simplify jam_mult formula

doc/JSON_INFO.md

@@ -3608,7 +3608,7 @@ ammo_effects define what effect the projectile, that you shoot, would have. List
 "count" : 0,                     // Default amount of ammo contained by a magazine (set this for ammo belts)
 "default_ammo": "556",           // If specified override the default ammo (optionally set this for ammo belts)
 "reload_time" : 100,             // How long it takes to load each unit of ammo into the magazine
-"mag_jam_mult": 1.25             // Multiplier for gun mechanincal malfunctioning from magazine, mostly when it's damaged; Values lesser than 1 reflect better quality of the magazine, that jam less; bigger than 1 result in gun being more prone to malfunction and jam at lesser damage level; zero mag_jam_mult (and zero gun_jam_mult in a gun) would remove any chance for a gun to malfunction. Only works if gun has any fault from gun_mechanical_simple group presented; Jam chances are described in Character::handle_gun_damage(); at this moment it is roughly: 0.000288% for undamaged magazine, 5% for 1 damage (|\), 24% for 2 damage (|.), 96% for 3 damage (\.), and 250% for 4 damage (XX), then this and gun values are summed up and multiplied by 1.8. Rule of thumb: helical mags should have 3, drum mags should have 2, the rest can be tweaked case by case, but mostly doesn't worth emulating it
+"mag_jam_mult": 1.25             // Multiplier for gun mechanincal malfunctioning from magazine, mostly when it's damaged; Values lesser than 1 reflect better quality of the magazine, that jam less; bigger than 1 result in gun being more prone to malfunction and jam at lesser damage level; zero mag_jam_mult (and zero gun_jam_mult in a gun) would remove any chance for a gun to malfunction. Only works if gun has any fault from gun_mechanical_simple group presented; Jam chances are described in Character::handle_gun_damage(); at this moment it is roughly: 0.027% for undamaged magazine, 5% for 1 damage (|\), 24% for 2 damage (|.), 96% for 3 damage (\.), and 250% for 4 damage (XX), then this and gun values are summed up. Rule of thumb: helical mags should have 3, drum mags should have 2, the rest can be tweaked case by case, but mostly doesn't worth emulating it
 "linkage" : "ammolink"           // If set one linkage (of given type) is dropped for each unit of ammo consumed (set for disintegrating ammo belts)
 ```
 
@@ -4156,7 +4156,7 @@ Guns can be defined like this:
 "sight_dispersion": 10,    // Inaccuracy of gun derived from the sight mechanism, measured in 100ths of Minutes Of Angle (MOA)
 "recoil": 0,               // Recoil caused when firing, measured in 100ths of Minutes Of Angle (MOA)
 "durability": 8,           // Resistance to damage/rusting, also determines misfire chance
-"gun_jam_mult": 1.25       // Multiplier for gun mechanincal malfunctioning, mostly when it's damaged; Values lesser than 1 reflect better quality of the gun, that jam less; bigger than 1 result in gun being more prone to malfunction and jam at lesser damage level; zero gun_jam_mult (and zero mag_jam_mult if magazine is presented) would remove any chance for a gun to malfunction. Only apply if gun has any fault from gun_mechanical_simple group presented; Jam chances are described in Character::handle_gun_damage(); at this moment it is roughly: 0.00018% for undamaged gun, 3% for 1 damage (|\), 15% for 2 damage (|.), 45% for 3 damage (\.), and 80% for 4 damage (XX), then this and magazine values are summed up and multiplied by 1.8
+"gun_jam_mult": 1.25       // Multiplier for gun mechanincal malfunctioning, mostly when it's damaged; Values lesser than 1 reflect better quality of the gun, that jam less; bigger than 1 result in gun being more prone to malfunction and jam at lesser damage level; zero gun_jam_mult (and zero mag_jam_mult if magazine is presented) would remove any chance for a gun to malfunction. Only apply if gun has any fault from gun_mechanical_simple group presented; Jam chances are described in Character::handle_gun_damage(); at this moment it is roughly: 0.05% for undamaged gun, 3% for 1 damage (|\), 15% for 2 damage (|.), 45% for 3 damage (\.), and 80% for 4 damage (XX), then this and magazine values are summed up
 "blackpowder_tolerance": 8,// One in X chance to get clogged up (per shot) when firing blackpowder ammunition (higher is better). Optional, default is 8.
 "min_cycle_recoil": 0,     // Minimum ammo recoil for gun to be able to fire more than once per attack.
 "clip_size": 100,          // Maximum amount of ammo that can be loaded

src/ranged.cpp

@@ -685,7 +685,7 @@ bool Character::handle_gun_damage( item &it )
     int gun_damage = it.damage() / 1000.0;
     switch( gun_damage ) {
         case 0:
-            gun_jam_chance = 0.0000018 * firing.gun_jam_mult;
+            gun_jam_chance = 0.0005 * firing.gun_jam_mult;
             break;
         case 1:
             gun_jam_chance = 0.03 * firing.gun_jam_mult;
@@ -707,7 +707,7 @@ bool Character::handle_gun_damage( item &it )
         mag_damage = it.magazine_current()->damage() / 1000.0;
         switch( mag_damage ) {
             case 0:
-                mag_jam_chance = 0.00000288 * it.magazine_current()->type->magazine->mag_jam_mult;
+                mag_jam_chance = 0.00027 * it.magazine_current()->type->magazine->mag_jam_mult;
                 break;
             case 1:
                 mag_jam_chance = 0.05 * it.magazine_current()->type->magazine->mag_jam_mult;
@@ -724,7 +724,7 @@ bool Character::handle_gun_damage( item &it )
         }
     }
 
-    const double jam_chance = ( gun_jam_chance + mag_jam_chance ) * 1.8;
+    const double jam_chance = gun_jam_chance + mag_jam_chance;
 
     add_msg_debug( debugmode::DF_RANGED,
                    "Gun jam chance: %s\nMagazine jam chance: %s\nGun damage level: %d\nMagazine damage level: %d\nFail to feed chance: %s",