The 170 mm Koksan, North Korea’s Not-So-Frightening Tool of Terror

By Mike Fredenburg

Thursday, April 05, 2018

Even if you typically don’t read defense-related publications, there’s a good chance you have read about North Korea’s fearsome self-propelled Koksan guns, which are perched along its border with South Korea. These 170 mm guns are, it is said, poised to rain thousands of deadly rounds upon the heads of the helpless citizens of Seoul if the DPRK is provoked.

Driving home the threat, the North Korean regime has for years threatened to use its artillery to turn Seoul to into a “sea of fire.” The threat is taken seriously by Western journalists. In 2003, Tony Karon wrote in Time that North Korea has the capacity to “flatten Seoul in the first half-hour of any confrontation.” In 2009, David C. Kang and Victor D. Cha speculated in Foreign Policy that a war on the Korean peninsula would cost about a trillion dollars in property damage and 1 million casualties, including 52,000 U.S. military casualties. In 2017, an article by Franz-Stefan Gady ran in the Diplomat with the description: “The first 24 hours of war on the Korean peninsula could cost hundreds of thousands of lives.”

But these articles and their ilk are long on emotion and short on knowledgeable analysis. They are playing right into the hands of Kim’s propaganda campaign, a campaign designed to convince an uneducated Western public and appeasement-minded politicians that even a North Korea with deliverable nuclear weapons and a history of attacking and killing South Koreans is preferable to the devastation the dictatorial regime can unleash with its conventional weapons.

Fortunately, there have been some good articles published by knowledgeable military analysts that offer a more realistic analysis. Particularly insightful is a study performed by Roger Cavazos, a retired U.S Army intelligence officer and an associate at the Nautilus Institute for Security and Sustainability. Cavasos shows that the North Korea artillery threat has been grossly overstated. But as excellent as his study is, it still makes a problematic assumption: that the Koksan is genuinely a 24-mile self-propelled gun capable of generating sustained artillery fire hour after hour, day after day, and capable of striking South Korea’s capital.

This is not just a technical discussion about any old weapons system. The Koksan has been the lynchpin of 20 years of threats by Kim Jong-Un and his father Kim Jong-Il. If it turns out that the gun is not quite as fearsome as is commonly accepted, then the American foreign-policy community ought to take that into consideration.

The Koksan was first identified by Western defense analysts in 1978. But it was not until 1985, when the gun was prominently featured in a North Korean military parade, that it came into the public eye. There is much conjecture on what design elements North Korea used to cobble together the Koksan: Some believe that it owes some of its design to WWII Japanese coastal guns, and others point to its similarities to Germany’s very effective WWII 173 mm K-18 gun. The best analogue, though, is to the M107, a 175 mm, self-propelled gun (SPG) used by the U.S. in the 1960s and 1970s (and still used today by several countries).

Some of what we know about the North Korean gun comes from a Koksan that was sold to Iran by North Korea and subsequently captured by Iraq in 1988. Iraq allowed a five-person exploitation team — composed of three engineers, an artillery officer, and Air Force intelligence officer Rick Francona — to examine the gun. In a recent interview, Francona, the team’s leader explained that while “the artillery analysts and engineers on the exploitation team were impressed with the engineering and design of the gun, including the metallurgy,” they “did not see anything that could be called new technology.” Though they were impressed with what appeared to be a competent design, the engineers did not identify anything revolutionary about it. In other words, it was a well-designed big gun built using technology comparable to guns designed in prior decades.

So there is no new technology in the Koksan — but the ranges claimed for the gun are revolutionary: 24 miles for an unassisted standard high explosive (HE) fragmentation round, and 35–37 miles for a rocket-assisted projectile (RAP) HE round. Though the public-domain provenance for these ranges is not well sourced, they are the ranges in common use. And they are validated by the publicly documented case of Iran’s using Koksan guns purchased from North Korea to conduct nuisance shelling of Kuwaiti oilfields from 35–37 miles away.

These ranges are significantly better than anything the United States or even the Soviet Union had in the 1970s or 1980s. They even exceed the range of the Soviets’ much larger (203 mm) Pion gun. Even when compared with modern artillery systems firing standard HE rounds, the Koksan gun’s cited 24-mile range still reigns supreme.

Giving more context to just how anomalous theses ranges are, the maximum cited range of the Koksan matches that of an Iowa Class battleship’s 16-inch guns. This seems unlikely, as all else being equal, larger rounds, having a higher cross-sectional density and being proportionately less affected by air resistance, should travel farther. Yet even our biggest currently active artillery is outranged by the Koksan. In fact, the only currently active combination of gun and artillery rounds the U.S. Army has capable of reaching 24 miles is the very expensive, but incredibly accurate, 155 mm precision-guided Excalibur round that uses glide-enhanced range to reach about 24 miles.

At first blush it might seem that North Korea’s gun designers came up with some secret way to get these extraordinary ranges — a secret that U.S gun designers did not have. But did they really, or is something else going on? Understanding why the latter is true requires delving a little deeper into the details. Despite Koksan guns having been examined and even captured by U.S. forces, our military has chosen not to publish their basic specifications. However, there are some very good side-profile pictures of the Koksan. Credible estimates put the total vehicle length, including the barrel stowed for travel, at 14.9 meters. So we can estimate the barrel length to be 11.2 meters (36.7 feet), give or take a couple of feet. In gun parlance this equals about 66 calibers, meaning the barrel length is 66 times that of the diameter of the 170 mm rounds it fires.

Compare that with the 37 to 39 calibers of our 155 mm artillery and we see that this is a very long barrel. But long barrels are not without precedent, as the naval artillery guns that are the standard for our Navy’s cruisers and destroyers are either 54 calibers or 62 calibers. Despite being 14 percent longer than the 54-caliber gun, the newer 62-caliber guns only enjoy a two percent advantage in muzzle velocity. While long barrels help increase range, it takes more than just a longer barrel to achieve large increases in both range and muzzle velocity.

More propellant is needed to take advantage of the potential for extra velocity that a longer barrel offers. Simply adding large amounts of propellant, however, seriously compromises the safety of the gun and causes the barrel to wear dramatically faster. Added wear on the barrel, in turn, degrades the accuracy and range of the gun. So stuffing a lot of propellant into a long barrel can add range, it will come at the expense of the gun’s performance.

The barrel of the Koksan is much longer than that of any of our current artillery. But it is only about 7 percent longer than the aforementioned M107, a 175 mm self-propelled gun which has a 60-caliber barrel. While the M107 is no longer in active use by our military, several hundred M107s are still being used by a number of countries. The primary reason the M107 was so popular for so many years is its tremendous range — it reaches up to 20 miles with inexpensive standard shells. And only by using costly and specialized extended-range shells can the most modern of artillery match or exceed its range.

M107 self-propelled gun on the left; captured Koksan on the right.

To improve effectiveness, the M107 uses a three-zone propellant scheme. Zone 1’s max range is 16,504 yards with a muzzle velocity of 1,675 feet per second (fps). Zone 2’s max range is 24,156 yards at 2,310 fps. And Zone 3’s max range is 37,740 yards (20 miles) at 3,000 fps. Each zone adds an increment of propellant. This improves the accuracy of the gun, extends its barrel life, and ensures that rounds fired with less propellant actually reach their targets faster and with more accuracy.

Yet here we have a U.S.-designed gun being outranged by 20 percent by the Koksan, a smaller-caliber gun whose barrel is only about 7 percent longer. Did North Korean engineers come up with a new propellant with better energy density and much better piezometric efficiency? Are North Korean gun designers better than those at Benet Labs and Watervliet Arsenal? Did they come up with some super steel far superior to what the United States and the rest of the world are using in their barrels? The answers to these questions are no, no, and no.

If not through superior technology, how did the North Korean Koksan get its extraordinary range? Combining what we know about artillery, using common drag-coefficient numbers for artillery projectiles, doing some standard ballistic simulations, and using the M107 as our baseline, yields the estimates shown in Table One. (For comparison’s sake, we also include an artillery system still in wide use by the United States, the M109A7 Paladin, a recently upgraded, modern 155 mm SPG capable of firing our most advanced, longest range, precision-guided artillery rounds.)

While the numbers we derived for the Koksan are just estimates, they are more than good enough for the kind of analysis we are doing. In running the simulation, we find that reaching 24 miles in range requires an increase in the Koksan muzzle velocity of 13 percent.

If the U.S. Army was willing to sacrifice safety and reliability to get the M107 to fire 24 miles, they could have added another firing zone and more propellant to the gun. Of course, that would have made the M107 much more dangerous to its crew while greatly diminishing the number of rounds the gun could fire before needing to have the barrel replaced. Few militaries are willing to make these kinds of compromises on safety, reliability, and accuracy. Apparently, the North Korean gun designers who were tasked to develop a gun a capable of reaching Seoul were. Such a large increase in muzzle velocity can only be achieved by cramming more propellant into the gun — conservatively, at least 30 percent more, along with a huge increase in pressure. In bumping up the pressure by such a large amount, the DPRK gun designers practically ignored the standards for reliability, endurance, and safety to which the world adheres.

By generally accepted gun-design standards, then, the Koksan is not really a 24-mile gun. Most likely, it is a 20-mile gun that, when fired at unsafe pressures, can reach 24 miles using normal rounds. But a 24-mile range is not enough to reach Seoul; reaching deep into the city would require a minimum of 35 miles of range. To get that additional twelve or so miles in range, the DPRK developed a rocket-assisted projectile (RAP). In a recent interview, a Ph.D. engineer with decades of experience in gun and artillery design told me, “The shell must be all rocket to get that twelve miles.”

And he is almost certainly right. Most RAP rounds give about a 20 percent boost in range. The U.S. M549A1 RAP rounds add about 22 percent more range over standard rounds, but they do so at the expense of accuracy and explosiveness. (The difference between an RAP round and a standard round amounts to the difference between 15 lbs. and 23.8 lbs. of TNT). The question, then, is what kind of trade-offs the DPRK made to get a twelve-mile boost in range. A significant-enough reduction in explosive payload would mean the Koksan RAP round is both less accurate and is significantly less lethal than a standard 170 mm HE fragmentation round. And its inaccuracy would greatly reduce its effectiveness against most military targets.

The North Korean gun designers met the political goal of having a weapon able to threaten Seoul. They did it by cramming unsafe amounts of propellant into their gun, enough to launch a shell fractionally as lethal as the standard 170 mm round. The result is a gun that, when used to reach extreme ranges, could be wildly inaccurate with miserable endurance. It should not be assumed that Kim Jong-Un has a gun capable of reliably shelling Seoul hour after hour, day after day. The Koksan, if used to target Seoul, would be at serious risk of exploding, becoming unusable, or losing its range within hours.

Consequently, the Koksan is not quite the threat to the civilian population that journalists and the foreign-policy community make it out to be. And that should be factored into any future analysis.