(Originally written in 2003 / 2004 - updated 09-29-2009)
I learned this technique close to a decade ago on an online forum. I don't recall who gave me the information, unfortunately I can't give credit where credit is due. I wrote the following review a few years later because I had gotten so many questions about the purpose of the "red stuff" on my charging handle.
Over the years many guys have asked me what the "red stuff" is in the gap on the charging handles closest to the upper receiver.
If you look at the charging handle on an AR15 from the top of the gun you will notice that there is a half circle shaped gap where the charging handle fits up agains the upper receiver.
If you look at this same area from the side, you will notice that there is a space between the charging handle and the upper receiver. This is where a majority of excess lube, gases, and debris escape out of the upper receiver and are directed into the shooters face.
By filling the half circle gap on the charging handle with RTV silicone you can make a cheap and very effective "gas buster" for your charging handle, and this prevents lube, gases, and debris from being blown back into your face.
Prep the charging handle. Clean the half circle shaped area on the charging handle with Acetone and wipe dry.
Get a tube of Permatex High-Temp RTV Silicone Gasket Makter #26B (can be found in the automotive section at Wal-Mart. It comes in a red tube and costs $2.97). When I first started using this technique I was only able to locate red RTV Silicon, it also comes in black. I use red RTV Silicon in the pictures below so that people reading this article could easily see what was silicon (red) and what was part of the carbine (black).
Looking at the charging handle from the top you will notice a gap that looks like a "C" or half circle. Ensure that the charging handle is locked in the forward position.
To make the gas buster modification, I filled in this gap between the upper receiver and the higher portion of the charging handle with the High-Temp RTV Silicone, using a plastic knife to level out the silicone and remove any excess.
Let the RTV Silicone dry over night, once the RTV Silicone is applied to the charging handle, DO NOT move the charging handle until the RTV Silicone has cured.
After the RTV cures, take a knife and slide the blade between the RTV and the upper receiver to loosen it up a bit. Then pull the charging handle back slowly, if it's still sticking to the upper receiver, use the blade of the knife to slowly and gently pull the RTV away from the upper receiver. You want the RTV Silicone to stick to the charging handle, not the upper receiver.
The RTV Silicone makes a perfect seal, nothing can excape from the upper receiver in the area of the charging handle.
The RTV makes a perfect seal around the upper receiver, it's much cheaper than a $90 charging handle.
Apply the RTV Silicon while the charging handle and bolt are both in the forward position:
Charging handle when complete and removed from the rifle:
The Do It Yourself Gas Buster Charging Handle in action at a recent LMS Defense 2 day carbine course (Sept 2009):