"I always have problem to properly calibrate my mouse when playing FPS games (Combat Arms)."
I know this answer is very late; but I've played quite a bit of Combat Arms over the years and I believe it's critical to OP's (former) issues.
Combat Arms in 2011 was using an engine similar to what is now available as "Combat Arms: Classic" (this is not the same as the one available on Steam called "Combat Arms:Reloaded"; which doesn't have these issues). In the old engine, there are a couple things to note about mouse movement.
Combat Arms had forced mouse smoothing (I can't find the reference for this anymore, it was said by a developer on Nexon forums when they were working on CA:Reloaded). The best way to help avoid major effects from that was to put a cap on the FPS that you could maintain constantly.
"My mouse is Razer Copperhead."
Razer Copperhead, like many gaming mice, has a polling rate up to 1000 Hz. Combat Arms doesn't like that. The aim will zig zag and not follow straight paths if the polling rate is above 125 Hz. Most gaming mice have settings to limit the polling rate in their software.
TL:DR The Combat Arms: Classic (or Combat Arms in 2011) engine does not work well for calibrating mice in general. The game has forced mouse smoothing so using fixed FPS is suggested. It also requires a polling rate of 125 Hz or below to allow the mouse to move in straight lines.
Update: If (for some reason) you do still want to play Combat Arms Classic with a 1000Hz mouse, I have found the workaround for that as well.
In your Combat Arms Classic folder there are two .exe's for the game. Engine and Engine2. Engine2 is actually a "beta" build that has been included with classic for awhile.
The Engine2.exe actually works to run the game, (and removes forced mouse smoothing) as long as the game is up to date (which means you need to run the normal launcher to update the game).
You'll want to make a shortcut for Engine2.exe. Then right-click on the shortcut and go to properties. Where it says Target, add -windowtitle to the end of the target. It should look something like:
"C:\Program Files (x86)\CombatArms Classic\Engine2.exe" -windowtitle
This shortcut will launch the game with the build which removes the forced mouse smoothing.
Best Answer
I've had no issues with lag/responsiveness, but this is very hit or miss depending on the user (the same reason why some people use gaming CRTs).
The only downside I've had is that the batteries make the mouse a little heavier, which causes faster fatigue for long periods of gaming.
Also, you run the risk of batteries dying on you in the middle of a frag fest.