What is the consequence of uneven bumping on precision/accuracy?

[jlow]

OK, before we start, I am not talking about significant headspace variability which can cause significant case stretch and case head separation.

The question here is effects on precision/accuracy. So, say for example we are getting a 3 thousands spread on bumping, I would think that this means even if the bullet was seated absolutely perfectly in terms of depth, the 3 thousands spread in headspace would allow the bullet to seat in the chamber with a 3 thousands spread and thus consequently a 3 thousands spread in terms of distance from the lands and the crown. To keep things simple, let’s assuming that we are seating to mag length and not slightly off or jamming into the lands, so avoiding that complication.

So what is the consequence of this 3 thousands spread? I assume that it would possibly mean a 3 thousands spread in when the bullet exits the crown (assuming absolute consistency in MV), does this have any effect on precision/accuracy and why?

 

[jlow]

Dave,

Sorry, this is not a question about seating distances.

As mentioned in my OP, I am assuming bullet seating depth is perfect, the question is if you have rounds that differ in headspace bump, does that affect accuracy/precision. For example, if the headspace is 3 thousands shorter, the whole round and of course the bullet can (but does not have to) move 3 thousands closer to the lands but that is not the same as seating the bullet 3 thousands longer as that not only affects the position of the bullet relative to the lands but also case volume.

 

[DennisH]

I am going to leave the bottom part alone,

If you have .003 headspace, your stating the brass will strech .003 before the bullet moves, therefore putting the bullet .003 further in the lands before moving, correct?

I have read everyone's responses and reviewed mine.

Everyone I shoot with try's to get their headspace within .002, not any more. They would rather have it tight vs loose. I believe factory ammo is punched at .005. It makes sense the more headspace you have, the more problems you will have and could very well affect accuracy as the brass will seat before the bullet moves.

If brass weight, bullet weight, and case length makes a difference, I would have to assume headspace will also make a difference

Good question, I am just giving an opinion, but I will definetly be interested listening to an experts opinion.


If I understand correctly: Here is my .02 cents,

I measure the bullet from the ojive when seating. Headspace should not matter, the bullet is going to be exactly where I want it.

I understand the issue as I do check my headspace with every FL sizing. I usually have a difference of 1/2 thousandths to .001, but most are dead on, .002 off. When I seat the bullet, if I want it .010 in the lands and that measurement is 2.287 at the ojive, then my bullet will be .010 in, every time, regardless of headspace.

Now will difference in headspace make a difference on accuracy, which I believe this is your question, I personally cant say, nor have I experience any problems, but after the below experience, I always check all my brass with every loading.

I started with 60 new pieces of 308 Lapua brass, my previous batch the headspace was always .002 off. I didn't check the new brass and left my sizing die set were it was always set, until on about the 6th loading, I had case head seperation. After checking, when I was FL sizing the new brass the headspace was off .012. All brass showed signs of failure and I threw these away. My RCBS Gold Medal die and press is pretty consistant setting the headspace at .002. I also have a set of die shims which comes in handy as setting the locknut gets tedious sometimes.

I started over with a new batch of 308 Lapua brass and had to readjust my die as the new brass was different than my old brass. So now I always check headspace everytime I load. The first one I run throught my FL die, I measure it. Sometimes I check the first four. If there ok, I proceed.

Shooting 308 Lapua brass that had .012 headspace didn't seem to affect the accuracy, it might have, I just didn't realize it. Nothing alerted me there was a problem. I did win overall one match and was always in the top three with this excessive headspace.

All brass is not created equal. All bullets are not created equal, if the lot number changes, measure it. I have had as much as .005 difference with Berger Bullets, different lots. I guess brass is the same way.

Even taking a new box of Lapua brass and just loading without checking, the bullet is where I want it, but sometimes the bolt is hard to close which is telling me the headspace is not consistant from the factory. I usually take the first round and use them in practice. After the first firing, I trim and check all measurements including concentricity of the neck and bullets-2nd loading. I usually have a few that fail. I mark them and set them aside for play.

 

[rjnallie]

I think you are questioning the accuracy if you had some loads at .003 and some at say .001 headspace. I have played with that some and have not had a noticeable difference with a 1 or 2 thousanths difference. I would think that if all you loads were the same, say .003, it would be better than having some at .001 and some at .003. I have noticed pressure changes when I get over .005 headspace.

 

[DennisH]

I have difficulty visualizing a chambered round with the shoulder firmly against the front of the chamber and a space between the case head and bolt face.

If you kiss the lands or jam the bullet, I would have to assume the gap will be in the front of the chamber. Even if it's jumped, I believe the gap will be in front, or it may be even on both the bolt and chamber. The ejector pin is putting a little pressure on the case. Could this make a .001 difference at the bolt head when the bullet is set off the lands? Probably so.

???

 

[bigedp51]

The ejector button and the firing pin push and drive the case forward and you end up with head clearance at the rear of the case. The reloading manuals tell you to never use cases that were used for reduced loads with full power loads because of the danger of case head separations. Its possible to make the case .001 shorter every time you pull the trigger. Try it with a fired case and watch it get shorter each time the firing pin hits the primer.

 

[edlongrange]

Posted for Lynn:

"Jlow

If your cases have 0.003 variation in the shoulder bump length your necks will no longer be trimmed to the same length resulting in larger extreme spreads.

Likewise if your cases length is not consistent your cases volume is no longer consistent again resulting in uneven velocity shot to shot.

Next thing that happens is your primers ignition varies because some of the brass moves forward in the chamber while the rest holds snug against the bolts face. This initial delay in setting off the primer again affects velocity variations.

To sum it all up your neck lengths will be off, your case volumes will differ and your ignition will not be as good as it could be.

Since this sport is all about consistency we try and get all our loaded rounds as alike as is humanly possible. Would 0.003 be big enough of a variation to worry about is an individual choice but as shoulder bumping is a relatively easy step in the whole reloading equation why risk a flier.

Lynn"

 

[BoydAllen]

I would think that this is one question that would best be answered by using a very accurate rifle to do a test. Experience has taught me that it is often the case that what we think will happen, and what actually happens are not the same thing. My experience also tells me that some things only make a difference if everything else is perfect, otherwise they get lost in the noise. If someone does a good test, I will be interested in the results.

 

[jlow]

Thanks guys, appreciate your feedback. Sorry for the slow response but the UPS truck dropped off the reverse osmosis unit we order today and so you can imagine where I was most of the day…

DennisH - The difference between what I am doing and your setups is that you are jamming and that of course can overcome a difference in headspace as the two contact point for the round would be the bolt face and the lands regardless of headspace. Your case can stretch and cause case head separation, but that is a different can of worms which is important but not for this discussion..

However, I am loading to mag length which means my ogive is nowhere close to the lands and so there could potentially be only one contact point i.e. the shoulder (unless the headspace is exactly chamber length – something to avoid). Yes my fired brass headspace being around 1.623” and my targeted bump to 1.620” (i.e. 0.003 bump since this is a gas gun).

To clarify what am talking about here is a 0.003 variance off that last number i.e. it is possible that the headspace varies between 1.619 to 1.622. It is unfortunately that there are two 0.003” number here but they are for two different things i.e. one is a targeted bump distance, the other is a variance.

In this case the only potential factor I think is the ogive of the bullet can be plus or minus 3 thousands from the lands (still far away) or the crown. I personally don’t think distance to the lands is significant but not sure about distance to the crown. I guess 3 thousands is probably too little to get you in and out of a node? I guess this is what I was driving at.

Rjnallie – yes, I you are correct in your intrepretation of my question and thanks for your answer.

Dave Berg – Please see above.

Bigedp51 – not sure what you say is relavent to my question – not trying to be rude, just saying.

Edlongrange – I use a Wilson trimmer which index to the head of the case and so the bump variance does not affect trim length.

BoydAllen – I completely agree with the comment. “My experience also tells me that some things only make a difference if everything else is perfect, otherwise they get lost in the noise” I think this is the nemesis of many reloaders trying to assess variables.

 

[edlongrange]

A follow up from Lynn:

"Jlow

If you're using a Wilson neck trimmer and all your over-all lengths are exactly the same your necks won't be the same length because the shoulder is not the same.

The reason for the neck variation is the 0.003 variation you have in your shoulder location. If you draw it out on paper leaving the over-all length the same but varying the location of the shoulder you will see it more clearly.

Boyd

Jack Neary has a video on You Tube you can watch.

Lynn"

 

[DennisH]

BigEd, I like your post.

The ejector button and the firing pin push and drive the case forward and you end up with head clearance at the rear of the case. The reloading manuals tell you to never use cases that were used for reduced loads with full power loads because of the danger of case head separations. Its possible to make the case .001 shorter every time you pull the trigger. Try it with a fired case and watch it get shorter each time the firing pin hits the primer.

I understand about the ejector and firing pin, but wouldn't the extractor limit this, considering your chamer only gives about .001 clearence to the body of the brass?

??? ??? ??? ???
Another question, I just FL sized my 6.0x47L rounds and my headspaced was set at .002. Most of the brass was there, and I didn't have more than .001 off on the rest.

I put these rounds in my vibrator cleaner, and just by choice, I rechecked the headspace. Almost all of them were more than .002 off. Could cleaning them in a vibrator cleaner cause this? When first checked they were .002. Now after cleaning, all are around .005?

PS: Definetly going to watch the YouTube video!

 

[jlow]

A follow up from Lynn:

"Jlow

If you're using a Wilson neck trimmer and all your over-all lengths are exactly the same your necks won't be the same length because the shoulder is not the same.

The reason for the neck variation is the 0.003 variation you have in your shoulder location. If you draw it out on paper leaving the over-all length the same but varying the location of the shoulder you will see it more clearly.

Boyd

Jack Neary has a video on You Tube you can watch.

Lynn"

Edlongrange – Thanks for the further posting, you are of course absolutely right, I was thinking of something else and missed your point. Regardless, this is a very good and important point. So shorter headspace means longer necks with the same case length and this means longer contact point for the bullet bearing surface and so MV variation.

The problem I have is I am not sure why I am getting this variation. I am using a good quality Redding Type S full length resizing due without the bushing, the case has been SS media cleaned and lubed with Imperial only at the body. I am also using their competition shell holder set. The only thing I can think of is I hand anneal the brass and this could reflect different degrees of annealing?

DennishH and BigEd – I thank you again for your input and understand your common interest but it appears to me to off topic. I would really like to keep on topic in this thread and not have it migrate to something else. Any chance you guys can discuss this in a new thread?

 

[BoydAllen]

I managed to figure out how to download Jack's presentation (that had to be split into six parts because of Youtube's maximum video length rule) right after it first became available on line. It is difficult to tell what will be significant for a particular shooter's program from the sort of abbreviated information that is typical of most questions asked on shooting forums. That is why I suggested a shooting test.

 

[DennisH]

jlow, I thing were all learning on this thread. I understand your position, but there is good information on this thread. I did mis-understand your question. I find all the replies interesting and informative. I will keep an eye on the replies only. Take care.

 

[jlow]

DennishH – not a problem with misunderstanding, it’s hard to get things across perfectly via posts and we all skim post – I’ve done it and so would not judge others but I do appreciate it when others makes an effort to lend a hand.

As for my concern, I have had threads hijacked (unintentionally) so that its direction is far off from what I was looking for when I started it. I understand that this is the nature of the beast in a board where the postings are serial and you cannot branch off but a little extra effort to move question to a new thread is always appreciated.

 

[jlow]

To continue, do you guys think my problem with the variable bump has to do with annealing or something elese?

 

[BoydAllen]

A couple of friends have been able to fix variable bump by "annealing" (not dead soft, but with good, and more uniform neck tension remaining) , so I would have to say that it is related to variability in the hardness of the metal, which probably goes back to the factory annealing.

 

[holstil]

I believe your issue will get better with annealing.

I can anneal some of your brass to see if it helps.

Jim

 

[jlow]

Thanks BoydAllen! It's always good to get feedback on an idea.

holstil - Thanks for your kind offer. I actually currently anneal but only manually and so will likely get something more consistent like a BenchSource. In the end, I need to test it side-by-side with my method to see if it makes a difference and so I appreciate your offer but I will end up doing it.

 

[BoydAllen]

Although I am sure that a machine is probably the best, one of my friends, that did not want to make the investment was able to achiever very uniform results by using Templaq to determine his time in flame, and an inexpensive electronic metronome to make keeping track of the seconds easier. After a couple of hundred cases, he was able to do as well by watching the color line come down the body of the case. He uses a vibratory case cleaner to clean and polish his cases some, and this makes seeing the color progression easier. Watching two things at once (the case and a timer) is not possible, so being able to hear the seconds tick off is a useful workaround. My friend used the Hornady annealing kit, and bought the metronome, for less than $30, at a local music store. They are common on Ebay.