The army has learned that the historical older guns and cannon have value, both in monetary and historical reasons. The practice of destroying them has almost ended. The old guns you have seen most likely have been transferred elsewhere, and not destroyed. There has been many movements of old guns between the national parks to balance off their collections at the parks. An example is the transfers from the older established national military parks to the new park being established at Pea Ridge Ark. and Maybe, at Wilson Creek MO. Also, loans of guns between parks have occurred at various times. I hope there are members who can expand on this short answer of mine. I will watch this post for any further info that I want.
Hope this helps.