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Head Clearance vs Accuracy

vernsker298

Fairly new to reloading and I've run into a situation I can't seem to figure out with my 25-06.

I started with some twice fired brass measured with a Hornady headspace comparator. I bumped the shoulders back .002 and ended up with a head clearance of 2.0415.

neck tension: .0035"
GM210M primers
case trimmed to to 2.475'
charged w/ IMR4350
bullet seated .006" off lands

I ended up with a 1/2" group at 100 yards with 47.5 grains and my max charge at 48.5 grains.

So here is where the problems starts.... I measure my brass that shot the max charge and the head clearance was much larger (2.0455). So at this point I started over by bumping the shoulder to 2.0435 and left all the other variable the same. This time at the range all my groups opened up ALOT but again the best set was at 47.5grains.

So then I stick with the larger head clearance and 47.5grains of powder but I played with bullet seat depth by +/- .006".
To my surprise I didn't find any improvement in accuracy and I'm stumped as to where to go from here.

I'm reluctant to go back to the smaller head clearance since I'm concerned about case life and having too much play there.
 
I do shoulders 0.003" back from their twice fired headspace. I end up with about 0.001" of spring-back.

If you're moving shoulders back 0.006", that is far too much. 0.010" ruins cases.
 
It is difficult to say if we can blame the second performance on the case prep changes between the two sessions, or if in fact the results from the two sessions are actually the same and you got fooled by statistics.

To stay organized, if we focus on getting the shoulder datum length setting dialed in first, and separately discuss reasons for group size changes, we may keep the thread from boiling the ocean....

Shoulder Datum Length first...
It often takes more than a few cycles to fully expand the brass to a chamber. There are examples of this happening in just one or two cycles, but more often than not it takes more than three to be certain. You don't use any rules of thumb for how many, you instead look for the numbers to trend up and then stabilize. You should start to feel difficulty in bolt closure about the same time.

Take your time with just a few cases and focus on getting those up to several cycles. Play it safe with the charge, and use your rifle chamber to tell you when you are in need of some bump. Keep good notes on the shoulder datum length on each piece for each cycle. This gets tedious, so guess a sample of three to five cases and at least three to four cycles on them before you draw conclusions.

BTW, I have been known to say this a lot to folks on the forum, but here is a reason why you would like to be portable and do all of this at the range without rounds trips home. YMMV

Good Luck.
 
Another thing to think about. Change only 1 thing at a time and retest in order to narrow down the issue. I bump my shoulder .0015 and that gives me a firm bolt closure every time. YMMV
 
In my very humble opinion, this is the problem today with all the focus of gadgets and theoretical procedures / numbers.

Yes, a shoulder bump of .001 to .002 is a good rule of thumb for bolt rifles and can be used once the cases have fully fired formed to the chamber of your rifle. I myself use this approach and it works quite well. But let's not forget why we size cases in the first place.

First, we size so the case will hold the bullet. I won't go into that any further since that's not germane to your post.

Second, we size so the case will chamber without excessive resistance and at the same time not over sizing thus overworking the case or worse, creating excessive case head space. So, you only need to size the case enough so it will chamber without having to force the bolt close. Removing the firing pin assembly is the best way to access the amount of resistance attributable to the case. It should close with only very slight resistance.

Often, as others have said, it takes several firings on new cases before you have to bump the shoulder. When you full size you are also sizing the radial dimension which greatly aids in chambering. So, what does this all mean?

Use your rifle chamber as the gauge to determine the amount of sizing you need. This is the way experienced reloaders did it to achieve optimum sizing before there were gauges. Therefore, try setting your die so you resize to the fired case headspace measurement, i.e., zero set back then check the case in the chamber of your rifle. If it chambers without resistance, that's all you need at this point. After a few firings you then can start bumping the shoulder using the aforementioned rule of thumb.

In order to obtain zero set back without extruding (lengthening) the case, the die should touch the shoulder of the case.

While I'm not a proponent of neck sizing because it doesn't size the radial dimension of the case, often cases can be neck sized after many firings before difficult chambering is encountered. This demonstrates the aforementioned point I was attempting to make; that the fired case head space is fine until it lengthens to the point that difficult chambering is encountered requiring full sizing, or the radial dimension of the case has expanded to the point of creating a problem to chamber the case.

The other problem with neck sizing is trying to predict when you'll need to full size. Of course, your rifle will tell you when, but you don't want to be in the field or in a match and find out your cases won't chamber. Thus, the functionality advantage of optimum full sizing.

As others have said, first focus on obtaining an optimum sizing procedure for your equipment. In this way, you will have cartridges that are functionally adequate. This now becomes your baseline to begin you load development.

In my experience, I have not noticed that sizing makes any significant difference, if any, in group size. In my experience, other factors such as bullet selection, powder selection / powder charge, bullet seating depth, and sometime primer selection produce the most pronounce effect on group size. Then of course there is the biggest variable, the shooter himself. So, I would be surprised if sizing affected your groups.
 
Just out of curiosity have you gone back to your original set back numbers and tested for accuracy? You're only talking about a .002" difference. Half the thickness of a sheet of paper. In 40+ years of shooting I have never used a gage and caliper to set the FL die. Too many mismatched dimensions/surfaces to get an accurate read. A stripped bolt and a sized case tells me all I need to know. Use one case. Creep down with the die until you get the feel you want. Try a second case as it will represent your normal loading process. You'll probably have to tweak the die again on the second case.
 
Your load development should result in repeatable round groups, the size will be dependent on barrel, bullet, distance,environmental and shooter ability(and other incremental factors). If you are seeing vertical or horizontal strings, bullets impacting in 2 separate locations you are still in testing/development stage. Repeatable round groups allow for true zeroing of the sights and the use of ballistic data. Grouping tendencies can often show you the direction your loading needs to go(increase/decrease charge,powder/bullet change,seating depth adjustments).
 
jlorenzo said:
Isn't that what your supposed to do? I may be a rookie myself apparently.
Click to expand...
It solely depends on how the brass behaves in your chamber. If the cartridge base-to-shoulder (CBTS) dimension continues to increase with the 2nd and/or 3rd firing, by setting the shoulder back approximately .002" after only a single firing you will be oversizing the brass by however much the CBTS increases with subsequent firings. If you're certain with the cartridges and brand(s) of brass you use that the cases are fully expanded after a single firing (I am, so it's not unheard of for this to be the case), setting the re-sizing die after a single firing isn't an issue. You just have to know what the brass does in your rifle, which means measuring CBTS for several rounds for at least 3-4 firings.
 
DaveTooley said:
Just out of curiosity have you gone back to your original set back numbers and tested for accuracy? You're only talking about a .002" difference. Half the thickness of a sheet of paper. In 40+ years of shooting I have never used a gage and caliper to set the FL die. Too many mismatched dimensions/surfaces to get an accurate read. A stripped bolt and a sized case tells me all I need to know. Use one case. Creep down with the die until you get the feel you want. Try a second case as it will represent your normal loading process. You'll probably have to tweak the die again on the second case.
Click to expand...
Thanks, I need to take time to do this!
 
My opinion and experience has been the area near the lands. 0.001 inch to 0.015 off the lands are really touchy and challenging to stay in the sweet spot. Jam or back it off. There are more tolerant jump distances. Just a thought.
 
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