A semi-automatic six-round pistol already exists.
It's an air repeater, and it's balanced just about the same as a flintlock pistol.
note: the long air repeater is listed as being one-handed, but it's called out as two-handed in both its own text ("the long air repeater has better range and ammo capacity than the one-handed variant") and the air repeater's text ("the air repeater and its longer-ranged, two-handed variant"), so I'm treating the long air repeater as two-handed.
This would use the "repeating" trait.
There are a handful of weapons with the repeating trait:
A repeating weapon is a type of ranged weapon with a shorter reload time. These weapons can't be loaded with individual bolts or bullets like other crossbows and firearms; instead, they require a magazine of specialized ammunition to be loaded into a special slot. Once that magazine is in place, the ammunition is automatically loaded each time the weapon is cocked to fire, reducing its reload to the value in its reload entry (typically 0). When the ammunition runs out, a new magazine must be loaded, which requires a free hand and 3 Interact actions (to remove the old magazine, retrieve the new magazine, and slot the new magazine in place). These actions don't need to be consecutive and are the same as Interacting to reload.
Let's compare a crossbow and a repeating crossbow, as well as a a flintlock pistol and air repeater.
Attribute |
Crossbow |
Repeating crossbow |
Flintlock pistol |
Air repeater |
Price |
3gp |
15gp |
6gp |
5gp |
Damage |
1d8 P |
1d8 P |
1d4 P |
1d4 P |
Bulk |
1 |
1 |
1 |
L |
Hands |
2 |
2 |
1 |
1 |
Range |
120' |
120' |
40' |
30' |
Reload |
1 |
0 |
1 |
0 |
Traits |
(none) |
Repeating |
Concussive, fatal d8 |
Agile, repeating |
Category |
Simple |
Advanced |
Simple |
Simple |
Group |
Bow |
Bow |
Firearm |
Firearm |
Rarity |
Common |
Uncommon |
Uncommon |
Uncommon |
A +1 repeating pistol would be largely equivalent to a +1 pistol.
Overall, you're basically describing the difference between a flintlock pistol and an air repeater. The flintlock pistol has to be reloaded each shot, while the air repeater has a six round magazine. The significantly higher cost to load a magazine compared to a bullet appears to be intended to balance out the benefits of not needing to reload between each shot, along with losing some beneficial traits.
Balancing an eight-round repeating pistol would depend on traits and any other modified attributes.
As we can see above, the tradeoff for adding repeating to a weapon seems to mostly be losing traits. The long air repeater compared to the flintlock musket is in a similar boat: it loses concussive and fatal d8 in exchange for gaining repeating.
If we consider the air repeater balanced with the flintlock pistol, adding +2 rounds to the air repeater and changing nothing else does not seem like a big enough difference to say a +1 eight-round repeater would be equivalent to a +2 flintlock pistol. If you wanted to have, say, a 120' range Colt 1911 with agile, repeating, and fatal d8, that should definitely come at a cost to your +X, but just increasing an air repeater to an eight-round magazine (effectively the printed long air repeater on AoNPRD) fits within the existing balance buffers.