When I first saw your post, I did not reply to it because I could not be certain about the correct ID for your fired civil war bullet, even after giving it extensive study and research. But since you emailed me a request for assistance, I'm returning to this discussion-thread, to give you my thoughts.
The weight your reported, 41.4 grams (639 grains) is in the correct range for a "short" .69-caliber civil war bullet. The presence of three very-narrow body grooves (not "rings" is a major ID-clue. (I should mention, shotgun slugs don't have those.)
The deep narrow hole in its base is another clue. That hole does indeed closely resemble the base-plug hole in a Williams "Bore Cleaner" bullet. However, a WC's base-hole has straight parallel sides, like a water-well in the ground, because it held the straight-sided body of the base-plug. Upon close examination of a super-enlargement of your photos, the hole's walls SEEM to not be parallel... looking instead like the hole is the narrow top of a cut-off bubble inside the bullet.
Because you can examine the hole in real-life and I can't, please stick the end of a toothpick into the hole to check whether the sides of the hole are straight, or whether the hole does get larger as it gets deeper. Also, the toothpick will tell you how deep the hole goes into the bullet.
I realize your fired bullet is significantly distorted by impact, which may have had a major distorting effect on the hole's shape.
And of course, it's length is also shortened by the impact. But still, please measure its length with your caliper, and tell us.
My initial guess was a Williams Cleaner... but the body-grooves on ALL of the versions of WC bullets are flat-bottomed instead of V-shaped like on most civil war bullets. In all my decades of closely examining civil war bullets, I've never seen even "extreme" firing-compression turn a Williams Cleaners body-grooves from flat-bottomed into narrow V-shaped. So, even in view of the WC-like hole in your bullet's base, I excluded it from being a Williams. Which, in summary, meant I could not ID it... and that, combined with being very-latenight weary when I saw it, I didn't feel up to composing and typing this looong response about it.
Please let us know the result of doing toothpick exploration of the hole, and the length measurement. Thanks.
Afterthought:
I suggest you post your pictures of it, with the weight and measurement and especially the dig-location info (which is always very important to include), in the Bullets discussion section at the Bullet-and-Shell website's Civil War Projectiles forum, here:
Civil War Projectiles Forum - Index
