Specific Impulse.

Let's talk about specific impulse for hand loading rifle cartridges. By following some basic rules, you can extract the most velocity and accuracy from any rifle. Here is what I have found to be the truths of hand loading cartridges.

Your cases, your powder, and your bullets all have to match a criterion. So here's some basics from which to start. The smaller the case, the faster the powder. The larger the case the slower the powder. The smaller the sectional density of a bullet, the higher the heat explosion of your powder, the higher the sectional density of your bullet the lower the heat explosion. The lighter the bullet the higher the heat explosion of powder. The heavier the bullet the lower the heat explosion of powder. 

The VMD (volume measure density) is given in thousandths, such as .0651 (example) and the density 15.337 (example). You must find a powder that fits this criterion, but also the powder will have to fill the rifle case to the neck.  Too small a density, filled to the neck may give too much powder, and too much pressure.  Also too large a density powder will not be enough to give the velocity you want.  

Specific Impulse is directly related to your VMD and the DENSITY of your powder, your BULLET weight, and your CASE size.  You may ask, what about ballistic coefficient (B.C.) of the bullet?  BC only relates to the bullet in flight.  The higher the BC,  .509 (example),  the flatter, and longer, the bullet will stay in flight.  Less air drag will be on the bullet, so the less drop the bullet will have. 

Even though all this is true, it is problematic because every powder is different, no two are the same. Each one will have a different effect. By following all the guide lines above you can achieve success.  As all hand loaders know, or should know, you must start 5% below your neck level and work up your load cautiously. You must take full responsibility for your actions.  I hope this gives you some knowledge of Specific Impulse.       

Mac