Running Count In Horizontal Row?

Is there any way to create a running count in one horizontal thread? I can create a running count in vertical columns,but not horizontal threads?

I have a spreadsheet that I want to keep track of how many rounds mulitple weapons have fired. I have a current rounds cell, a last fired cell and total rounds cell. The problem is that when I enter the number of rounds shot in the "last fired cell", the sum of the total rounds cell goes up (via the sum function) however, is there a way to keep that number there, even though I will replace the last fired cell with a new number the next time I fire the weapon?