Shoulder Bump and Base to Ogive

A

akinswi

New member
Is it common for the base to ogive measurement to differ if you change your shoulder bump?

I was fiddling around with changing my shoulder bump for 30-06 and I noticed that it changed my base to ogive measurements from 2.717 to 2.724 . I had to adjust my seating die down 7 thousandths to match the base to ogive I wanted

The batch of Sierras Im loading the “bullet length” is sorted to 1.204-1.205. The previous batch was sorted from 1.206 - 1.207. That batch was right on the money 2.717-2.718.

My barrel harmonics are perfect at that BO. Im going from a -0.006 shoulder bump to a .-0.003
 
Yes, and I had fits with getting the consistency I wanted when I tried to change the should bump by adjusting dies.

I switched to using Redding Competition Shell holders and it made a big, beneficial difference. SDs in the sub 5 range across several calibers, accuracy and consistency overall improved. Was skeptical, but a believer now.
 
Im using a Dillion RL550 so not able to use shell holders. I did notice that when I used my Forster FLS die my shoulder bumps were not near as consistent than using my Lee FLS with the expander ball removed. They would vary .003 thousandths using an expander ball. Vs non expander ball varied 1thousandths or less
 
Bart B,

I did a test too see on different seating depths in My M1 and the new criterion barrel , and how it would affect accuracy (Harmonics). When I loaded my first batch at sierra’s recommended OAL of 3.320 for 168 MKS, and at 50 yards they were producing almost 1 hole groups so I saved a unfired round and measured the base to ogive and it landed at 2.7175 so I figured I didnt need to change anything.

So really the answer to your question is I dont know because I ended the test there because I didn’t think I could shoot better than that.
 
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Here is the Target at 100yds SR-1

Im trying a -0.003 shoulder bump too see if I can tighten it up just a wee bit
 

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akinswi said:
Im using a Dillion RL550 so not able to use shell holders. I did notice that when I used my Forster FLS die my shoulder bumps were not near as consistent than using my Lee FLS with the expander ball removed. They would vary .003 thousandths using an expander ball. Vs non expander ball varied 1thousandths or less
Click to expand...
More lube, polish the expander die and the body die.
 
I don’t see how shoulder bump effects seating depth . If both are measured from the base , where the shoulder is shouldn’t have any effect where your seating die puts the bullet . It can effect jump distance but not where the bullet is seated in relation to the base of the case .

I could be missing something here , please help me understand.
 
Metal god said:
I don’t see how shoulder bump effects seating depth . If both are measured from the base , where the shoulder is shouldn’t have any effect where your seating die puts the bullet . It can effect jump distance but not where the bullet is seated in relation to the base of the case .

I could be missing something here , please help me understand.
Click to expand...
--agree--
 
The measurements dont lie. Has to be a difference somewhere with the resizing process whether or not its related to shoulder bump or something else I dont know …
 
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I expect what you are seeing is that, first, because the expander pulls the neck forward in a somewhat irregular fashion due to minor dimensional differences in the brass and the friction between it and the neck, you get the shoulder pulled forward in slightly different amounts. You want to be sure to lube the inside of the neck to use the expander. Before I discovered expanding with mandrels or using a Redding Body Die to set the shoulder and sizing the neck separately with a Lee Collet Die, I used a Redding standard resizing die in which I had replaced the steel expander with one of the carbide ball expanders. Using graphite or motor mica as the inside neck lube, that combination reduced the pulling friction significantly and improve the case runout quite a bit.

A case shoulder and body that have been stretched forward by the expander may be acting like a spring that is compressing -0.007" while you seat the bullet, then returning back up. A non-expanded case can do that, too, but may just be doing it to a different degree.
 
Agree Unclenick. Why I suggested the lube and polish. If that does not work, yep, probably getting new dies, which I have done in a few cases as well.
 
What’s interesting Nick, When I went back to my old setup, A Lee FLS “No expander ball , Then using a lee collet die to re expand the neck the measurement stayed the same (2.724 instead of 2.717) but I did adjust it to -.003 shoulder bump this go around instead of -0.006.

I think you are correct that the brass is springing back causing the issue. Lol I guess thats why they give you the ability to adjust seating dies.
 
akinswi said:
The measurements dont lie. Has to be a difference somewhere with the resizing process whether or not its related to shoulder bump or something else I dont know …
Click to expand...

when seating a bullet the case should be raised to the same height every stroke. The die's stem should also be at the same spot vertically unless it has been adjusted. Those are the only two factors that can affect case base to ogive on a seated bullet. Case head to meplat measurements will vary according to the consistency of the ogive which will cause the seating stem to contacts the bullet in different places. A dirty seating stem can also do that as can damaged
or uneven/dinged case heads
 
Bart B. said:
How many rounds erodes the throat at bullet contact diameter .010 inch further down the barrel?
Click to expand...

I am surprised you would ask this, it depends on the cartridge. a 22-250 Win will erode a lot faster than a .223 Remington and a 300 Win mag will erode faster than a .308 Win. The more powder per round you push through a bore diameter the faster the erosion
 
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I seem to recall reading an article where someone had done some testing to this effect. It was some time ago and I do not remember where I saw it or who authored the article. I am sure someone has that answer, or at least an answer relative to that test.
 
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