I bought my first TopGun in last year, and my second TopGun in last month. I find that the version of the two TopGun are different. It is marked "BT1" at the handle of the second gun, while there is nothing at the same position of the first gun.
Recently, I install the gun in my new computer (XP sp2). I am using Smog's GunCon2PC driver. I find that my first gun is very accurate, but my second gun is not accurate for the lower half of the screen.
After tons of testing and debugging, I finally solved the problem by the software calibration provided by the GunCon2PC driver.
In the GunCon2PC panel --> calibration, I unclick the "LCD Gun defaults" and set the Edges as follows:
Top : 31 Left : 137 Bottom : 255 Right : 711
This setting is good for the newer TopGun (BT1 version). For my old TopGun, I just click the "LCD Gun default" box and it works very well.