How much shoulder bump?

[Heavies]

I have recently acquired a Larry Willis tool to measure shoulder bump, and I have found that my method of trial and error 'feel' has produced a .001 - .0015 bump on certain cases. Due to the shell holder not making full contact with the die, not all cases are sized consistently, some long and some short. This made me realize why sometimes I would have a intermittent tight bolt before and after firing.

So, I got a set of Redding Competition Shell holders. This set is supposed to come with .002, .004, .006, .008, .010" over sized shell holders, so that one may have full contact on the die, thereby, creating a dead length sizing on all the cases. Now the problem, The set came with two .008" holders instead of the .006".

In my rifle the .010" does not produce any sizing, the .008" holder produces a .0005" set back on the shoulder, and the .004" comes out to .003" set back. Obviously I need the one holder that is missing (.006") to give me the .0015-.002" set back. :'(

In the mean time, while I am waiting to get this straightened out with Redding, is .0005" enough 'bump'? The fired cases go into the chamber with very, very little perceivable resistance, but will I have a sticky bolt after firing? Will this small amount of sizing give a better accuracy advantage over bumping at the more popular number of .002? And finally, why is it that .002" bump is the default number when speaking of shoulder bump?

Thank you all for your continuing advice.
-B

The rifle is a Savage PTA, single shot boltgun, so minimal case sizing is acceptable, however, I hate that stiff bolt after firing.

 

[CanusLatransSnpr]

Heavies: The .001-.002" bump is the default because you don't want to overwork the brass. If a guy continuously bumped the shoulders back say .004", after awhile he may experience a case head separation. With continued firings, the brass in a case flows toward the neck, thus making the brass thinner down around the case head. With the .002" less bump, thats that much less the shoulders, or the case itself to move forward. If you have two .008" shell holders, take some emery cloth and put it on a piece of glass and surface it down to .006"! Pretty simple and easy to do!

Mike

 

[CanusLatransSnpr]

Actually, if you just took it to .007" that would be sufficient!

Mike

 

[RonAKA]

You have to keep in mind that many that just neck size do not bump the shoulder at all. Typically you can get away with no bump for quite a few loadings. So even if you only get a 0.0005" bump that is probably enough, and even zero is OK at least for a few loadings.

 

[CanusLatransSnpr]

I didn't even think about it from that aspect Ron. Good call!

Mike

 

[Terry]

I made my own bump gage with a piece of aluminum bar stock. Using it I measured the distance from the base of the case to the end of the gage on a once fired case. When cases get hard to load in the chamber I bump them back to the once fired measurement. Works Great!
Terry

 

[Heavies]

Heavies: The .001-.002" bump is the default because you don't want to overwork the brass. If a guy continuously bumped the shoulders back say .004", after awhile he may experience a case head separation. With continued firings, the brass in a case flows toward the neck, thus making the brass thinner down around the case head. With the .002" less bump, thats that much less the shoulders, or the case itself to move forward. If you have two .008" shell holders, take some emery cloth and put it on a piece of glass and surface it down to .006"! Pretty simple and easy to do!

Mike

Actually, if you just took it to .007" that would be sufficient!

Mike

Mike,
Thank you, that is a very good Idea, and I could probably get it closer to where I want it than if I wait for Redding to get me a marked holder. The two .008's may have been a Blessing in disguise.

So, .002" bump is probably a maximum figure when sizing cases for a bolt action?

You have to keep in mind that many that just neck size do not bump the shoulder at all. Typically you can get away with no bump for quite a few loadings. So even if you only get a 0.0005" bump that is probably enough, and even zero is OK at least for a few loadings.

Ron,
also a superb idea. I was thinking that maybe after firing .0005 would cause that click at the top of the bolt. But perhaps I should try it out first and see how it performs. The less working of the brass, the better.

I made my own bump gage with a piece of aluminum bar stock. Using it I measured the distance from the base of the case to the end of the gage on a once fired case. When cases get hard to load in the chamber I bump them back to the once fired measurement. Works Great!
Terry

Terry,
I must apologize, but I am not quite grasping at what you did. Do you have any pics? Sorry, I'm just a simpleton. ;D


This is why this site is the best!
When I get frustrated it is best to get some second opinions, and open up to new ideas.
Thanks,
-B

 

[artbosco]

I have recently acquired a Larry Willis tool to measure shoulder bump, and I have found that my method of trial and error 'feel' has produced a .001 - .0015 bump on certain cases. Due to the shell holder not making full contact with the die, not all cases are sized consistently, some long and some short. This made me realize why sometimes I would have a intermittent tight bolt before and after firing.

The mixture of alloys in a case is not perfect, which is why some are harder than others, and spring back can and does vary. To think that each and every case could be set to a precise .0005" [5/10,000th inch] threshold all the time is a pipe dream. I strive for a .001" to .002" range and let the shooting gods take care of the rest.

 

[Terry]

Pics

 

[CanusLatransSnpr]

Heavies: Yes .002" bump would be max with a bolt action rifle! If you can get it to .0015" that would be better. Now if you were sizing brass for an AR type rifle, I would say bump em back .003-.004" then. Auto loaders need a little more wiggle room when it comes to head space.

Mike

 

[Heavies]

Thanks guys! Ill shoot for .001. I'm gonna stone one of the extra shell holders another .0005 or more if need be.

Thanks for the pics Terry, I see what you mean now. I was thinking something way different, but I get it now.

 

[RonAKA]

I may be thinking about this all wrong, but for what it is worth, here is what I did when I initially sent up my Forster Bushing/Bump die.

I used the die itself as a gauge to measure a bunch of fired and not sized brass for length. Just took the top works off and stuffed them into the die and measured each one - base of cartridge to top of die. I then picked the shortest case, and essentially set it up in the die to have zero headspace with the press at full stroke (Lee Classic Cast). I locked the die in that location. Other brass varied from the same short length to 1-1.5 thou longer. When I sized them the short brass had their shoulder left where it was, and the long brass had it pushed back.

I guess the point is that I did not get a consistent set back of the shoulder from case to case, and only the longer ones got set back. I measured again of course to see if the long ones got set back. My thoughts are that the chamber size in the gun is not changing, so I don't see why I can't leave the die set that way for the life of the gun (I only have one 6BR, so don't have to adjust to another chamber).

Long story, but I don't see anything wrong with only a 0.0005 bump, or perhaps even a near zero bump as long as that only happens on the shortest as fired cases, and those cases chamber well.

 

[CanusLatransSnpr]

One mistake I made was that I didn't ask what chambering this die was being set up for? No two cartridges behave the same when being fired. I.E. a 22-250 being fired and a 6mmbr being fired will behave differently. Some cartridges "inherently" stretch brass. Thats part of the reason Ackley started improving cartridges, to try to eliminate cartridge stretch when being fired. A 22-250 AI with 40 deg. shoulder won't stretch near as much as a std. 22-250 will.

Mike

 

[Heavies]

This is for 6mmbr, so there isn't too much strech. ;D

 

[bozo699]

Pics

Terry,
What did you do run a chamber reamer in the aluminum bar stock?
Wayne.

 

[tenring]

I use a Redding Instant Indicator to measure bump to .0005".

If you can, find a Savage firing pin/bolt shroud removal tool, use it to disassemble those components. Sinclair does not list it for a Savage, but I would call them.

Then replace the bare bolt in action. On a properly sized case, the bolt handle should drop about half way down with ease, then very slightly resist as the brass shoulder touches the chamber. At that point, you have perfect bump. If the boilt handle flops down, you pushed shoulder back too far. If bolt is difficult to put down, bump another .0005"

 

[moosetracker]

I use the Larry Willis "Digital Headspace Gauge" as well. I try to set up my FL, Body or Bump dies with .001" bump for target work and .002" for hunting. I also use Sinclair shims under the die so I never need to readjust the die once set up. With this approach, I can change the shim if I want more or less bump. When resizing for an identical cartridge in another gun, I just use the shim suited for that chamber without readjusting the die.

 

[Heavies]

I use the Larry Willis "Digital Headspace Gauge" as well. I try to set up my FL, Body or Bump dies with .001" bump for target work and .002" for hunting. I also use Sinclair shims under the die so I never need to readjust the die once set up. With this approach, I can change the shim if I want more or less bump. When resizing for an identical cartridge in another gun, I just use the shim suited for that chamber without readjusting the die.

Funny thing... I found out about those Sinclair shims after I purchased the Redding comp shell holders. I was kicking myself! Would have got those if I had known about 'em. They are cheaper too.

 

[Terry]

Pics

Terry,
What did you do run a chamber reamer in the aluminum bar stock?
Wayne.

It was made on a lathe by drilling the hole slightly larger than the case neck and cutting the angle to match the sholder of the case. The gage is the cut off with a cut off tool. We later faced the end cut off.

 

[bozo699]

Terry,
It ought to work good thanks for the info.
Wayne.