Yeah, give up on the idea. Autonumbers should not be used for this purpose - there is just too much that can go wrong, aside from not always being sequential, which is what I think you are saying. You cannot delete a record and expect Access to reuse the autonumber you deleted in order to keep the sequence going. There are better ways.
One would be to find the last saved value in an indexed (no dupes) field and increment by whatever you want and use that. In a multiuser db, that can cause write conflicts when two people are in create mode and one has not saved the record they're working on IF you try to start the record creation with that number. Create the number when the record is completed and saved, otherwise, they are each starting a record with the same ID.
Another way would be to store the NextInvNum in a table. This value could be updated when a record is started or saved. On start, a concurrent user updates this value thus starts with a different ID.
In either case, committing the ID at the start means the potential drawback is that if one user cancels the record, the ID is not sequential. If that is really that important, the record could exist but the ID could be statused as Cancel. Even paper based systems did not enforce sequential ID's. It's impossible to guarantee that a numbered paper document would not get totally screwed up and end in the waste basket, so why try enforce sequental IDs? The only way I know of that you can 99.9% ensure sequential IDs is to assign someone the task of meteing them out one by one, only when the record is ready to be committed as complete. Otherwise, the second method should work IF the record ID is created when the record is finished and will NOT ever need to be deleted.
