I managed to get one of the few brand new original Colt R0635 9mm submachine gun (SMG) buffers still available. Let’s see how it works!
This information is intended solely for entertainment purposes only. Do not perform any action based on any of the following information. The accuracy of this information is not guaranteed or warranted. All copyrights and trademarks are the property of their respective owners.
The Colt SP62913 is a 5.5oz buffer designed by Colt as a heavy “deadblow” buffer for their new (at the time) 9mm R0635 select-fire submachine gun.
Deadblow buffers use sliding weights or a moving mass to help dampen or eliminate “bolt bounce”. Bolt bounce happens when the bolt in a semi-auto slides forward, chambers a fresh round, slams into the chamber face, and the bolt “bounces” backwards off of the chamber face. This can cause misfires and out of battery burst cartridges as the cartridge is partially extracted from the chamber by the bounce.
The gas-operated AR carbines used a lightweight 3oz. aluminum buffer with 3 steel sliding weights inside that act like a deadblow hammer. The three internal weights are separated by three rubber disks.
Colt’s new blowback-operated 9mm version of the AR needed a lot more weight in the bolt and buffer to tame the blowback system. It used a 16oz bolt and a 5.5oz. buffer, providing 21.5oz. of total blowback-resisting mass.
Colt decided the “commercial” models (what they called semi-auto civilian models) could get away with using a solid steel 5.5oz. buffer because most people of the day wouldn’t shoot their rifles fast enough for bolt bounce to be a problem. Competition and casual shooting at the time focused on perfect accuracy, not speed. The only triggers widely available were the old, gritty, slow, factory mil-spec AR triggers.
The SMG, with full auto select-fire, could experience case ruptures (out of battery discharges) from bolt bounce, so they needed a deadblow buffer, but it still needed to be much heavier than the standard AR-15 buffer.
Colt could have used sliding tungsten weights in their carbine buffer (like an H3 buffer) but tungsten is expensive and difficult to machine. Instead they came up with the clever idea of using the buffer body itself as the deadblow weight. Enter the 2-piece buffer!
In this buffer design, the head and body are separate but held together by a roll pin that “floats” in a 1/4″ hole in the sides of the buffer body. This allows the head and body to move separate from each other, but just a little bit (less than 1/10″).
Inside the buffer body is a small cavity that holds 3 rubber disks to cushion the blow between the head and body.
When the recoil spring launches the bolt and buffer forward, the head is tight against the rear of the bolt. The body is getting pulled along behind the head with just a little bit of space between the two because of the loose fit of the two parts. When the bolt and buffer head stop, the body slams forward a fraction of a second later, negating bolt bounce. Simple, clever, effective, and less expensive than tungsten weights.
However, they are no longer being produced.
Colt apparently now lists the cheap solid 1-piece buffer as the replacement part for all their 9mm AR’s. They decided to stop producing the 2-piece buffer years ago, likely because of cost cutting measures related to their bankruptcy and a lack of SMG sales. A widely available 5.5oz. H3 carbine buffer could act as a simple drop-in replacement for the older 2-piece buffer if one were to fail or break in an old Colt SMG.
Even though the two-piece buffer is now obsolete, it is a neat bit of history.
9mm AR design has changed over the years and 5.5oz is no longer sufficient buffer mass for most 9mm AR’s. Almost all modern 9mm blowback bolts weigh less than the original Colt 16oz. and require a heavier buffer. If you need help selecting a buffer for your modern 9mm AR, go here: What 9mm buffer do I need??? (and spring).
Blowback9.com 