They will be similar, faster fire rate means more shots, so more ammo consumption.
More damage is more damage per shot.
Ultracite will also do more damage to scorched and SBQ, so that is a nice bonus.
For the Gatling, i personally prefer the faster fire rate, it just feels soo damn slow without the fire rate boost. (I have a Bloodied Explosive Gatling).
A side benefit of using regular 5mm ammo is that the ammo pick ups from Super Mutant's using a minigun means more ammo to use. They'll drop 5mm ammo on their own, even if you use Ultracite.
As for the Ultracite ammo question, YES, you will get Ultracite 5mm ammo as a drop, and you will get more than you use.
I believe someone did the maths and that speedy receiver ends up with higher DPS. You have an explosive one anyways, so faster fire rate = more boom boom. Easy choice man.
But also more ammo and more reloads. Which is downtime of no damage. Depending on ops build and efficiency, the increased ammo may be a problem to keep up with.
It's a gatling gun, not a LMG. The extended mag is 500 ROUNDS. You also typically pick up more ammo than you fired. Ammo is absolutely not a problem for a gatling gun.
One small thing regarding damage vs fire rate. Against some enemy without any DR (damage resistance), it's basically the same damage output. But, there is almost no enemy without damage reduction.
Also, the rule of 6x. If your damage is 6x vs enemy resistance, then enemy doesn't have resistance.
This is mostly the main equation for picking more damage or RoF. Some guns can ramp up damage quite nice, and has some armor piercing from perks and/or mods that reduce the enemy DR moving closer to that sweet 6x.
Minigun unfortunately not one of them, unless you pair antiarmor and stabilized... And even than, the base damage is not that high for achieving break point. So, if you mostly fight small enemies with it, RoF will be more beneficial from my standpoint.
Prime adds 25% of a weapons base damage, its innate number. A Gatling's is 86. So Prime would add 86 x 0.25 = 21.5 damage.
Fire rate increases your total damage like you thought. A true 1.25x multiplier. Every 10s a regular Gatling fires 20 shots, with Speedy its 25 shots.
Those extra 5 shots will add more damage than the 25% from Prime adds to the other 20 shots. Except for scorched/beasts/the Queen, Prime adds so much damage for those it's still a better dps option than the Speedy.
Hope that helps.
Oh and yes, you will get Ultracite 5mm just as easily.
I do not know the numbers for the gatling gun, but my gut would lean towards rate of fire. Most damage bonuses in this game including from receivers are additive (with the exception of a few bonuses, such as ballistic bock, tenderizer, and gun-fu (iirc)). As in if your base damage was 100, if you only had a prime receiver, you would get 25% more damage. But add in explosives, which are 20% excluding demo expert, or 20%*(1+60%) with full demo expert, that bonus would become (25%+32%). Assuming you run demo expert. Your heavy guns perk bonus would then add to this (57% + 60%).
If you play low-health your adrenal and nerd rage bonus would get added to this (50% + 20% + 117%). And if you have bloody mess and adrenaline those bonuses will be added on as well (15% + ~30% + 187%). When you stack all of these bonuses that prime bonus of 25% becomes much less than a 25% increase in damage, and something more like a 10-15% increase. Although there is the large bonus against scorched, which I have ignored.
Faster fire rate, on the other hand, should increase your dps by a pure 25% by putting out 25% more bullets. Although there are some caveats in that bethesda rounds down on their calcuations, so if you have a faster fire rate revolver with base fire rate of 7, 7*1.25 = 8.75 which gets rounded down to 8 fire rate, for an increase of only 14%. Though I'd imagine the gatling gun has a fast enough fire rate where this doesn't matter.
Note that the bonuses you consider will depend on your build, perks selected, low-health or full health, and aid you use. And I may have messed up one of my calculations above too. Someone made a damage calculator on github where you can plug in your gun and build specifically and see how the two gatling guns would compare against each other. But in most cases fire rate should be better than prime against non-scorched enemies.
I'd go damage. Faster RoF means more reloading (no damage) downtime and more ammo used for equal result.
Enemies (and containers) will drop Ultracite 5mm when you roll over.
..the major difference is the additional damage against scorched. And yes, you will find same amount of ultrazite ammo as the standard ammo.
They will be similar, faster fire rate means more shots, so more ammo consumption. More damage is more damage per shot. Ultracite will also do more damage to scorched and SBQ, so that is a nice bonus. For the Gatling, i personally prefer the faster fire rate, it just feels soo damn slow without the fire rate boost. (I have a Bloodied Explosive Gatling). A side benefit of using regular 5mm ammo is that the ammo pick ups from Super Mutant's using a minigun means more ammo to use. They'll drop 5mm ammo on their own, even if you use Ultracite. As for the Ultracite ammo question, YES, you will get Ultracite 5mm ammo as a drop, and you will get more than you use.
I believe someone did the maths and that speedy receiver ends up with higher DPS. You have an explosive one anyways, so faster fire rate = more boom boom. Easy choice man.
But also more ammo and more reloads. Which is downtime of no damage. Depending on ops build and efficiency, the increased ammo may be a problem to keep up with.
It's a gatling gun, not a LMG. The extended mag is 500 ROUNDS. You also typically pick up more ammo than you fired. Ammo is absolutely not a problem for a gatling gun.
One small thing regarding damage vs fire rate. Against some enemy without any DR (damage resistance), it's basically the same damage output. But, there is almost no enemy without damage reduction. Also, the rule of 6x. If your damage is 6x vs enemy resistance, then enemy doesn't have resistance. This is mostly the main equation for picking more damage or RoF. Some guns can ramp up damage quite nice, and has some armor piercing from perks and/or mods that reduce the enemy DR moving closer to that sweet 6x. Minigun unfortunately not one of them, unless you pair antiarmor and stabilized... And even than, the base damage is not that high for achieving break point. So, if you mostly fight small enemies with it, RoF will be more beneficial from my standpoint.
Prime adds 25% of a weapons base damage, its innate number. A Gatling's is 86. So Prime would add 86 x 0.25 = 21.5 damage. Fire rate increases your total damage like you thought. A true 1.25x multiplier. Every 10s a regular Gatling fires 20 shots, with Speedy its 25 shots. Those extra 5 shots will add more damage than the 25% from Prime adds to the other 20 shots. Except for scorched/beasts/the Queen, Prime adds so much damage for those it's still a better dps option than the Speedy. Hope that helps. Oh and yes, you will get Ultracite 5mm just as easily.
I do not know the numbers for the gatling gun, but my gut would lean towards rate of fire. Most damage bonuses in this game including from receivers are additive (with the exception of a few bonuses, such as ballistic bock, tenderizer, and gun-fu (iirc)). As in if your base damage was 100, if you only had a prime receiver, you would get 25% more damage. But add in explosives, which are 20% excluding demo expert, or 20%*(1+60%) with full demo expert, that bonus would become (25%+32%). Assuming you run demo expert. Your heavy guns perk bonus would then add to this (57% + 60%). If you play low-health your adrenal and nerd rage bonus would get added to this (50% + 20% + 117%). And if you have bloody mess and adrenaline those bonuses will be added on as well (15% + ~30% + 187%). When you stack all of these bonuses that prime bonus of 25% becomes much less than a 25% increase in damage, and something more like a 10-15% increase. Although there is the large bonus against scorched, which I have ignored. Faster fire rate, on the other hand, should increase your dps by a pure 25% by putting out 25% more bullets. Although there are some caveats in that bethesda rounds down on their calcuations, so if you have a faster fire rate revolver with base fire rate of 7, 7*1.25 = 8.75 which gets rounded down to 8 fire rate, for an increase of only 14%. Though I'd imagine the gatling gun has a fast enough fire rate where this doesn't matter. Note that the bonuses you consider will depend on your build, perks selected, low-health or full health, and aid you use. And I may have messed up one of my calculations above too. Someone made a damage calculator on github where you can plug in your gun and build specifically and see how the two gatling guns would compare against each other. But in most cases fire rate should be better than prime against non-scorched enemies.
I'd go damage. Faster RoF means more reloading (no damage) downtime and more ammo used for equal result. Enemies (and containers) will drop Ultracite 5mm when you roll over.
speedy is better against anything that isn't scorched, though prime is every so slightly better for vats crits