On paper, this mastery seemed really weak given the short range of pistols and ammo problems. However, the more I played with it, I found this build to be surprisingly good.

For ammo, most people would get scavenger, but this will delay to you from getting Hellrunner 3 as soon as possible for the dodge and movement speed. Like Onslaught, this mastery uses movement speed to boost damage, so movement speed is crucial as defense and offense.

After experimentation, the solution I came up is to not use .44 revolvers but use the 7.62 sidearms where ammo is plentiful in Europa and Io. The 9mm calibrated pistol's damage is really poor for the endgame. The other solution to the ammo problem is to make sure that you get the most damage per bullet, which is getting crit chance as close to 100%.

Obligatory link to sylph's guide for basics.
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2714349654

2. Hellrunner.
3. Son of a Gun.
4. Son of a Gun.
5. Gunslinger.
6. Gun Kata
7. Hellrunner.
8. Hellrunner.
9. Son of a Gun.
10. Gun Kata.
11. Dash.
12. Dash.
13. Dash.
14. Gunslinger.
15. Gunslinger.
16-17. Gun Kata/Dodgemaster.

If you really feel like you need the 3rd weapon slot, you can get Hoarder at level 2 and get the three levels of Hellrunner at 7-9.

As mentioned in introduction, Hellrunner 3 is a high priority after getting Gunslinger and Gun Kata.

Possibly experiment with getting Hacker at 9 and Whizkid 11 if you want to add crit 10 to your JS armors and not be at the mercy of the RNG on the AV3 amp.

After maxing Hellrunner and SoG, the next priority is dash to increase movement speed further. Dashing will also reduce damage taken. The way to proc dash is:

"can you see an enemy?
will you move in the same direction you were moving before?
then you will dash."
(Credit Dr. Granola from the JH discord.)

The final priority will be maxing out Gunslinger for faster reloads.

Gun Kata level 3 is not required.

You could invest the last point into Dodgemaster to retain some dodge chance when reloading.

The best options here are the Rift then Mines. Rift is far superior because of the guaranteed armor chest at the end. There were some runs where I went through 3 level of Mines and did not get any JS armor sadly.

If you are not comfortable with the difficulty of Mines and there is no Rift, you can try Docking Bay for a possible JS visor.

Be careful with the MDF sidearms. You do not want to calibrate 1 and auto-calibrate these guns because the extra range actually wastes bullets since you are more likely to trigger Gun Kata when enemies are in cover, and easily lead to ammo exhaustion.

Europa L1:
Make sure to print two 7.62 sidearms from the manufacturing station if you don't have them already.

With the 7.62 sidearm plan, the recommended branch option is Refueling Base/Pit because you will need the backpack to avoid ammo exhaustion. The dual pistols should provide adequate firepower to clear the difficult Refueling Base. The AV3 pistol amp is badly needed and you are looking for the crit chance 20% or 40% perk to get at least 100% crit chance.

Depends on how happy were you with your AV3 pistol amp, you might need to get 3 red keycards for Io Black site or CRI labs (ouch) to reroll your perks. If you don't have a problem with your amp, just head to the Io Warehouse/Infernal lock. There is also a good chance you might find a level 2 mfg station in a vault to reroll into the 20% crit chance amp perk. I cannot recommend Shadow Halls because of the potential for ammo exhaustion and the Mines as well since it is unlikely that you will have poison resistance or immunity.

Ammo exhaustion will be a real concern now since 7.62 ammo drops will be rare at this point. I recommend just beelining to the exits. 500 bullets should be plenty. If you want to try to kill as much as possible, you might be need to pack 700 bullets, and even more if you want to try Dante Inferno (I can't recommend because of archwarlocks).

For the 7.62 sidearms, add Speed-loader (bulk), Calibrate 1 (accuracy), auto-calibrate and leave one slot open for possible cold/emp/vampiric mods, or you can print AV1 7.62 sidearms and gamble on the AV perks.

DO NOT GET Calibrate 1 if you are using MDF sidearms.

JS armor wise, this is the only mastery where you can defend using Carrier instead of Meshed for the power pack because you really need the inventory space for 7.62 ammo, but there is nothing wrong with going for standard Meshed (power), Durable (bulk), Swift (accuracy). Swift is a high priority mod.

Just Durable for your JS helmet is necessary.

If you do get whizkid 1, add Crit 10 (accuracy) as the 4th mod slot if your luck with the pistol AMP was bad.

For pistol amp:

Pistol accelerator/Pistol hit tracker (+20%/+40% critical chance): High priority. Keep in mind the critical damage up from Gun Kata level 1.

Pistol booster/Pistol capacitor (+20% damage/+25% critical damage): Very good perks but less of a priority than the critical chance perks.

Pistol speed-loader (75% faster reload action if weapon is empty): Really useful when you are stuck at Gunslinger level 1 with the slow reload speeds.

Optimal range is not that useful since Gun Kata procs depend on the optimal range printed on the weapon.

The ideal AV3 Pistol amp would have Pistol booster and Pistol accelerator.

If you are having ammo problems at Io, grab a level of Hoarder and use any plasma weapon or launcher in Io that you find. It might be prudent to just beeline to the exits as well since Dash and Gunslinger aren't critical.

For UV and below, where ammo is not plentiful, you might need to get 2 levels of Scavenger after Hellrunner 3.

https://youtu.be/KvmehAWN9Tk
Thank you for reading this guide.