# Compression gun

sid_galt
If the gunpowder in a gun is detonated in a chamber consisting of compressed air such that the top of the chamber is formed by the bullet, wouldn't the efficiency of energy extraction from the gunpowder increase as the pressure ratio will be much higher? Would such a bullet be able to achieve much higher velocities?

Gold Member
Hi sidgalt!

To say the truth I have no idea about ballistics. Anyway, I thought you will find interesting this link:

http://www.aa.washington.edu/AERP/ramac/ [Broken]

Last edited by a moderator:
Mentor
The pressure generated by the burning propellant is orders of magnitude higher than you could provide with any reasonably-sized pump, so it really would help much to add that tiny bit of energy.

sid_galt
Clausius2 said:
Hi sidgalt!

To say the truth I have no idea about ballistics. Anyway, I thought you will find interesting this link:

http://www.aa.washington.edu/AERP/ramac/ [Broken]

Thanks for the link Clausius2. It's fantastic. I had no idea they were using ramjets to power projectiles.

russ_watters said:
The pressure generated by the burning propellant is orders of magnitude higher than you could provide with any reasonably-sized pump, so it really would help much to add that tiny bit of energy.

Assuming detonation gives a pressure ratio of 18:1, the theoretical efficiency is 68%. If we compress the air beforehand to a pressure of say 4:1, then the theoretical efficiency rises to 81%, a 13% difference. I would say it's quite a lot. Of course the casing would also have to be 4 times thicker and a method of injecting gunpowder pellets will have to be arranged.

Last edited by a moderator:
Mentor
Where did the 18:1 come from?

sid_galt
It was off the head. I did a google search but I coudln't find the compression ratio generated in a gun

Antiphon
sid_galt said:
It was off the head. I did a google search but I coudln't find the compression ratio generated in a gun

It's about 50,000 atmospheres. Good luck trying to compete with that
using a pump.

LunchBox
Or you could do it the simpler way... more powder and a longer barrel...

Cheers...