# What would be the Joule requirement to a railgun launch a conventional bullet at mach 7?

The railgun project from the US Naval Surface Warfare Center Dahlgren Division shoots a projectile of 3.2kg at a speed of mach 7 with 18.4 megajoules of energy.

How many joules would a railgun need to shoot a rifle-sized bullet of 3 grams at same speed of mach 7?

By the way, I'm asking about muzzle energy, for the little I think I know, railguns have a maximum efficiency of 47% and mostly is lost due to inneficiencies in the system, heat and other physical requirements (like accelerating something from 0 to mach 7 around 0.0004 seconds).

Of course,due to this, the final result is impossible to tell, so you can see the answer as "at least x joules of energy, not counting all these factors".

Well, since the Megajoule railgun projectile weights 3 kg, and the rifle projectile weights 3 grams, 1000 times less, then I simply divided 18 megajoules by 1000, resulting in 18 kilojoules. But I don't know if that makes sense.

– L.Dutch
Commented Dec 14, 2023 at 11:22
• @L.Dutch as a moderator I'm sure you must be aware that this question does not at all match the example question of “I use ______ for ______, what do you use?”. The fact that this question would remain the same if the OP removed their answer makes it clear that it does not fall under that rule, and adding your own answer but stating you're not sure whether it's correct is, if anything, helpful to answerers as it shows the querent's thought process.
– M S
Commented Dec 14, 2023 at 12:09
• @L.Dutch I understand this as "I have this math. Is it correct?" instead of "I have this answer, please give me more". It feels like the OP is providing their research (something we want, even if it is quite basic), and as such the question would be on-topic. Commented Dec 14, 2023 at 13:35
• Keep in mind that the muzzle energy of a projectile will always be less than the energy that was used to accelerate it. The energies in your post describe the muzzle energy, yet it seems like you're asking for the energy required per shot. Can you edit this post to clarify what you're trying to accomplish? Commented Dec 14, 2023 at 17:47
• Subsidiary to the answer (kinetic energy is kinetic energy), a 3g rifle bullet is going to experience a lot more resistance per unit mass from air than a 3kg railgun slug.
– jdunlop
Commented Dec 14, 2023 at 20:13