The SD is without a doubt one of the coolest firearms in history. Whether it was in your favorite ‘Merica 80’s action film, a SEAL recruitment poster, or seeing it blast Iranians in Operation Nimrod,this gun has become a “bucket list” item for some people. And why not? Its horribly cool looking, Jedi quiet, and theintegral suppressor makes it the most compact platform with which to lay hate. Or does it?
The MP5SD was developed by Heckler & Koch in 1976, at the request of military units who needed an incredibly quiet firearm but with all the performance capabilities not possible with suppressed pistols. The SD was a revolutionary concept because it allowed readily available standard NATO supersonic 9mm ammo, and still achieved subsonic performance.
It did so by porting the barrel at the base, which allowed gas into the two stage silencer. The first portion is an expansion chamber that reduced the volume of propellant gas pushing the round. This gas then entered the silencer portion for the rest of the work.
The porting reduced the velocity of the round by up to 25%. For you math people out there, you have probably realized that it also means the energy of the round was reduced, which due to the physics involved, means that a 25% reduction in velocity results in the kinetic energy being cut in half.
Original patent drawing from Germany
Those that have used this gun in harm’s way will attest to the SD’s lack of stopping power; which in those situations is addressed by holding the trigger down until your target is sufficiently ventilated.
So the entire purpose of the gun was to get noise reduction performance on par with standard silencers using subsonic ammo, but with ammunition that was readily available in a war zone.
Now, fast forward 30 years, and replace a strategic military group with a regular civilian, or even a police agency. The price difference between 115 gr Blazer brass and 147 gr Speer Lawman (a subsonic round) is only about 5 cents a round, and readily available. Additionally, bullet manufacturing has drastically improved reliability and performance of 147 gr rounds, which was admitably quite a feat in the middle of the 20th century. So that initial need that drove production is all but gone for most buyers in the U.S.
So how does the SD stack up to modern options? We will use the Griffin Revolution 9 silencer for this comparison, because it is a well built silencer, the tri-lug adapter adds no additional length, and we like those guys over there; they’re former co-workers in a fairly crappy part of the world.
The average dB rating of the SD is about 127 dB. Out of our BAP9C with the Griffin silencer and using subsonic loads we have witnessed dB levels between 124-135 dB depending the ammo brand.
Even in the worst case, it is less than a 10 dB difference. That is still significant, as the dB rating is not a linear but logarithmic scale; very loosely speaking a 10dB difference would be about twice as loud. From a comparison standpoint OSHA compares a 130 dB sound to a thunder clap, and a 140 dB sound to a jet 100 ft overhead (a fairly useless comparison as both result in hearing damage.)
Skipping any further nerdery on the subject; it is suffice to say they are comparable, as long as subsonic 9mm is used. But not on the SD. Because it probably wont be able to cycle the next round. Yeah, forgot to mention that; using subsonic ammo in the SD will not help much in the sound suppression department, but it will almost certaintly lead to a malfunction.
There is more to consider than just sound; lets look at what happens in front of the muzzle. a 147 grain delivers on average about 10% less kinetic energy than a 115 gr bullet, assuming everything else is identical. But don’t forget, the aforementioned velocity reduction leads to a 50% reduction in energy on target. Again, for comparison, a .380 ACP averages about 20% more energy than what is coming out of the SD.
Still stronger than your SD
Ballistic improvements have made the 9mm round useful again, but all those advancements become useless when the round is slowed down that much. All those super cool, exploding, DNA erasing rounds need that velocity to expand, liquefy, vaporize etc. on target. The velocity is a crucial component in any terminal ballistic design; it is not something that can be changed without a massive performance impact.
Other Important Dimensions
we have already poked some holes into the lure of the SD, but we at Brethren Arms are not ones to stop beating the horse until it is undeniably dead. As stated above, another great aspect of the SD was its comparable size to unsuppressed submachine guns. So lets do it by the numbers:
MP5SD BAP9C w/ Griffin Revolution9
weight 6.2 lbs 5.5 lbs
Overall length 21.7″ 21.3″
( end cap only, no stock)
Keep in mind that those values are with the silencer, so if the world is ending, and you have already blown out your ear drums, with a twist of the wrist your gun now weighs 4.8 lbs and is only 13.7″ long.
Its not a clear victory from a dimensional standpoint; the SD does have both a longer sight radius and more front end real estate for accuracy. However, the addition of optics and a variety of foregrips does diminish those advantages. Which brings us to the final fact to consider; modularity. With AR-15s being so modular that “light weight polymer dust cover” is a legitimate product, we have realized that some of those accessories are pretty useful and even life saving. Even with our Mlok handguard winning the title of shortest handguard in existence, There is still enough space to put a vertical foregrip, a flashlight, and even a laser, quick detach sling mount, pencil sharpener, etc.
The SD handguard currently has to be welded to the receiver, and the only ergonomic upgrade is a rubber sleeve which loosely resembles a bicycle tire. We can add light and foregrip mounts, but it is a custom and frankly pricey modification. Without a welder and an engineering degree, this is what your SD looks like when you want to be able to use a light.
Yes, those are hose clamps on a gun that retails close to $4000. If this was your issued gun, you could care less. But as a regular consumer, it ventures into buying-a-mercedes-and-putting-walmart-hubcaps-on territory. To be fair, there are some other options for mounting lights, but better is relative in this instance.
The SD Look in Other Platforms and Calibers
Before we really start offending you by looking at other guns, even with the roller delayed platform the only caliber that would benefit at all from being “SD’d” would be the 300 BLK. None of the other calibers have any functionality whatsoever at subsonic speeds without rifle length barrels, most would not even function, regardless of how much money we wasted trying to develop it. The 300 BLK also suffers from the same fate as modern 9mm ammo; subsonic loads are as easy to obtain and comparably priced to supersonic rounds. Even from the perspective of the few military/government units utilizing 300BLK; the situation of having piles of supers and no subs is highly unlikely. It needs to be adopted by a much larger portion of dozens of militaries before it could it could conceivably happen.
And now for the fun part of the program; Bashing AR-15’s and others sporting the uncircumsized look.
We believe that form follows function, but you can still make well performing gun look bad ass. HK did it back back in the day, and several companies are doing it now. And then there is this…
Does it look “cool?” Were not the masters of aesthetics, so we’ll leave that to you. Does it have any purpose? That we will shout a resounding “no” to. You will notice that this is actually a .22lr gun, because no AR style gas system on earth could reliably function this firearm.
Even on something less ridiculous, we strain to see the purpose.
A common method of achieving the “SD look” is to put a shorter barrel than what would be normally dictated by the handguard; then finish it off with a silencer to make up the difference. Again aesthetics aside, all that has been created is a gun that can not be fired without the suppressor; not because of operation, but because of your weak, fleshy digits.
Purely cosmetic “SD” guns are even prevalent in the HK community. If you haven’t seen a press release about the work and time involved in integrally suppressing any caliber other than 9mm, rest assured it is just a standard gun with an SD handguard welded on, and yet another way to lose ones hand.
Yup, that is where your hand goes
The suppressor on here is a regular AAC; with an overall length of 7.66″ , almost all of which is sticking out past the hand guard. What has been acheived? A 16″ overall length gun, that is still classified as an SBR; a potential limb remover if fired without the silencer; an awkward, thick, permanently attached hand guard with no functionality; and all at a price greater than the “non-SD” version of the exact same gun. You can hear the crying all the way from Oberndorf.
The SD was an amazing accomplishment, and has rightfully earned its place in miliary history. Its existence is a bittersweet memory of a time when the German Powerhouse could solve any problem thrown at them, and still got it to look good. A look so good in fact, that its purpose has been all but forgotten, in favor of stylish appearance. We still love the SD, and do not want this article to convince you to erase it off that bucket list. We just want you to recognize what it is; not some mythical implement stored in the gun safe next to Thor’s hammer and Excalibur, but a useful tool design to fit a specific niche at a specific time.
We leave you with the words of the foremost expert on the subject.