Burned on the River

Burned on the River is a side quest in the Fallout 4 Creation Club content "Solar Cannon."

Synopsis
On the trail of valuable prototype pre-War weaponry, investigate a Gunner named Sarge who has been amassing a stash of loot, apparently including one such weapon.

Quick walkthrough

 * First clue at the South Boston military checkpoint on a corpse in a trailer.
 * First safe opened by pressing bell 4 times at the Red Rocket near Big John's Salvage.
 * Second safe opened by entering code 5-3-6.
 * Third safe opened by code 1-2-11-12.

Detailed walkthrough
Find three notes, one on a Gunner at the South Boston military checkpoint and the others in two safes near Big John's Salvage. Use Sarge's clue #3 ("lining up the cards") to unlock the final safe.

First clue
The note "Sarge's clue #1" can be found on the body of a Gunner named Jack, located in a trailer at the military checkpoint. Follow the note's instructions and go to the nearby Red Rocket filling station north of Big John's Salvage. In the garage, a locked wall safe can be found, wired to a bell on the checkout counter, with the body of Queen nearby. From the instructions given by Sarge's clue, the wall safe will open if the bell is pressed the same number of times as the amount of leaves on Queen's facial tattoo; in other words, four times. Alternatively, the safe can be unlocked by 'hot wiring' it with Hacker rank 1 or an Intelligence of 5 or greater.

Second clue
The safe in the Red Rocket contains a note titled "Sarge's clue #2", which then leads to a small blue-painted house to the east of the Red Rocket, with the body of Ace in front. A floor safe will be found in the red dog house behind the house itself, requiring a three-digit code to unlock. Sarge's clue points to "the rockets pinned in the alley, then the craters holed in the moon, and then finally, the stars underneath." This refers to the billboard for Back Alley Bowling on the roadside, namely the number of bowling pins, holes in the bowling ball, and stars to either side of the word "Bowling". The code is 5-3-6.

Third clue
The second safe contains Sarge's third and final clue, "Sarge's clue #3", pointing to Sarge's stash safe, located in a bus inside the underpass just south of D.B. Technical High School. Deuce is inside and will attack. The safe requires a four-digit code to unlock; Sarge's clue hints that "It's as simple as lining up the cards." The code is 1-2-11-12, corresponding to the playing card values associated with the names of Sarge's Gunner crew, in the order he listed their names in his third note. Alternatively, the code can be bypassed with Locksmith rank 4.

Once the safe is unlocked the quest will immediately complete. Inside the safe is a legendary solar cannon with the "Exterminator's" legendary effect, as well as Sarge's note and some other loot.