Looking good!
Since we are revisiting this thread, I might as well mention a new revision of the G&P M500.
As previously stated, you absolutely need a 3 pieced barrel to be able to simply drop the old revision of this light. It was basicly designed from the ground up for the old TM M4.
Now, it seems G&P finally woke up, and released a new revision, as seen here:
http://cgi.ebay.com/G-P-M500-Airsoft...QQcmdZViewItem
Seems to be based on their R500 light, has three activation modes as opposed to the older version's one, rechargeable batteries stock, etc.
It also seems to be compatible with any M4 lenght foregrips.
Sweet!