As noted here, veracity of bomber gunner claims is even worse than fighter claims by a huge factor.
But
There is some hard intel. Naturally I cannot find it now, but in researching Forgotten Fifteenth I found a 1943-44 document showing demonstrated accuracy of various HB gun positions expressed in mils (group dispersion) at 500 yds or so. The most accurate was the ball turret possibly because that was the first HB position to receive a computing sight. Also, counter-intuitively to I Saw It On The Internet, BTGs had the fewest casualties of all bomber aircrew. Heaviest losses were bomb/navs, who of course were right up front.