I replied to you at TreasureNet about this shell.
No offense intended, I'm only trying to be helpful... your measurement of it, at 6.5-inches diameter, absolutely cannot be precisely correct. If your shell actually measures 6.5-inches in diameter, it would not fit into a 6.5"-caliber cannon.
Please try to do an extremely precise measurement, with a better device than a tape measure. Your shell's actual very-exact diameter will answer your question, i.e., was it fired by Fort Donelson's 6.5" Rifled Columbiad. Because .10-to-.15-inch will tell the tale, you'll need to clean off even very small bits of rust-encrustation from the raised band (called a bourrelet) at the the bottom of the shell's sides. Make measurements across the base in several directions and tell us the result.
A 6.4"-caliber cylindrical shell typically measured 6.25-inches in diameter. A shell made for use in Fort Donelson's 6.5"-caliber Rifled Columbiad would measure 6.35 to 6.4-inches in diameter.
Please keep in mind that you'll be dealing with the thickness of rust-encrustation on two surfaces, meaning, the two sides of the shell. If there is a mere .1-inch (1/10th-inch) of encrustation on each side of the shell's base, it adds up to increasing the shell's original diameter by .2-inch. That would transform an actual diameter of 6.25-inch upward to 6.45... which is extremely close to the 6.5" measurement your tape measure gives you.
Emike asked "Does anyone (Bill, Pete, etc.?) know the earliest appearance of big Brooke ratchet sabot shells in the field?"
Regarding Digger42071's shell, the question isn't the caliber's earliest appearance, but the length. The longest Brooke 6.4" shell in the Bell book on civil war Heavy Artillery projectiles in 12.87-inches in length (including the sabot). Digger42071's SABOTLESS shell appears to be at least that long, compared with the length of his foot in the photo. (Please give us a precise length measurement of your shell, and its precise weight.) My point is, the longer -- and therefore, MUCH heavier -- versions of 6.4" Brooke shells appear later in the war, with the advent of the double-banded and triple-banded Brooke Rifles, capable of withstanding the firing of such very heavy long-bodied projectiles. Therefore, I do not think Digger42071's very long-bodied Broke shell existed at the time of the Fort Donelson battle (February 1862). But of course, if its actual no-rustcrust iron-only diameter turns out to be 6.4" instead of 6.25" I will be proven incorrect.
Another clue would be the CS Navy Archer Percussion fuze in this shell's fuzehole. Contrary to what was first believed, that version of CS Navy fuze dates back to at least very-early 1862. Some have been found in the vicinity of the very-early 1862 shelling near Hatteras, North Carolina. So the presence of one in Digger42071's shell does not remove the possibility that it is from the February 1862 Fort Donelson artillery engagement.
Regards,
Pete