There are major changes coming to the virtual rifle building tool, Gunstruction, with the next update. There will be new options and new parts but the biggest news is that there will be more free content in the app. The new version (4.6) will give you access to all weapons (AR-15, AR308, and R700) for no cost along with save slots to save your creations.
The changes are not in the respective app stores yet but should be available soon.
Doesn’t seem to work on an iPad.