They might have been just put to weapon storage, can you check by trying to mount the mg by clicking the MG mount on Wolverine for example and then again the mg mount (same one) - this will open a special menu where you can select which mg to mount

About store to buy weapons - incoming. We need to do this as soon as possible

All outdoor machine guns will stop displaying after a certain amount of time. It should be enough to quit and restart the game and the machine guns will show up again for a while.