Bullet Nose Profile and Seating Depth Sensitivity

[Laurie]

Here's a question for Bryan.

Bryan, back in September 2009, you wrote these words in a thread on the US Rifle Teams Long Range Forum in which Eric Stecker was advising those shooting VLDs who were having problems to try really long jumps if soft seating into the rifling wasn't working.

http://www.usrifleteams.com/lrforum/index.php?/topic/10924-jumping-berger-vlds/&

specifically on page 2, you wrote:

Posted September 17, 2009

We've got plenty of evidence just in this thread that secant ogive bullets can achieve their best accuracy with long jumps; something previously thought to be impossible or unlikely by many.



So we've learned something new. Some of us learned it sooner than others, but the point is that this subject of seating depth is not well understood. Sure we know that seating depth is a knob we can tune to refine the precision capabilities of our shooting sticks.



But does anyone know why seating depth affects precision?



Does anyone really know how seating depth affects precision?



I would love to know!



Here are a few facts that can be stated about seating depth and precision:



1. The precision of tangent ogive bullets tends to be less sensitive to seating depth than secant ogive bullets.



2. Internal case capacity is affected when seating depth varies.



3. The initial pressure spike on ignition is higher if the bullet is seated into the riflings compared to off the riflings.



4. The best seating depth for one rifle isn't always the best seating depth for another rifle, even when the rifles are very similar in construction.



If we could establish a cause/effect relationship between the variables and the result, imagine how huge that would be.



So who has some more facts to add to the list above? Anyone disagree with any of the existing 4? I'd like to change the direction of this thread from "It's possible to jump VLDs" to a more meaningful inspection of seating depth and precision in general. If we put our heads together, we just might come up with something and advance our collective knowledge of this mysterious tuning knob.



-Bryan

Did you get an answer to your questions? ....... or are we still as much in the dark today?

(One thing I've never understood is just what mechanism can improve bullet-to-lands alignment by making a long jump. The only guess I can make is that the overtaking powder gasses centre the bullet in the leade before it reaches the start of the rifling.)

 

[6BRinNZ]

Point 1 - how is sensitive defined?
Hybrids are pitched as less sensitive but I find that I get the best and most repeatable precision within a specific seating depth and all held to a specific tolerance when seated.

 

[Laurie]

Nobody got any thoughts on the reasons behind the behaviours?

 

[JRS]

I don't know why, but have always had better luck with accuracy using tangent ogive bullets.

 

[6BRinNZ]

Nothing that's not speculation.
Some thoughts
Case capacity. Probably a small influencer as seating depth has a large impact on group size with very small changes and the magnitude of effect is similar in small and large capacity cases.

Seating depth sensitivity needs to be defined. I think that definition might bust some thinking and lead down another path

Seating depth changes alter timing in that the bullet has more or less distance to travel before exit. Some suggest certain barrel lengths perform better than others so maybe some relationship here

How does nose shape and land angle interact. I think 1.3 deg is common but I don't think it's commonly understood why.

 

[6BRinNZ]

Have test been run that show a correlation between loads that have run out vs not? Imo if a large jump is simply allowing the bullet to be straightened then it would be possible to get the same group size by shooting straight ammunition at a different seating depth.