The PSG1 shoots pretty quickly already. If this is for the campaign mode, put on outrange. If you're playing raid, put on Fire Rate because # of bullets far outweighs power in raid.
This is for Raid Mode.
So Fire Rate 4 outputs more damage per second than Outrange 3 does?
I believe it'd be a largely similar output of damage per second but Fire Rate 4 is more consistent as it in effect all the time where Outrange 3 only kicks into optimal damage at a certain range.
In Raid Mode, its less clear cut than people make it about to be about what's really better. Some on here say rate of fire matters more but it depends on what stage you're doing. A long stage like the Ghost Ship or the later stages of Abyss mode may stretch your ammo supply. If you intend to do a long route through the Ghost Ship, a genocide run perhaps, then you want to consider tailoring your weapon to do as much damage per shot as you can. The PSG1 already has a relatively fast natural fire rate which is far superior to the bolt action rifles.
That's said, I usually have both Fire Rate 4 and Outrange 3 on my PSG1. I do have the Full Burst/ Auto Loader/ Gluttony PSG1 but I prefer to only use that on Stage 20 Abyss where it's kill time truly shines. On a long haul like the Ghost ship or any of the later Abyss stages I prefer to run this:
That set there gives you fast, powerful shots but allows you a lot more control over your ammo expenditure which is why I prefer it for the longer Abyss stages over the FullAutoGlut-PSG1. You don't really lose out from not having Full Burst. Auto Loader could be replaced by something of your preference but you generally want to be laying down continuous fire as the PSG1 usually takes a couple of shots to down a higher level ooze and you don't want to be trying to take on a mob of them and reloading every 2 seconds. Gluttony allows you to use the PSG1 as a battering ram that keeps on going. --- Well, I suppose it's time to get kidnapped again...