Why are the two .69 caliber bullets different?
Do you have any idea of how many different .69 caliber weapons were used in the CW?
Not only would you have the regular Union army using them, but then there's all of the various Union militias that would have been issued them. And then you had a rather large contingent of southern forces roaming the area too. You said you found them within the city limits or your town, would that be St. Joe? If so, just up Highway 169 about 12 miles you go through the little town of Rochester. If you take Highway "V" east from Rochester, you'll come to a wide spot in the road known as Helena. In the fall of 1861, (before there was a Helena there), camp Highly was founded just on the outskirts of Helena, and it was a camp of instruction for the MO. State Guard. Some of the weapons these men were given had come from the Union arsenal at Liberty, Mo. That early in the war, I would assume most of these weapons were of .69 caliber. After a bit of training, the MSG at camp Highly left to take part in the siege of Lexington.
I don't think they look fired to me, but perhaps if they were shot out of a smoothbore, they wouldn't have any rifling on them, would they? The larger bullet looks cast to me, as the mold lines don't look like the exactly match up. These bullets have been lying in the ground for 150 years, and no telling how many times they've been stepped on, plowed over etc. so damage can happen to a dropped bullet much less one that has been fired. The difference in color? I'm going to assume you didn't find them in the same exact location and the difference in oxidation is due to the various chemicals in the soil.
I don't want to know where you found them, but I would like to know what city you live in. It may help in learning the possible history of these bullets. For example, if you live in St. Joe, they had a strong Union presence throughout the war. But if you live in Camden Point, then there's a good chance these bullets have a southern connection. If you live in Helena, then we're probably related.