Shoulder Bump does it impact ACCURACY?

[Mulligan]

I have tested shoulder bump uniformity on accuracy and can say without a doubt, inconsistent shoulder set-back or bump will show up on target at 600 yards. I get smaller groups with a uniform shoulder set-back (resizing really).

I have searched the web looking for the results of testing different shoulder set-backs on accuracy. For example, .0005" verses .002", vs .004" shoulder set-back tested head to head.


Do any of you know of such a test?

CW

 

[hicap40]

 

[CharlieNC]

Interestingly we frequently see posts stating the fire forming loads when blowing out to an Ackley or such give comparable accuracy vs the subsequent load. That represents quite a shoulder difference.

 

[Mulligan]

Interestingly we frequently see posts stating the fire forming loads when blowing out to an Ackley or such give comparable accuracy vs the subsequent load. That represents quite a shoulder difference.

exactly!
CW

 

[TRexF16]

Interestingly we frequently see posts stating the fire forming loads when blowing out to an Ackley or such give comparable accuracy vs the subsequent load. That represents quite a shoulder difference.

But not a headspace difference, since a correctly cut AI chamber should be a perfect fit at the neck to shoulder junction on a SAAMI parent case. Ought to hold the case neck centered well in the chamber.

Cheers,
Rex

 

[Mulligan]

I wonder how much bump Tony Boyer used..... might have to dig out that book and take a look. I thought it was around 1/2 thou?
CW

Edit
Thanks @hicap40 for sharing this information.
CW

 

[CharlieNC]

But not a headspace difference, since a correctly cut AI chamber should be a perfect fit at the neck to shoulder junction on a SAAMI parent case. Ought to hold the case neck centered well in the chamber.

Cheers,
Rex

I do not have personal knowledge to know how much headspace is typically experienced, but like any new brass it likely is not an ideal fit. This coupled with a non fitting shoulder would seem to be the perfect storm. Interested to hear first hand experience.

 

[RegionRat]

But not a headspace difference, since a correctly cut AI chamber should be a perfect fit at the neck to shoulder junction on a SAAMI parent case. Ought to hold the case neck centered well in the chamber.

Cheers,
Rex

Yes, an AI design won't have as much leeway in the chamber, but what about a 6 BR going to a 6 Dasher?

A club mate won matches, more than once, while fireforming his 6 Dasher brass, which really begs the question of the effect Clay is asking about.

ETA: Is it the shoulder datum issue, or is it really the jam or neck controls that have a dominant effect?

Happy New Year!!!

 

[Dusty Stevens]

Consistency is key. Dragging an expander or varying hardness makes the bump not the same. Think of one case slamming forward .001 and another going forward .005 and how much difference in ignition that will cause. Good vibrations man, and when i say good i mean consistent

 

[jackieschmidt]

In my 6PPC and 30BR, I have found that the most consistent agging capability is found when I get more aggressive with the sizing. I actually bump the shoulder as much as .003.

Through the years, I have also found that more than a few shooters have no idea how to determine exactly how much they are pushing the shoulder back. Many Benchrest Shooters do not rely on commercial available tools, and certainly not “feel”.

We use what many refer to as a “Thingy”, which is simply a piece of barrel about 1” inch long with your chamber‘s shoulder established on one end. using it with a set of dial calipers allows you to establish the shoulder set back to an exact dimension.

I have also made up windowed Thingys for each of the Benchrest cases I use, 6PPC, 6BR, and 30BR. This allows me to see exactly what the case looks like in the actual chamber.

It is just my opinion, but I think a shooter should have his Gunsmith make him a “Thingy” as part of the barrel joh. Not only does it allow you to establish your shoulder bump in a precision manner, you can also establish your bullets seating depth in a fresh chamber and throat By simply inserting the loaded round and giving it a little twist against the lands. You can then visually see the amount of contact and work from that point.

 

[Mulligan]

I Reread that section of Tony’s book, looks like he starts with .001” and adjusts a tiny bit either way from there to get set-back to where the bolt handle will fall just short of halfway.
CW

 

[Mulligan]

In my 6PPC and 30BR, I have found that the most consistent agging capability is found when I get more aggressive with the sizing. I actually bump the shoulder as much as .003.

Through the years, I have also found that more than a few shooters have no idea how to determine exactly how much they are pushing the shoulder back. Many View attachment 1304258 Benchrest Shooters do not rely on commercial available tools, and certainly not “feel”.

We use what many refer to as a “Thingy”, which is simply a piece of barrel about 1” inch long with your chamber‘s shoulder established on one end. using it with a set of dial calipers allows you to establish the shoulder set back to an exact dimension.

I have also made up windowed Thingys for each of the Benchrest cases I use, 6PPC, 6BR, and 30BR. This allows me to see exactly what the case looks like in the actual chamber.View attachment 1304260

Dan Dowling refers to those as “Critters” or “chamber critters”. I have never seen one with a window, that is awesome.

CW

 

[Alex Wheeler]

Yes it does. I'd bet you find a little more room is not a bad thing. But you will have to test it. Remember how the bump is effecting seating depth as well.

 

[Mulligan]

Yes it does. I'd bet you find a little more room is not a bad thing. But you will have to test it. Remember how the bump is effecting seating depth as well.

Doggone, the dynamics of this sport!
Thanks Alex, the value of this forum to my testing is priceless.
CW

 

[DaveTooley]

I've never shot a rifle accurately when I was fighting the bolt handle. I've never focused on a specific number. So whatever bump it takes to free up the bolt handle is what I go with.

Jackie's Gizzies work.

 

[jackieschmidt]

I've never shot a rifle accurately when I was fighting the bolt handle. I've never focused on a specific number. So whatever bump it takes to free up the bolt handle is what I go with.

Jackie's Gizzies work.

Dave, that is true with an experienced shooter such as yourself.
less experienced shooters need to understand that other things can affect the feel of the bolt handle falling, the principle culprit being the web diameter. Another the actual alignment of the chamber with the bolt way.

I have seen shooters sizing cases until the bolt fell free, only to end up with an exorbitant and unsafe amount of headspace.

I suppose it is the Machinist in me that demands I measure everything.

 

[jimmymac]

I don’t know anyone who shoots group or score that doesn’t use a barrel stub or thingy to measure shoulder bump. I only bump enough to ensure smooth gun operation. That’s usually on the order of .0015 to .002.

 

[P1ZombieKiller]

I suppose it is the Machinist in me that demands I measure everything.

Jackie,
Is "Machinist" another word for OCD???

 

[Northridge]

I have tested shoulder bump uniformity on accuracy and can say without a doubt, inconsistent shoulder set-back or bump will show up on target at 600 yards. I get smaller groups with a uniform shoulder set-back (resizing really).

I have searched the web looking for the results of testing different shoulder set-backs on accuracy. For example, .0005" verses .002", vs .004" shoulder set-back tested head to head.


Do any of you know of such a test?

CW

shoulder bump or consistent shoulder bump will affect overall affinity for shots to land in a similar quadrant. like so many other things we can ladder test shoulder bump is no different, ladder three different bumps and you will find one will likely perform or shoot cleaner. there really is not a "the perfect bump" though it is of my opinion from managing this situation for a long time that a little more say .002-.003 bump vs .0005-.001 is less finicky and will help develop a lazier tune window with less random popped shots. I am also of the opinion that less say .0005-.001 is just as reliable if you measure ever single case to catch the piece that doesn't spring or rebound to your .0005 but stays just long enough to slightly bind or drag in the chamber causing a shot that will leave the static quadrant or point of impact or developing a "popped" shot. the same applies to looser bumps but the popped shot will be less detectable. grouping is relative as well if you are tuning for half MOA at distance at best you may not realize the benefit vs managing sub half MOA for group shooting. also a note when I tune a rifle for 5 shot or light class I can manage less bump with success or lack of popped shots but for 10 shot grouping or a string with more heat i'm more successful with more bump reducing late string popped shots this go's for F-class situations as well where we can lay up a lot of bore heat . like everything develop a couple comparison ladders all the same procedures except the bump being different at distance a winner will show itself. look how the ladder forms as it comes in to tune and leaves tune not just the smallest grouping but a lazy grouping window.

Shawn Williams

 

[D-4297]

Jackie,
Is "Machinist" another word for OCD???

Absolutely , says the retired T& D guy . Drives people around me nuts . Just like I go straight up a wall when I hear the phrase ,"That's good enough" .