I've been looking at the "Big Guns" that Rogues have available in their class. There are currently 4:
1) .45 Thompson
2) Chicago Typewriter
3) Beretta M1918
4) Sawed-Off Shotgun
Big Guns are useful to kill bosses and mini-bosses.
After reading through the reviews on this website for each of these 4 cards, it appears that the summary is:
The .45 Thompson and the Chicago Typewriter are better served in the hands of investigators with a naturally high Strength. Sharpshooter is helpful, but not a "must include".
The Beretta M1918 and the Sawed-Off Shotgun are probably better served in the hands of investigators with a naturally high Agility. Sharpshooter is very helpful, quite possibly a major contributor to the success of these guns.
Let me explain:
.45 Thompson and Chicago Typewriter
Neither of these two guns have a "succeed by" motif, so you don't need to aim exceptionally high. Assuming a 3 Strength Rogue with Lonnie Ritter and Delilah O'Rourke in play, they are already at a 5 base fight. If they are fighting a 4 Fight enemy, and the worst modifier in the bag is -5, then they need to be testing at a difficulty of 9. That means they are missing 4 more skill to beat the test. That can be garnered in different ways: Hard Knocks, Well Connected, High Roller, skill cards, etc.
Beretta M1918 and Sawed-Off Shotgun
These guns have a "succeed by" motif. The Beretta wants to over succeed by 4, and the Sawed-Off Shotgun wants to over succeed by at least 4, but ideally 6 for maximum value. Keeping with the same math as above (3 Strength Rogue, vs 4 Fight enemy, worst modifier is -5), then testing at a difficulty of 9, if you pull the -5 token, gives you an over success of 0. So, the Beretta wants you to test at 13, whereas the Sawed-Off Shotgun wants you to test at 15. WOW! Those are BIG NUMBERS!
Sharpshooter can help in these instances. Of course, cards like Well Connected, High Roller and Hard Knocks are helpful. But Sharpshooter can have a bigger impact, in just one card.
So right there, Sharpshooter is giving a bonus of +4 to +5, depending on the investigator.
Sharpshooter is capable of giving even more bonuses than that.
- your attack now uses Agility instead. Rogue cards have a lot of Agility icons, more than Strength icons. This means you are more likely to have cards with Agility pips to commit to boost the test. There are a handful of double-Agility icon cards in Rogue.
- you can choose to use the enemy's Evade value rather than their Fight value. I have not done a thorough search through bosses and mini-bosses stats, but I will estimate that enemies generally have equal or lower Evade values, compared to their Fight values. So Sharpshooter can possibly give extra bonuses from this difference in values.
TLDR: Sharpshooter can be a very strong card for a Big Guns Rogue who wants to be able to effectively kill bosses. If equipping the Beretta M1918 or Sawed-Off Shotgun, then Sharpshooter is a strong enabler to land those over-succeed by targets.