet completely mutilated." How would Walker (or anyone else) determine that "a bullet completely mutilated" was .30 caliber, much less .a 30-06. BTW, remember that ".30-06" specifies a cartridge, and not necessarily a bullet. How would anyone be able to just look at the mangled metal that Walker describes and say "oh, this is from a .30-06" and not from (say) a .308 or 7.62x54 Nagant or .303 Enfield or .300 savage or .300 Winchester Magnum or .30-40 Krag?