diff --git a/shmup.cpp b/shmup.cpp
index 8adcdce1..b79279a4 100644
--- a/shmup.cpp
+++ b/shmup.cpp
@@ -12,7 +12,7 @@ namespace shmupballs {
   int xmove, xfire, yb, rad;
 
   void calc() {
-    rad = vid.radius * (vid.mobilecompasssize ? vid.mobilecompasssize : 14) / 100;
+    rad = min(vid.radius, min(vid.xres, vid.yres) / 2) * (vid.mobilecompasssize ? vid.mobilecompasssize : 14) / 100;
     xmove = max(vid.xcenter - vid.radius - rad, rad);
     xfire = min(vid.xcenter + vid.radius + rad, vid.xres - rad);
     yb = vid.ycenter + vid.radius - rad;