Increasing COL experience

[6.5 forever]

I lengthened my bullet seating to get in front of the case doughnut that starts developing around 10 firings. My velocity increased 10 fps. I thought increasing case capacity would decrease velocity. No problem but I would like to understand why.
I use a carbide mandrel. Any experience?

 

[VA_XTC_Shooter]

As your bullet gets closer to the lands, pressure increases, thus velocity increases.

I believe it is in the book Firearms Pressure Factors that shows this relationship between bullet seating depth and effect on pressure. Pressure from seating bullet deeper didn't actually increase until (in example of book) you went deeper than where the cannelure was at case mouth. And in the opposite direction from the baseline, cannelure at case mouth, pressure increased.

My takeway was, the baseline is the standard COL, anything shorter or longer increases pressure, but extending toward lands increase higher per inch of increase.

 

[DLT]

I lengthened my bullet seating to get in front of the case doughnut that starts developing around 10 firings. My velocity increased 10 fps. I thought increasing case capacity would decrease velocity. No problem but I would like to understand why.
I use a carbide mandrel. Any experience?

My experience has been since I got a chrono is starting at the lands velocity decreases the further I got from them. I’m sure it stabilizes just don’t know where

 

[dellet]

A couple of other possibilities
Dirty chamber, carbon ring that you are now seating the bullet into.

Compressed load, it seams like most powders only like to be compressed so much. Some not at all. What can happen is the velocity kind of stalls or falls off due to poor ignition of the packed powder. Relieving the compression raises velocity.

 

[Richard Jones]

What was your ES before? Most folks would be happy to have only a 10fps spread.......

 

[6.5 forever]

What was your ES before? Most folks would be hpy to have only a 10fps spread......

The speed is a increase for the series. Not a spread.

 

[Straightshooter1]

I lengthened my bullet seating to get in front of the case doughnut that starts developing around 10 firings. My velocity increased 10 fps. I thought increasing case capacity would decrease velocity. No problem but I would like to understand why.
I use a carbide mandrel. Any experience?

I used to think like that, where decreasing case capacity would increase velocity due to less case volume. I thought that should be the overriding factor that influences velocity. But my data would show that my velocity actually decreased when seating bullets deeper. It wasn't unit I read an old college study that showed how changes in the blow buy and chamber volume had more influence on velocity as seating depth changes. Here's a graph that kinda sums it up for me:

 

[358WCF]

I lengthened my bullet seating to get in front of the case doughnut that starts developing around 10 firings. My velocity increased 10 fps. I thought increasing case capacity would decrease velocity. No problem but I would like to understand why.
I use a carbide mandrel. Any experience?

I dont think 10fps is a statistically meaningful increase. How much of an OAL change? How far to touch/jam? How many shots in the samples? Try it again next week. How did it shoot?

 

[Richard Jones]

The speed is a increase for the series. Not a spread.

OK, I don't understand. Just looking to learn what I don't know, which is a lot....

 

[6.5 forever]

OK, I don't understand. Just looking to learn what I don't know, which is a lot....

Same here lol

 

[6.5 forever]

I dont think 10fps is a statistically meaningful increase. How much of an OAL change? How far to touch/jam? How many shots in the samples? Try it again next week. How did it shoot?

This increase was meaningful because it was permanent. 7-8 hundred rounds.

 

[6.5 forever]

I used to think like that, where decreasing case capacity would increase velocity due to less case volume. I thought that should be the overriding factor that influences velocity. But my data would show that my velocity actually decreased when seating bullets deeper. It wasn't unit I read an old college study that showed how changes in the blow buy and chamber volume had more influence on velocity as seating depth changes. Here's a graph that kinda sums it up for me:
View attachment 1617562

Thanks for your insight!

 

[William Vogel]

I lengthened my bullet seating to get in front of the case doughnut that starts developing around 10 firings. My velocity increased 10 fps. I thought increasing case capacity would decrease velocity. No problem but I would like to understand why.
I use a carbide mandrel. Any experience?

How many thousandths did you increase the seating depth? I find that by increasing the throat of the chamber to accommodate heavier bullets (example is 223AI chamber) I gain velocity without pressure signs. I shoot 88 grain bullets at 2900 fps with no flattening of primers and 75 grain bullets to 3150 same effect. GRT predicts pressure at approximately 56000. COAL is 2.640 for 88 and 2.630 for 75.

 

[6.5 forever]

How many thousandths did you increase the seating depth? I find that by increasing the throat of the chamber to accommodate heavier bullets (example is 223AI chamber) I gain velocity without pressure signs. I shoot 88 grain bullets at 2900 fps with no flattening of primers and 75 grain bullets to 3150 same effect. GRT predicts pressure at approximately 56000. COAL is 2.640 for 88 and 2.630 for 75.

OAL Was 2.779
Now 2.820

 

[David Christian]

Very simple. As you seat deeper into the case and move away from the lands, more gas escapes down the barrel ahead of the bullet, therefore reducing the pressure curve. In general, deeper seating=less enclosed pressure=slower speed.

 

[6.5 forever]

Very simple. As you seat deeper into the case and move away from the lands, more gas escapes down the barrel ahead of the bullet, therefore reducing the pressure curve. In general, deeper seating=less enclosed pressure=slower speed.

That makes sense!