Ok, then... It may make more sense after you read the original post in the AoMr discussion topic by Fingerzam, but I'll try to re-explain.
Advanced pistols in Angel of Marksmanship are no longer 2d5; they are 2d4 like normal pistols (though they can have spoilies attached, which can end up making them far better than what you'd originally have from the 2d5 pistol). This means that instead of having one 2d5 pistol (which does 2-10 damage) and one 2d4 pistol (which does 2-8 damage), you have two 2d4 pistols (which both do 2-8 damage), which results in lower average damage than what you'd get from previous AoMr games, as well as the loss of the advantage in taking Son of a Gun 3 before Dualgunner 1.
When you get Son of a Gun 2 in the vanilla 9.8.5 release, you get +4 to your pistol damage rolls, and this means that for a 2d5 pistol your damage is upped from 2-10 HP of damage to 6-14 HP of damage. However, for a 2d4 pistol you only get your damage upped from 2-8 HP of damage to 6-12 HP of damage. If after getting Son of a Gun 2, you choose Dualgunner 1, in the vanilla release you are shooting one pistol that does 6-14 HP of damage and another that only does 6-12 HP of damage. This averages out to 10 damage from the 2d5 pistol and 9 damage from the 2d4 pistol for a total average damage of 19 HP of damage (per shooting of both pistols). However, in the beta both pistols are 2d4, so both pistols have an average damage of 9 HP, which leaves you with a total damage of 18 HP per shooting of both pistols. When you empty both clips (by firing both guns 6 times), in the vanilla release, you'll do on average 114 HP of damage (19 x 6), whereas in the beta you'll do on average 108 HP of damage (18 x 6). In both versions, Son of a Gun 2 lowers the energy used (a marker of how long actions take; the lower the number the faster the action happens and the more time you have before the enemies react) from 1000 to 600, but the addition of a second gun with Dualgunner 1 multiplies the amount of energy used by 1.4, increasing the energy usage from 600 to 840 per fire action, and the total energy usage for emptying both clips is 5040 energy (840 x 6).
Compare this to what happens when you take Son of a Gun 3: You get +6 to your pistol damage rolls, and your shots take 400 energy (200 less than with Son of a Gun 2, and 440 less than with Son of a Gun 2/Dualgunner 1). In the vanilla release, this is a superior choice even when taking reloading into account (reloading taking 1200 energy), because you are using a 2d5 pistol: With the same amount of energy it takes to fire 12 shots with Dualgunner 1 (5040 energy), you can fire 9.6 shots with Son of a Gun 3 (5040 - 1200 [one reload] = 3840, 3840 / 400 [energy it takes to fire with Son of a Gun 3] = 9.6 [total number of shots possible]), and while those 12 shots on average will add up to 114 HP of damage on average, the 9.6 shots you take with Son of a Gun 3 will add up to an average damage of 115.2 (damage of a 2d5 pistol after Son of a Gun 3 is 8-16 HP; average damage out of that is 12; 12 x 9.6 [the total number of shots you can take with 5040 energy] = 115.2). However, with the beta your primary pistol is only 2d4, which means that with Son of a Gun 3 your damage range is 8-14 HP. Average damage out of that range is 11, and 11 times 9.6 is 105.6 HP worth of damage, which is 2.4 HP less damage on average than going Son of a Gun 2/Dualgunner 1.
:sigh: I hope that explains it a little better; if it's the formatting that turns you off I'm afraid I may not be able to help you there... :/ Just the way I write.