Pressure/Velocity Not Rising with Increasing Seating Depth

[MEHavey]

It would seem counter-intuitive that pressure/velocity wouldn't increase as the bullet is seated deeper into a case, but I had read recently that this was indeed what would happen -- backed up w/ pressure transducer data. So today when I was running a test to find best standoff distance for a moderate velocity match load, I graphed the results to see this....


Granted this is small-numbers, but the trending is clear even w/ the error bars taken into account.
`Anyone want to take a shot at the physics at work?

 

[Kernel]

Two things are happening simultaneously:

1. As case volume decreases (seating depth), pressure increases.
2. As freebore increase (distance from lands), pressure decrease.

Number two is winning.

 

[NCsmitty]

You'll find that it is less an issue in bottle neck cases than straight cases like handgun cartridges.
They can create large pressure spikes with just small increments of depth because of the powder's burn rate.


NCsmitty

 

[steve4102]

http://www.barnesbullets.com/resources/newsletters/september-2007-barnes-bullet-n/

 

[gamestalker]

That's so contradicting of what we learn from most of the books. But I can understand the physics as presented. Myself and probably multitudes of other hand loaders, load with the pretense that deeper means increased pressures with most any cartridge. But as has been pointed out, bottle neck cases are actually less inclined to react as a straight walled case would, where as even a slight increase in seating depth can produce significant pressure increases with small straight walled cartridges. I do have one question though. At what depth will pressures start to increase?
Good post!

 

[Ridgerunner665]

There are sooooooooooooooo many variables in play here its impossible to imagine them all and how they interact in any particular round...

Neck tension, bullet construction, powder characteristics...even primers.

 

[eldon519]

Pages 25 and 26 of Hornady Handbook of Cartridge Reloading 7th edition basically state that within reason for a bottleneck rifle round, giving the bullet more jump to the lands (ie deeper seating) produces lower pressure and lower velocity while a smaller jump or contact with the lands will produce higher pressure and velocity, possibly dangerously so.

Makes sense though, you're basically giving our bullet a running start to engage the rifling, and most rifle powders will be slow enough not to spike.

 

[DickM]

Regardless of the larger question, your data clearly show that velocity does increase with increasing seating depth. The last data point, possibly the last two, indicate that there may be a point beyond which it doesn't continue to increase, but up to then there's a clear positive relationship.