diff --git a/src/bullet.lua b/src/bullet.lua
index 88184fb..7b8682a 100644
--- a/src/bullet.lua
+++ b/src/bullet.lua
@@ -7,19 +7,34 @@ local gfx <const> = playdate.graphics
 
 class("Bullet").extends(gfx.sprite)
 
-function Bullet:init(size, friendly)
+function Bullet:init(power, friendly)
+   Bullet.super.init(self)
+   self.power = power
+
+   local size = 1
+   if power == 1 then
+      size = 2
+   elseif power == 2 then
+      size = 4
+   elseif power == 3 then
+      size = 6
+   elseif power == 4 then
+      size = 10
+   end
+
    local img = gfx.image.new(size, size)
    gfx.pushContext(img)
      gfx.fillCircleInRect(0, 0, size, size)
    gfx.popContext()
 
-   Bullet.super.init(self, img)
+   self:setImage(img)
    self:setCollideRect(0, 0, self:getSize())
+   self.collisionResponse = gfx.sprite.kCollisionTypeOverlap
 
    local friendly = friendly or true
    if friendly then
       self:setGroupMask(0x4)
-      self:setCollidesWithGroupsMask(0x9)
+      self:setCollidesWithGroupsMask(0x8)
       self.direction = 1
    else
       self:setGroupMask(0x10)
diff --git a/src/kani.lua b/src/kani.lua
index b1b1917..673825e 100644
--- a/src/kani.lua
+++ b/src/kani.lua
@@ -7,8 +7,14 @@ import "bullet"
 
 local gfx <const> = playdate.graphics
 
--- the amount of charge needed to increase the shot size by 1
-local SHOT_INCREMENT <const> = 10
+-- the amount of charge needed to increase the shot power by 1 at each power level
+local WEAPON_CHARGE_LOOKUP <const> = {
+   [0]=5,
+   [1]=15,
+   [2]=30,
+   [3]=49
+}
+
 -- maximum value the reserve charge can reach
 local MAX_CHARGE <const> = 100
 
@@ -24,7 +30,7 @@ function Kani:init()
    self:setCollidesWithGroupsMask(0x19)
 
    self.reserveCharge = 100
-   self.shotCharge = 0
+   self.weaponPower = 0
    self.firingMode = false
 
    -- input handlers
@@ -39,37 +45,42 @@ function Kani:init()
       rightButtonUp =   function() self.vector.x = 0 end,
       cranked = function(change, accelChange) self:chargeReserve(change) end,
       AButtonDown = function()
-         self.firingTimer = playdate.timer.keyRepeatTimerWithDelay(0, 500, self.chargeShot, self)
+         self.firingTimer = playdate.timer.keyRepeatTimerWithDelay(500, 1000, self.chargeShot, self)
       end,
       AButtonUp = function() self:fire() end,
    }
 end
 
 function Kani:chargeReserve(change)
+   if change < 0 then return end
    self.reserveCharge = math.min(self.reserveCharge + change / 15, 100)
    print("Reserve charge at " .. self.reserveCharge)
 end
 
 function Kani:chargeShot()
-   if self.reserveCharge > SHOT_INCREMENT then
-      self.shotCharge += 1
-      self.reserveCharge -= SHOT_INCREMENT
-      print("Shot charged to size " .. self.shotCharge)
+   if self.weaponPower >= 4 then
+      return -- weapon is fully charged
+   end
+   local requiredPower = WEAPON_CHARGE_LOOKUP[self.weaponPower]
+   if self.reserveCharge > requiredPower then
+      self.reserveCharge -= requiredPower
+      self.weaponPower += 1
+      print("Shot charged to size " .. self.weaponPower)
    end
 end
 
 function Kani:fire()
    self.firingTimer:remove()
 
-   if self.shotCharge <= 0 then
+   if self.weaponPower <= 0 then
       return
    end
 
-   print("Creating bullet of size " .. self.shotCharge)
-   local bullet = Bullet(self.shotCharge * 2)
+   print("Firing bullet of size " .. self.weaponPower)
+   local bullet = Bullet(self.weaponPower)
    bullet:moveTo(self.x+16, self.y)
    bullet:add()
-   self.shotCharge = 0
+   self.weaponPower = 0
 end
 
 function Kani:addInputHandlers()
diff --git a/src/main.lua b/src/main.lua
index d83f738..6d295fa 100644
--- a/src/main.lua
+++ b/src/main.lua
@@ -26,14 +26,15 @@ end
 function makeWalls()
    makeWall(200, 0, 400, 1)
    makeWall(0, 120, 1, 240)
-   makeWall(200, 0, 400, 1)
-   makeWall(0, 120, 1, 240)
+   makeWall(200, 240, 400, 1)
+   makeWall(400, 120, 1, 240)
 end
 
 function makeWall(x, y, w, h)
    local wall = gfx.sprite.new()
    wall:setSize(w, h)
    wall:setCollideRect(0, 0, wall:getSize())
+   wall:moveTo(x, y)
    wall:setGroupMask(0x1)
    wall:add()
 end