Shoulder bump, how much is too much ?

[Jeff Porter]

Using my Hornady headspace tool to measure 5.56 Starline shows a number of 1.4500” to the datum line. Same number on a .223 Go gauge.

1x fired in a .223 FTR chambered barrel shows 1.4520” so a fairly tight chamber.

Brass annealed and sized with target of 1.4510”. Most are coming out 1.4500”-1.4520” range.

But for some reason I have yet to figure out, I have been getting some that are sized down shorter in the batch. These are 1.4400-1.4440”. 6-10 thou under sized from the min spec.

These are getting light primer strikes and “Click no boom” ftf.

I presume this is because it is only 1x fired in this chamber and different softness is having less spring back from the die.

I am going to go through the bolt and change the striker spring and check firing pin protrusion and try the rounds in another rifle but wanted to ask before I go down that rabbit hole….

How much below the 1.450” is too short ?

 

[FeMan]

There isn't a hard and fast answer. Excessive shoulder bump causes case stretch and can lead to early failure. If you get them too far back, you can end up with pierced primers too.

I would say the goal is set shoulders back no more than 0.002". I have seen cases fired with around 0.005" or 0.006" setback fail in five or six firings and seen pierced primers develop with bumps in the 0,010" to 0.020" range.

I think you are well served by replacing your firing pin spring. If that doesn't work, check to make sure you have adequate firing pin protrusion. A 0.010" setback shoulder shouldn't result in the type of ignition change you are describing.

 

[C.E.Smith043]

Make sure you are not getting any sizing lube built up or on the shoulder area. This can cause you to get inconsistent shoulder bump when resizing.

 

[Rocketvapor]

Short cases might cushion pin impact.

 

[Webster]

Using my Hornady headspace tool to measure 5.56 Starline shows a number of 1.4500” to the datum line. Same number on a .223 Go gauge.

1x fired in a .223 FTR chambered barrel shows 1.4520” so a fairly tight chamber.

Brass annealed and sized with target of 1.4510”. Most are coming out 1.4500”-1.4520” range.

But for some reason I have yet to figure out, I have been getting some that are sized down shorter in the batch. These are 1.4400-1.4440”. 6-10 thou under sized from the min spec.

These are getting light primer strikes and “Click no boom” ftf.

I presume this is because it is only 1x fired in this chamber and different softness is having less spring back from the die.

I am going to go through the bolt and change the striker spring and check firing pin protrusion and try the rounds in another rifle but wanted to ask before I go down that rabbit hole….

How much below the 1.450” is too short ?

Don't understand how you can size cases in the same die with the same adjustment and get a wide range of set back? What am I missing? When I size with a bump gauge they are all within 1.5 thou of each other.

 

[Jeff Porter]

Don't understand how you can size cases in the same die with the same adjustment and get a wide range of set back? What am I missing? When I size with a bump gauge they are all within 1.5 thou of each other.

That is what I am asking too. How can the same die setting give 0.010” different bumps. Current suspects are lube on the shoulder and different metallurgical properties of the Starline 5.56. The batch is all 1x fired in the same gun but was from two different purchases of the brass. ( different manufacturing lots ).

I notes that to get the 0.0015 set back I was needing to count to 5 (dwell time with press ram down) and would get consistent results. My theory is some of the brass was softer and didn’t spring back after being compressed.

 

[fisherman987]

I'm not sure why you are getting those few outliers, but 12 thousandths shoulder bump is WAY to much! Your case is sliding forward that far when the firing pin strikes it which will cause a delayed fire or even misfires. For cases that size, I would keep shoulder bump at .002 max. If you only have a small number of cases that sized that way, I would toss them out, or if you want to save them, you can try seating your bullet .020 into the lands with the primer fully seated and see if you can get the shoulder blown back out, then back your die off and slowly adjust until you are bumping shoulders back .002 on those cases. Keep those cases separate until they harden up enough to start sizing like the others.

 

[NorCalMikie]

Don't understand how you can size cases in the same die with the same adjustment and get a wide range of set back? What am I missing? When I size with a bump gauge they are all within 1.5 thou of each other.

Some cases may be softer than the others, making measuring inconsistent. And short cases can and will give you FTF or soft strikes.
And IMHO, a .020 bump is too much. When fired, case stretch can start to be a problem, leading to case head separation.
Make sure you have/keep a broken case extractor tool on hand.
A .002 bump would be nice. And I see no need for a spring change, That's NOT an issue. You brass is the issue and it's not really that bad.
Just another bump in the learning curve.

 

[fisherman987]

That is what I am asking too. How can the same die setting give 0.010” different bumps. Current suspects are lube on the shoulder and different metallurgical properties of the Starline 5.56. The batch is all 1x fired in the same gun but was from two different purchases of the brass. ( different manufacturing lots ).

I notes that to get the 0.0015 set back I was needing to count to 5 (dwell time with press ram down) and would get consistent results. My theory is some of the brass was softer and didn’t spring back after being compressed.

Your theory is correct, most likely due to lot to lot variance.

 

[JFrank]

There’s three kinds of bumps~
not enough
too much
looks about right

 

[G Liles]

What kind of lube are you using and how much? I try and make sure all cases have the same amount of lube on them.

 

[Florida Wally]

There is a possibility that the shorter brass went in the die shorter. You could try measuring your brass before sizing. The shorter brass may be from the different lot you mention and after one firing haven’t fully formed to your chamber.

I had a similar issue a few years back. While practicing, after firing a live round, I would leave the fired brass in the chamber and dry fire the rifle several times. Later, when I sized the brass, I would get a few that seemed to have excessive shoulder bump, just like what you are experiencing. The dry firing was bumping the shoulders, about 0.001” each time I dry fired, so I had some brass that was 0.005 “or 0.006” shorter than the others. I was also perplexed and it wasn’t until recently that I realized what was happening.

 

[SD guy]

Possibility of inaccurate & or inconsistent measurement of shoulder should be considered as well. Some are more adapt at using our measuring equipment more than others. For me, when working up a load for one rifle only, the go gauge is rather irrelevant.

Again if I am loading ammo for a particular chamber my no go measurement would be a fired case from that chamber. I would recommend de-priming the fired case & insuring the shoulder is not bumped. Now you may measure your de-primed case 10 times. why 10 times? you are developing a feel for taking this measurement. With light pressure your goal would be to achieve the same measurement every time. If you like you may wish to compare this measurement to a factory go gauge. Yet for this chamber the fired brass is most relivent.

Some one, new to this process may see .005 difference in measuring the same brass 5 times. You are measuring the same sample each time so we should expect the same measurement 5 times in a row. If not your technique needs refining.

Ok so we have our sample no go reference brass. Goal going forward for me is to achieve .002" shorter head space than the sample no go brass. This is with reliability & brass life in mind.

 

[Jeff Porter]

Ok more data. Measure FP protrusion using depth plunge end of my caliper and it is .053” range so in spec for a R700 action.

Got home from church and put a new chrome silicon FP spring in the bolt. Pulled the bullets and dumped the powder on the loads that did not fire yesterday. Some of these went bang ( primer only). Others still did not ignite. Popped the bad primer( CCI 450) and put a 400 in. Bang , put a federal 205MAR in … bang , put a different 450 in. Bang….

While I have an issue with sizing repeatability, this may be primer lot related too.

I’ll do some more research later with different bumps to see where too short for a CCI 400 is.

 

[Jeff Porter]

What kind of lube are you using and how much? I try and make sure all cases have the same amount of lube on them.

Creedmoor version of Imperial wax. Dab my finger about every third case.

 

[G Liles]

Creedmoor version of Imperial wax. Dab my finger about every third case.

I guess I’m old school, I use RCBS lube on the RCBS pad.

 

[Webster]

Some cases may be softer than the others, making measuring inconsistent. And short cases can and will give you FTF or soft strikes.
And IMHO, a .020 bump is too much. When fired, case stretch can start to be a problem, leading to case head separation.
Make sure you have/keep a broken case extractor tool on hand.
A .002 bump would be nice. And I see no need for a spring change, That's NOT an issue. You brass is the issue and it's not really that bad.
Just another bump in the learning curve.

I cannot image 0.010" spring back.

 

[M-61]

I’ll do some more research later with different bumps to see where too short for a CCI 400 is

I’m totally lost on how you’re going to do this, and the purpose.

 

[M-61]

There’s three kinds of bumps~
not enough
too much
looks about right

How about none at all? (unless needed)

 

[Bob3700]

Jeff,
Two possible causes with ur issues.
FTF or delay in the bang.
We have seen new brass have some contamination from the factory. Teammate Had a bunch of Peterson 223 brass that, when loaded. New out of the box, had FTF and hang fires. Disassembled rounds n found powder clumped together. Took the rest of the new brass n washed it in soap n water before loading. Issues disappeared.

Shoulder bumps: I measure my fired brass from datum to base. Usually about 4 random fired pieces from the box. That will give me a good idea what my fired case measures to the datum. I set the dial caliper/ bump gauge to “0”. Using Imperial die wax, I hit the neck n base of the case.. My bushing/ sizing die is set for .0015” to .002” shoulder setback.
When sizing, I do let the case “dwell” for a count of five before bringing the case down.

Like u, I anneal the cases after each firing, making sure to set my flame on one torch to hit the shoulder of the case. For the most part, I am using Lapua, Win. , and Remington brass.

Sometimes, a case won’t bump to my target dimensions ( usually not enough bump) and it takes a second pass thru the die to get there.

,010” variation in bump is a lot. Only other issue maybe over or under annealing. Causing variations in brass hardness.

Just for grins, try cleaning ur die out every ten sizings n see if that makes a difference.

Bob