So, did a little analysis of my 709 creature database vs. a 6th-level (minimum for variant human STR 20 with standard array/point-buy and GWM feat) with a greatsword +2, and here are the results of using Action Surge for 4 attacks. FYI, this was a Battle Master build with Precision attack and any other maneuver which would allow you to add the superiority die for damage.
The table below shows the % of opponents of a given AC or CR that the fighter would kill in the Action Surge round.
For example, of the 56 CR 3 monsters in my file, the Fighter would kill 86% of them (only 8 have sufficient HP to survive).
For example, of the 102 AC 15 monsters in my file, the Fighter would kill 55% of them in the Action Surge round.
As you can see, once you get to a certain AC or CR, and the creature is either too hard to hit enough and/or has too many hit points to kill in one round with Action Surge.
Now, this is the type of optimized build I don't typically see in my own games, but for others it might be interesting.
Finally, depending on AC, when GWM is used typically damage over the 4 attacks ranges from about 50 - 90 in the round. Oh, and I did include critical hits on the 20.
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