Shoulder bump

[tenring]

How far can shoulders be safely bumped? Fiddling with die depth and some went back .08 - .010". How will that affect accuracy when others are pushed back .001-.003"?

 

[Ned Ludd]

How far can shoulders be safely bumped? Fiddling with die depth and some went back .08 - .010". How will that affect accuracy when others are pushed back .001-.003"?

I'm assuming you meant you pushed the shoulders back on a few cases from .008" to .010". In my rifles, the shoulders on Virgin Lapua brass can move from about .004" to .005" (.308 Win) to as much as .008" to .009" (.223 Rem) on the first firing. You really don't want to ever bump them back equal to virgin brass because it works/stretches the case-wall region slightly above where the cases transition from thick case-head to thin case-wall. If you do that even as few as a couple or three times, you greatly increase the risk of case-head separation, which is very bad news. Because the fired case expansion is limited by the chamber, bumping the shoulder back .001" to .002" is usually sufficient to allow them to easily fit back into the chamber without excessive force required for bolt closure.

In any event, you have apparently resized a few of your cases back close to what they were as virgin brass, maybe even a tick more. I typically find that virgin brass requires anywhere from 0.1 to as much as 0.3 gr more powder to achieve the same velocity as fired brass. If I do a preliminary load workup in virgin brass, I know I will have to drop the charge weight by about 0.1 to 0.3 gr to keep the same velocity after the brass has been fire-formed. Presumably, this is due in part to some amount of the total combustion energy going toward expanding the virgin cases and pushing the shoulder forward. If we only push the shoulders back from .001" to .002" when resizing, that same amount of energy is no longer required for case expansion and some of it translates into increased velocity. The slightly decreased "effective" case volume of virgin brass, or in your situation, brass which has had the shoulders bumped farther than necessary, doesn't seem to be as much of a factor in the load differences between virgin and fire-formed brass. 50,000 to 60,000 psi is sufficient to expand it to closely fit the chamber, which is the limit of the true pressure cell. My guess would be that rounds loaded with your "excessively-sized" cases may show some small velocity anomalies as compared to the other cases that were sized properly. If they follow what I usually observe with virgin brass in my rifles, those loaded rounds might be a bit slower than the others. As I mentioned, it might translate to a change in charge weight of something like 0.1 gr to 0.3 gr, which is not huge, but it's not zero, either. That's why I'd just use them as fouling rounds, where precision doesn't matter that much, anyhow.

I usually keep plenty of fired commercial ammo brass around, which is good for various things like the initial setup of sizing dies, trimmer, etc., so you don't mess up your good brass adjusting the equipment until the settings are already fairly close to where you want them. Again, I'd suggest using the pieces that were bumped more than about .002"-.003" as foulers. Segregate or mark them clearly to make absolutely sure you don't accidentally bump the shoulders on those pieces that far a second time. They should then be ok to mix back in with the rest of the brass following the next full case prep with the resizing die set appropriately.

 

[Drolds72]

I have inadvertently bumped that much, and would dent primer, but not fire even with a good Bullet jam.

 

[Uncle Ed]

How far can shoulders be safely bumped? Fiddling with die depth and some went back .08 - .010". How will that affect accuracy when others are pushed back .001-.003"?

Your question depends on chamber pressure, brass strength, chamber dimensions and resizing die dimensions. The thumb rule of bumping the shoulder back .001 to .002 is to insure longer case life and preventing the case from stretching beyond its elastic limits.

At 38,000 cup or 42,000 psi the chamber pressure is not great enough to make my 30-30 cases stretch. And I have cases 25 to 30 years old that fail of split necks and never of case head separation. On these 30-30 cases the primer always protrude because the chamber pressure is not great enough to make the base of the case stretch to meet the bolt face.

This is where making a workup load and starting at the suggested starting load and working up will be very informative. You will reach a point with new brass as the load increases that the primers are flush with the base of the case. And at this point the brass is stretching and does not spring back to it original length. And this varies between case brands, and the hardness and thickness in the base of the case.

Below these .308 cases were fired in a brand new Savage rifle with the die setup to make hard contact with the shell holder. And no information was give on how far the case shoulder where pushed back during sizing so this info is very ball park. But please note on the bottom chart it tells you at what firing the cases started to stretch.

Below is a example of excessive head clearance, meaning bumping the case shoulder back too far in a old worn British Enfield rifle at max military headspace. In my rifle there was .016 head clearance and the case separated on its 3rd firing when full length resized and the die making contract with the shell holder.

Below a properly fire formed case that is now headspacing on its shoulder and does not stretch with minimum shoulder bump.

Bottom line, In 1973 I bought a Remington 760 .270 Win pump action that I full length resize with the die making hard contact with the shell holder and never had a single case head separation. "BUT" had a case head separation with a .303 British Enfield rifle the 3rd time the case was full length resized. It is the amount of shoulder bump and the chamber pressure that makes the case stretch and fail. And with reduced loads and neck sizing only I got over 30 reloads with the same cases before the necks started cracking.

So again it is the amount the case shoulder is pushed back every time and higher chamber pressures that cause the case to stretch and fail. And your chamber and sizing die will have the greatest effect on case life and why minimum shoulder bump extends case life.

Bumping the shoulder back .001 to .002 below the red dotted line will prevent case stretching when full length resizing. And the further the shoulder is pushed back toward the green dotted line increases the chances the case will stretch and have a case head separation.

 

[D-4297]

Kudos to both Ned Ludd and Uncle Ed for posting excellent explanatory information that teaches ...Thank you for helping to advance new shooters knowledge and information with viable and understandable posts .

 

[K22]

The degree of shoulder bump should always be based on a fired case in the rifle you are loading for. As others have stated, a .001 to .002 bump for bolt rifles is optimum. The key is to size the case just enough to permit reliable chambering but not oversize thus creating an excessive head space condition.

On new brass I run them though a full length sizing size set a "0" cam over*. This straightens the case without pushing the shoulder back. From that point on the degree of cam over* is based bumping the shoulder back .001 to .002 on a fired case for that specific rifle.

I full length resize (fired cases) all my rifle cases using a .001 to .002 bump and except for a rare neck split, I get about 15 to 17 reloads per case without any signs of incipient head separation. Of course careful inspection of each case should always be done after each loading to detect and fatigue defects. After 15 to 17 reloads I retire the cases even if there is no fatigue defects. Figured I got my money's worth of those cases.

I used Skip Shims to adjust the amount of cam over* for each rifle of the same caliber, i.e. I have several 223 Rem's and 243 Win's and some require a different degree of cam over* to achieve the desired bump.

*cam over - the amount of travel of the press's ram (w/ shell holder) to the sizing die; zero cam over being the ram just touching the sizing die.