I remember that thread, or post vaguely. From my readings, not only U.S. arms were collected from the citizens, but such gov't property as blankets, knap sacks, bayonets, cartridge boxes, what ever could be reused. They would ask, but not enter unless they had a good idea the citizen had gathered and was holding gov't property.
In her book, Tilly Pierce, a 12 year old Gettysburg resident tells of a soldier giving her a keep-sake musket just after the battle. When word reached the provost she had a musket, it was taken. She and her parents protested, giving the soldier's name and unit who gave it to her. As I recall, it was returned to her later the same day after the story was verified. No such leniency was shown to gawkers and onlookers who flocked to Gettysburg after the battle who picked up muskets and other property to take as they pleased. I recall the story of a judge who tried to sneak a musket away as a souvenir from his trip to the battlefield, he was made to bury horses for a day as punishment.
Some soldiers, or whole units took the opportunity to up-grade their arms on the field. A N.J. regiment issued faulty Enfields did just that, stacking their Enfields neatly on the field and reissuing themselves Springfields found among the dead. One of the top enlisted men in the regiment picked up a Richmond musket, declaring it was every bit as good as any Springfield and would be happy to use it against it's former owners. Detachments were usually sent to the field with wagons to gather muskets and other gov't property for reuse. As far as I know it was their responsibility to make the weapons safe for transport to Washington Arsenal which meant unloading them in the field.
There was a lot to do after the battle, bury the dead, care for the wounded, send P.O.W.'s off to interment camps, and glean the field for useable weapons and supplies for re-issue. To the misfortune of several Gettysburg citizens, fired artillery shells were not something the gov't gathered after the battle. They would buy any (lead?) balls contained in them, but didn't want the entire shell costing the lives and limb of several, (usually boys) Gettysburg citizens who were scrapping the shells for extra money. On a more positive note, a pair of enterprising Gettysburg lads borrowed money to set themselves up in a temporary suttler business selling tobacco to the troops. As I recall, they paid back the loan to their parents and gained a tidy sum for themselves.
This from Greg Coco's and Tilly Pierce's books.