Shoulder bump and group size

[rwj]

Jim is a UBR world record holder. He tests extensively.

Sure does! I’ve shot next to him on several occasions at our home club… his groups form very small holes.

 

[LCazador]

Neither am I. That is a fine group, but you could do the same without running that tight and potentially sacrificing gun operation. Been there, done that. I tested the theory and it was white noise for me. If it works for you have at it, but in the previous post made by @Janeau he indicated that zero bump or .001 bump is the norm for BR. I say that is not the case at all. There are no absolutes in this stuff and there are better ways if people do the testing for themselves to see what matters and what does not matter.

Since we are posting targets to prove a point, here are four consecutive match targets with .003 bump. Not a single tight chambering round in the bunch. Did not give up anything.


View attachment 1709156

Jimmy that's some mighty fine shootin"!

 

[rwj]

Sucks. It is better when I just read and not participate. I am wasting my time defending myself for nothing more than bringing a different perspective for people to think about.

I enjoy reading differing view points… missed yours over the past few months!

 

[Murray Brook]

Sucks. It is better when I just read and not participate. I am wasting my time defending myself for nothing more than bringing a different perspective for people to think about.

Whatever you do don't stop posting. One of the reasons I'm here is to read and learn what shooters like yourself have to say. Your posting really matters.

 

[Schippergreg]

Sucks. It is better when I just read and not participate. I am wasting my time defending myself for nothing more than bringing a different perspective for people to think about.

You’re not wasting your time, I for one appreciate when those who can walk the walk share their experiences.

 

[D Coots]

getcha a 9mm empty case
Insert it over the neck onto the shoulder and use that as a chamber gauge... measure OAL end to end
Compare this to one you know is of correct shoulder bump
The 9mm case hits a nice index spot on the shoulder of most LR cases
I don't have an empty 9mm case here at the shop but you get the idea.
I used to make chamber gauges, now all I use are 9mm and 10mm empty cases to gauge shoulder bump
if you have a bullet seated and need to double check - use the longer 38 special case
---
Have your required OAl written on the case you use as a tool, THAT CASE ONLY
dont just grab any emtpy case laying around as they all will be different lengths
use the same one
I have several with the numbers written on them for various calibers.

That'll never work.....DOESN'T COST ENUF

 

[D Coots]

To much resistance will throw flyers i know that for a fact I've had it done to me more than once, how much is to much I dont know all I can tell you is since I've started the Wheeler method it ain't happening.

Just wondering out loud here.

If you're shooting from a bench, would the bolt closing hard be part of the flyer problem?

 

[M-61]

Sucks. It is better when I just read and not participate. I am wasting my time defending myself for nothing more than bringing a different perspective for people to think about.

I agree. I’m actually posting way less and don’t read as much as I used to on this site.
I used to write lengthy posts on Veterans Day but I’ve even limited that. And I’m tired of defending myself with a keyboard.
Slipping away is way less painful.

 

[L.Sherm]

Not su

Just wondering out loud here.

If you're shooting from a bench, would the bolt closing hard be part of the flyer problem?

Not sure of your question, do you mean because it screws up the way the gun is tracking or whatever it does inside the chamber?
All I know is ive had times doing load development that I took a fired case measured with a comparator took that number bumped backed .002 and once in awhile during load development it would cause a flyer. I do not get those occasional rounds that dont close without resistance now. Just about all my rifles now that I do the Wheeler method take .003 to get no resistance 1 takes .004.
At the end of the day the results your after is what matters.

 

[rwj]

I agree. I’m actually posting way less and don’t read as much as I used to on this site.
I used to write lengthy posts on Veterans Day but I’ve even limited that. And I’m tired of defending myself with a keyboard.
Slipping away is way less painful.

Well… if people quit posting… there won’t be anything to read!

 

[Straightshooter1]

Depends on if you believe the case has to have support for the primer to ignite.

If CTBO is cartridge base to bullet ogive, then you can place the shoulder any where you want and the measurement won’t change.

The neck will get longer or shorter, and the same for body.

Jump/jam is the distance form shoulder to land. If the bullet is set to touch and you move the shoulder back .002”, you have a .002” jam. Move the shoulder the other way ant it pulls the bullet away.

IMHO, I don't think there's a correct blanket answer to the questions OP presented, except for. . . "it depends" as an answer. For example, gas guns are going to have very different issues than bolt guns. With bolt guns, it depends on the amount of jump and shouldn't be compared to those that are touching or jamming or even those with a jump less than .010".

It's like does the change in jump have an effect on group size and not looking at the effect throat erosion has on group size. Change in jump can have an effect, but "it depends".

So, as I see it, shoulder bump having an effect on group size just depends on how you set up your cartridge to fit into your guns chamber.

 

[Evan]

Sucks. It is better when I just read and not participate. I am wasting my time defending myself for nothing more than bringing a different perspective for people to think about.

It seems all the knowledgeable voices around here fall silent after a while. I appreciate you sharing your experience. I use the ignore button extensively - it helps.

I have the rifle with a stripped bolt at the bench and size cases to fit. The bolt ideally falls 90% closed and takes a whisper of pressure to close the last bit. I err on the side of it falling all the way closed because too tight and it shoots bad. Typically 0.001” of bump is the difference between the bolt doesn’t fall and the bolt falls all the way. The actual amount of bump is irrelevant though - the cases have to fit properly and I check every one every time on any gun I expect the utmost accuracy from. If the metric is 1 MOA, then you can size one case until the bolt falls freely, add a tiny bit extra bump for safe extra clearance if a hard case comes along that won’t bump as far, then run all your cases blindly after that.

 

[LCazador]

Just wondering out loud here.

If you're shooting from a bench, would the bolt closing hard be part of the flyer problem?

Yes sir! Everytime for me! Too much gun movement and you're forcing headspace/

 

[dellet]

IMHO, I don't think there's a correct blanket answer to the questions OP presented, except for. . . "it depends" as an answer. For example, gas guns are going to have very different issues than bolt guns. With bolt guns, it depends on the amount of jump and shouldn't be compared to those that are touching or jamming or even those with a jump less than .010".

It's like does the change in jump have an effect on group size and not looking at the effect throat erosion has on group size. Change in jump can have an effect, but "it depends".

So, as I see it, shoulder bump having an effect on group size just depends on how you set up your cartridge to fit into your guns chamber.

The question I was responding to really didn’t address accuracy except that it hinted that bumping the shoulder back .002” was equivalent to changing the seating depth .002” because CTBO, (cartridge base to ogive in my mind) would change. The only way that can happen is if your seating die indexes off the shoulder. Or I have some kind of mental block keeping me from understanding how moving the shoulder would change the base to ogive distance.

My point is that what changes is shoulder to ogive, which will effect jump.

Maybe it’s too much like saying changing the base to shoulder measurement changes headspace. Or measuring in tensions when seating a bullet.

 

[JMayo]

Sucks. It is better when I just read and not participate. I am wasting my time defending myself for nothing more than bringing a different perspective for people to think about.

I want easy bolt operation no matter what it takes in bump. With .001 some of the 200 cases won't be 001 and I'll have to upset the rifle, sometimes having to beat the bolt closed, that's a hard no. So i find a happy medium that works with all 200.
Sholder bump, unless real excessive, is not for accuracy.
it's for smooth operation & brass care IN MY OPINION

 

[jelenko]

use the ignore button extensively

^^

 

[D Coots]

Not su
Not sure of your question, do you mean because it screws up the way the gun is tracking or whatever it does inside the chamber?
All I know is ive had times doing load development that I took a fired case measured with a comparator took that number bumped backed .002 and once in awhile during load development it would cause a flyer. I do not get those occasional rounds that dont close without resistance now. Just about all my rifles now that I do the Wheeler method take .003 to get no resistance 1 takes .004.
At the end of the day the results your after is what matters.

Just wondering if the hard closing of the bolt was upsetting the gun on the bags causing the shot change? Kinda like going from the sighter to the record change.

After every agg I check my brass In The Rifle. You'd be amazed of the difference between the actual fit in the chamber and my measurement. Most generally a couple need more thou bump to conform.

 

[Ccrider]

I knew when I made my first post in this thread it was probably a mistake even though my intent was good.

Nope. Not a mistake at all. Thanks for sharing and please don’t let one or two ignorant people screw things up for everyone.

 

[tom]

You can use any comparator larger than the neck and smaller than the shoulder to take comparative measurements. But remember that is only comparable at whatever diameter that is. It's an angle thing, and how they match between actual chamber and die. I prefer a comparator made with my chamber reamer for serious stuff, but using a stripped bolt to find "touch"(i would advise to NOT load there), you can than give them some clearance from that. I prefer 1-2 of "jump" myself. Cases still last for multiple barrels, and you don't get unexplained flyers from this topic of discussion.

Tom

 

[D Coots]

Just an FYI.....If I bump my brass to where the bolt drops with no resistance I will have "The Click." It's happened with different dies. Less bump, the click dissapears.

If this post derails the intent of the op, no problem. I'll delete.