In the bottom right (underneath my weapon) it shows 3 green bars and 7 red bars - what do these represent? The same bars are displayed regardless of which weapon I have equipped.
The green bar indicates the number of rounds for your BFG 9000, and the red bar indicates the amount of fuel for your Chainsaw.
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It's worth noting that your BFG 9000 only consumes one "bar" of ammunition per shot, put your chainsaw may consume multiple "bars" of fuel depending on the enemy slay with it. – PausePause Mar 31 '20 at 16:28