That Broken GWM/SS "Fix"

The most notorious feats in 5th Edition D&D are Sharpshooter (SS) and Great Weapon Master (GWM). These feats are considered the most powerful because of a mechanic they share in common.

The RAW Mechanic

The GWM/SS mechanic allows a character to subtract 5 from their attack roll in exchange for adding 10 to their damage roll.

This effect is pronounced when the attacker rolls with advantage, which mitigates a fair portion of attack detriment. As a result, the GWM/SS problems can be exacerbated when combined with other feats that affect or simulate advantage. Elven Accuracy gives you an additional advantage die to roll when you have advantage (i.e. "superadvantage"). Lucky allows you to basically simulate advantage (or turn disadvantage into superadvantage, unless you're playing RAI).

The Common "Fix"

The most popular "fix" I've seen for this problem is to change the math on the -attack/+damage mechanic. That's a good place to start, but the devil is in the details.

The gut-check problem with these feats is that +10 is just a LOT of damage to add to a single attack at low levels. But, it seems less crazy at high levels. So people think the result is to scale the penalty/bonus. This is a little fiddly and atypical of 5e weapon damage design, but not a bad intuition.

A lot of homebrewers hear "scaling" and think "proficiency." This is rooted in good design instincts, because it's something that is pegged to and balanced around character level progression. So, they decide the penalty/bonus should be based around proficiency.

Well, you can't just add proficiency to damage and subtract it from attack, because that's going to make almost every attack worse. A flat attack bonus will generally outperform a flat damage bonus under 5e's math.

Instead, they subtract proficiency from attack, and add double proficiency to damage.

This creates some issues.

The Problem With Proficiency

Homebrewers love shoehorning proficiency into everything, when they really shouldn't. Proficiency should be a standard number added when you have proficiency in something. Otherwise, you are muddying the term, which can confuse players at the table.

Another issue is that classes and items are generally already balanced around including proficiency. When you add it somewhere else, it creates a multiplicative effect. Or, in the case of a GWM/SS "fix" that subtracts proficiency from attack, it "zeroes out" the bonus. You never really experience the penalty, since it is matched with commensurate proficiency increases.

Generally, there are better candidates in the cases of items. In keeping with design convention, a number of charges should be fixed to a flat number. If you want scaling, then using an ability modifier can promote player choice.

Unless you have a really creative implementation, you should avoid leveraging proficiency for other design purposes.

The Problem With the "Fix"

You haven't changed the ratio.

You're reducing the attack penalty, which has a greater impact on damage output than an equivalent damage bonus. This takes all the guesswork out of using the feat. Instead of a tactical choice, it becomes an "always on" mode. Let's see how this bears out in the feats.

Sharpshooter

For the purposes of Sharpshooter, we're going to make a few assumptions to simplify the math:

• You take Sharpshooter at Level 4 (no Vuman).
• You max Dexterity after taking Sharpshooter.
• You have the Archery fighting style.

These numbers will change a bit if you're playing a Rogue, who can't get access to the fighting style, and has a lot more damage dice to account for. In general, Rogues prefer to make sure they hit so they can proc sneak attack, so reducing your attack odds is not a good idea. If you're building around Sharpshooter, you should look into Fighter or Ranger instead.

We need to examine the progression every time we get an attack or damage boost:

• Level 4 (Sharpshooter feat)
• Level 5 to 7 (Proficiency Boost)
• Level 8 (ASI boost Dexterity to 18)
• Level 9 to 11 (Proficiency Boost)
• Level 12 (ASI boost Dexterity to 20)
• Level 13 to 16 (Proficiency Boost)
• Level 17 to 20 (Proficiency Boost)

The problem is that the fix smooths out the curve across the armor classes. So instead of becoming a great tool for one-shotting low-AC mooks, it deals double or triple the damage against high-AC foes. Since Sharpshooter previously presented a tactical decision only against high-AC foes, the change makes it so that "always on" should be the default.

Notably, when you hit Level 13, the math is identical to Sharpshooter. When you get your last proficiency boost at level 17, the curve slightly inverts and the "fix" becomes more useful against low-AC targets and less useful against high-AC targets. However, by this tier, Sharpshooter is pretty much "always on" anyhow, so the change is making little tactical difference. The numbers are also very close.Great Weapon Master

The numbers bear out in a similar way for Great Weapon Master (GWM). We need to run them a little differently. Unlike Sharpshooter, it's hard to make safe assumptions here. You might be running a Fighter/Paladin with a fighting style or you might be running a Barbarian with Rage damage. Either way, these add a damage boost, unlike Archery, which adds an attack bonus to mitigate the attack penalty of Sharpshooter.

Since these myriad variables are hard to parse, let's just take a look at a plain 2d6 weapon with no fighting style at level 8 for illustration:

Generally, with lower attack bonuses and higher damage dice, the AC benchmark where it makes sense to use the feat gets lower. Lower attack bonuses mean you are suffering a relatively greater detriment from the attack penalty. Higher damage makes it more important to hit, since a smaller percentage of your damage is coming from the boost.

When applying this to the GWM "fix," we really see where the tactical decision comes into play. It can be really damaging to use the feat against the wrong foe. Since the "fix" smooths out that dynamic, it removes most of the situations in which you would have to make a tactical decision. Otherwise, it mitigates the mistake you would make by making a tactical blunder.