Optimal Shoulder Bump Question

Desert Dan

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Fellas,
One of the benefits of opening up my 6.5 PRC chamber is now it is unmistakable when my brass shoulder is still barely touching the front of the chamber versus when it is clear. I decided to re-confirm my optimal shoulder bump using the same method as Broz’s video. When fully fire formed, my brass measures 1.646”, and the bolt is getting very tough to close. Sized 2k down to 1.644 the brass barely touches the front of the chamber. With the firing pin and ejector removed the bolt will move smoothly but will not drop freely so I know it is still contacting. Sized to even 1.6435 the bolt drops without any resistance.
I figured sizing to 1.643 (3k bump and 1k clear of any contact) would work well but I wanted to ask if that’s leaving me with too little headspace. Thanks in advance.

 
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I approach is this way: as long as I can consistently chamber my rounds with little or no pressure in closing the bolt, I'm good. If there's too much resistance, then I might have bolt lift and/or extraction issues. Like you, my fired cases are at 1.646" and I bump them to 1.644 and my bolt closes the sized cases without any noticeable resistance and then extracts just fine afterwards (this is a drop in barrel from Preferred Barrel Blanks).

Having a little extra headspace in a hunting rifle can be a good thing, IMHO, in the event that dust or dirt get in where it shouldn't be. For a target rifle, some people like to have a little light touch in the freebore in order to have the axis of the bullet lined up with the axis of the bore for more consistent accuracy.
 

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If you can consistently hit 1.643" and you are getting results on target you like, I'd say that'll do. Least that is how I'd see it.
 

surfbturf

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I always do .002 back. Just in case sand or dust get on my brass.

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Broz

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Fellas,
One of the benefits of opening up my 6.5 PRC chamber is now it is unmistakable when my brass shoulder is still barely touching the front of the chamber versus when it is clear. I decided to re-confirm my optimal shoulder bump using the same method as Broz’s video. When fully fire formed, my brass measures 1.646”, and the bolt is getting very tough to close. Sized 2k down to 1.644 the brass barely touches the front of the chamber. With the firing pin and ejector removed the bolt will move smoothly but will not drop freely so I know it is still contacting. Sized to even 1.6435 the bolt drops without any resistance.
I figured sizing to 1.643 (3k bump and 1k clear of any contact) would work well but I wanted to ask if that’s leaving me with too little headspace. Thanks in advance.

A couple things. One, don't forget if you still have the ejector pin in the bolt, that will create some felt resistance and may stop the bolt handle from free falling. Two, if the bolt is closing firmly on a case and you measure that case, understand that case is longer than the chamber and the resistance we feel closing the bolt is the case actually being compressed in the chamber. In other words, that case is indeed over chamber length. So you need to size a case like that back .001 or .002" at a time, until you find the spot with very little reststance or just to the length where resistance closing is gone. Lets call that "zero Head space" That is the true chamber length and from that measurement of the zero HS case, we can now bump back the .002" to have an actual case with .002" of free head space. Hope this makes sense. Brass has spring back and we need to address that.
 

Masterdunbar

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A couple things. One, don't forget if you still have the ejector pin in the bolt, that will create some felt resistance and may stop the bolt handle from free falling. Two, if the bolt is closing firmly on a case and you measure that case, understand that case is longer than the chamber and the resistance we feel closing the bolt is the case actually being compressed in the chamber. In other words, that case is indeed over chamber length. So you need to size a case like that back .001 or .002" at a time, until you find the spot with very little reststance or just to the length where resistance closing is gone. Lets call that "zero Head space" That is the true chamber length and from that measurement of the zero HS case, we can now bump back the .002" to have an actual case with .002" of free head space. Hope this makes sense. Brass has spring back and we need to address that.

Broz the annealing process helps to mitigate springback, correct ?


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Broz

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Broz the annealing process helps to mitigate springback, correct ?


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Yes it sure does. However we want to be consistent with that step of our process. So if you anneal, do it always at the same intervals. And I would not suggest you wait more than 2 firings so the changes are minimized between anneals.
 

Desert Dan

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Thanks Jeff.
Yes, both the firing pin and ejectors were removed.
So if I understand correctly, the point at which I am getting that very slight resistance (1.644”) is zero HS and from that point I should bump it .002 more for proper headspace. Glad I asked, thank you.
 

Broz

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Thanks Jeff.
Yes, both the firing pin and ejectors were removed.
So if I understand correctly, the point at which I am getting that very slight resistance (1.644”) is zero HS and from that point I should bump it .002 more for proper headspace. Glad I asked, thank you.
Yes, I do believe from your description that zero would likely be 1.644"
 
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Thanks Jeff.
Yes, both the firing pin and ejectors were removed.
So if I understand correctly, the point at which I am getting that very slight resistance (1.644”) is zero HS and from that point I should bump it .002 more for proper headspace. Glad I asked, thank you.
The only time I remove my exactor pins and looking for that free fall by the bolt handle is when I'm mounting a new barrel and setting my chamber's headspace with my go-gauge. Once that headspace is set, I simply size my brass according to the measurement of my fire formed cases. In my 6.5 PRC my fired cases are 1.646 (understanding that's the length after springback of my annealed cases) and so I bump .002 to 1.644 as that amount of bump gives me pretty consistent chambering and extraction without having to work the brass too much.

I know of a few 6BR people who only bump their brass by just .001 and they seem to do alright, and a could who don't bump their brass at all . . . at least, not until they can't chamber their brass. That's just something I would not feel comfortable with; probably because my guns are factory and not so custom. 🤷‍♂️
 

OSOK - Crash

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Yes it sure does. However we want to be consistent with that step of our process. So if you anneal, do it always at the same intervals. And I would not suggest you wait more than 2 firings so the changes are minimized between anneals.

Listened to Avery’s latest podcast with ADG, and ADG recommended annealing every time. Each time you fire it work hardens the brass. So they recommended to anneal after each firing.

Not questioning anyone’s methods here at all. Just adding something I heard from the manufacturer.
 

Desert Dan

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I am now in the habit of annealing every time. Even with once fired brass I’m almost always either developing a load, gathering extra data on an existing load, or shooting an existing load after doing all that work developing a load for the most accuracy possible, so there is no reason for me not to. With components being what they are right now I don’t want to send a single round just for the sake of shooting. Last weekend when I shot a few extras to even out a few oversized cases, I still shot them over the V3 to collect some data. Besides, the AMP makes it so easy and I usually never have to do more than 100 pieces at a time. Usually it’s closer to 50.
 

Masterdunbar

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That’s going to be my new practice. Unfortunately I screwed up and FL sized my brass for a new load work up ……..and then bought my annealing machine. It’s 3x fired so I’m sure it could have REALLY used a good fire bath. Live and learn.


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Desert Dan

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I got pretty nerdy and did a lot of research about annealing before I bought my machine. It was a real eye opener. I ended up going down a rabbit hole of data and comparisons but came away with a much better understanding on how it plays into consistency as you start stretching out the distances.

 

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