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If you follow defense related matters even casually you have certainly been seeing tons of articles and analysis around hypersonic cruise missiles and hypersonic missiles in general. You may even have read an article or two about the hypersonic missile gap. And if you follow defense matters a bit more regularly you will likely have heard about the Navy's Railgun and the hypervelocity projectile (HVP) that came out of the railgun program that is now being used in conventional guns such as the Navy's 5" MK 45 guns or the Army's 155 mm howitzers.

You probably also know that the "hyper" in hypersonic missile is an indicator for missiles that can travel at speed of 5 times or more the speed of sound, i.e. greater than Mach 5. Given this, you might also think that the "hyper" in hypervelocity also stands for projectiles that can travel at speeds greater than Mach 5, and this thinking would only be reinforced by the fact that the HVP was developed as part of the Navy's railgun program which has executed a number of Mach 7 test firings . And you would be kind of right and kind of wrong.

Indeed, if you do a Google search you will find plenty of citations stating that hypervelocity refers to objects traveling at speed greater than 3000 meters per second, Mach 8.7 or about 6711 mph. Mach 8.7 is very fast and in fact faster than what anyone expects the muzzle velocity of any deployable railgun to be able to achieve in the foreseeable future. This fact could produce some discordance over the name Hypervelocity Projectile as it hasn't ever really been fired at hypervelocity speeds. Adding to this discordance is that going forward most uses of the HVP will involve firing it from guns at muzzle velocities of around 3300 fps or about mach 2.9. Certainly a respectable speed, but there are quite a few guns with higher or similar muzzle velocities including, but not limited to the M1 Abram tank guns (greater than 5000 fps), the Phalanx 20 mm CIWS (3650 fps), the A-10's GAU-8 Avenger 30 mm cannon (3324 fps) and even Navy's own 5"Mark 45 gun that can fire the Mark 80 projectile ( EX-175 cartridge) at a muzzle velocity of 3450 fps.

So, to say the least, the muzzle velocities of the HVP projectile are a bit underwhelming when you are thinking of hypervelocity as being 9800 fps or greater. At first blush this seems like the HVP is not being accurately marketed, and in a de facto sense you are probably right, but in a de jure sense you are mostly wrong. You are wrong because it turns out that that the

the Dictionary of United States Army Terms defines "hypervelocity" as:

1. Muzzle velocity of an artillery projectile of 3,500 feet per second or more.

2. Muzzle velocity of a small arms projectile of 5,000 feet per second or more.

3. Muzzle velocities of tank cannon projectiles in excess of 3,350 feet per second.

So, if we use the Army's definition, as the Navy apparently does, then the HVP, when fired from the Navy's experimental railgun, is certainly achieving hyper muzzle velocities and the velocity is close enough when being fired from conventional guns that the name isn't really all that misleading. Further, there is little doubt doubt that the Navy's 5" Mark 45/62 gun firing an HVP round with an EX-175 cartridge would exceed 3500 fps.

Hence, now we know that hypervelocity projectiles do not meet the most common definition of hypervelocity, but that they do meet the Army's definition. What we don't know is why the vast majority of promos for the HVP fail to provide the definition for hypervelocity and that none of them provide context for just how fast, or slow, is the HVP as compared to plain old rounds without the word "hyper" attached to them.

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