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300 RUM load development questions

 
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  #85  
Old 03-15-2013, 09:21 PM
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Re: 300 RUM load development questions

On the bushings and neck tension what I was getting at is that with bushing you can adjust the tension in .001 increments. With a std FL die, you get what you get. if you don't use an expander ball you will get probably 5 or thou of tension. With the expander ball, you get maybe 2 thou give or take. That's what I shoot for when bushing sizing. The theory is, less tension = more consistency. More consitency in tension itself would come from neck turning, producing consistent neck thickness.

It is definitely possible for necks to become thicker at the base if the brass grows as it's fired. How much of an issue it is, I don't know. If you have long necks and are not seating to the junction, probably no issue at all. That would be another good argument for using the expander ball. My 300 WSM brass did not grow at all, but my 25-06 brass does. The difference is probably the shoulder angle.

I was also just thinking about the .001 runout on my 22-250 and 25-06 brass. That is unturned brass so some of that "apparent" runout is probably due to thickness irregularity and not runout at all.
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  #86  
Old 03-15-2013, 09:33 PM
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Re: 300 RUM load development questions

Quote:
Originally Posted by cerwin View Post
So, are you suggesting that standard FL dies, rather than bushing dies may be the preffered method of correctly sizing consistently every time?

I'm pretty sure your saying that partially sizing necks is of no benefit. I realize that the shoulder is the contact point when fired and necks dont factor in to the contact point, so long as necks are centered to the case.
Some top level competitors lap the necks out of standard full length sizing dies. They believe that such modified dies have necks better aligned with the die shoulder and body chamber. Bushings have a slight clearance tolerance but that doesn't seem to matter much. I lapped out my die necks decades ago long before bushing dies were even a dream in their designers head. I've got five .308 Win. RCBS full length dies all with different neck diameters for different case neck wall thicknesses.

Partial neck sizing ends up makeing cases bind up the bolt when they're chambered. I think the reason partial neck sizing's so popular is it typically make better ammo than when case shoulders are set back way too far; something that happens so often when sizing dies are against the shell holder when the case is all the way into the die.

I believe that the best neck tension is what's the least that will keep bullets in place during normal handling and loading. If case necks' elasticity changes, use a different bushing diameter. A few folks buy two sets then lap one set out half a thoiusandth so they'll have more exact control over neck tension. But I think one thousandth steps is good enough.

I also believe than bullet runout under 1% of bullet diameter is good enough. Having shot enough 1/2 to 5/8 MOA test groups at long range with 30 caliber bullets having up to .003" runout, why beat your brains out trying to get it smaller? Seat the bullet to engage the rifling when chambered and it'll center well in the rifling then shoot the darned thing out the barrel; it'll fly just right.
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  #87  
Old 03-15-2013, 09:36 PM
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Re: 300 RUM load development questions

Some good info...
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  #88  
Old 03-15-2013, 11:29 PM
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Join Date: May 2012
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Re: 300 RUM load development questions

[QUOTE=Bart B;779362]

Therefore, I suggest you do something to ensure your fired case shoulders are set back at least .001" when sized. Here's a link to something very handy for making very accurate changes in sizing die height:

The Firing Line Forums

The above forum is some good stuff. A whole other level than I've seen here as of yet. I'll read the post's there over, and over, and then I'll read em again, and again.
Answer me on this. I dont know what my headspace actually is. All I know is what I measure after firing with a Sinclair bump gauge. I set my die to Redding directions to firm contact with .010 comp shell holder with ram in uppermost position. I size a stroke, if bump gauge measurement does not decrease, I go .080 shell holder, and repeat until I've bumped my measurement 1-2 thou. I'm using Nosler brass, which is too short and soft to start with. I have noticed that fired brass is longer after 2nd firing, than the first. Is this too much headspace or too short brass? Nosler brass new is measuring about 006 shorter than all other brass. Didn't want to use it because of that, but all I had. Right now I've gotten 2 firings out of this brass, and think I can get another. As mentioned on the above forum, I do happen to have a precision mic. Haven't used it, but maybe I need to. Maybe I need different brass.

Then again, why worry. I'm getting the best groups I've ever had at 200. Still need a bit warmer load though. I'll probably hit the range tomorrow, either that or do some shed hunting. The Elk are starting to drop.

Last edited by cerwin; 03-16-2013 at 12:29 AM. Reason: Deleted text
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  #89  
Old 03-16-2013, 07:07 AM
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Re: 300 RUM load development questions

Quote:
Originally Posted by cerwin View Post
I dont know what my headspace actually is. All I know is what I measure after firing with a Sinclair bump gauge. I set my die to Redding directions to firm contact with .010 comp shell holder with ram in uppermost position. I size a stroke, if bump gauge measurement does not decrease, I go .008 shell holder, and repeat until I've bumped my measurement 1-2 thou.
Your chamber headspace is probably somewhere between the 2.4742" and 2.4842" dimensions shown in the following:

http://www.saami.org/PubResources/CC...a%20Magnum.pdf

It doesn't matter what it is as all you need to do is set fired case shoulders back about .002". If you want to know what it actually is, you might need a chamber headspace GO gauge to calibrate your measureing tool, then compare fired case headspace to that. With max loads, the fired case headspace will typically be .001" to .002" shorter than actual chamber headspace. Without an adjustable chamber headspace gauge or set of gauges in .001" increments, it's gonna be a task to see exactly what it is.

Quote:
I'm using Nosler brass, which is too short and soft to start with. I have noticed that fired brass is longer after 2nd firing, than the first. Is this too much headspace or too short brass? Nosler brass new is measuring about 006 shorter than all other brass. Didn't want to use it because of that, but all I had. Right now I've gotten 2 firings out of this brass, and think I can get another. As mentioned on the above forum, I do happen to have a precision mic. Haven't used it, but maybe I need to. Maybe I need different brass.
What dimension is longer? Case length which is spec'd between 2.830" to 2.850" or case headspace which is spec'd between 2.4742" and 2.4842"?

Cases typically get longer (head to mouth) after each shoot and reesize cycle; it's the nature of the beast. How much depends on the diameter differences between case and chamber; the difference in your chamber and die diameters effect this; a greater difference means more case lengthening with each cycle. Some of the change is caused by how much the fired case shoulder's set back each time; minimal setback reduces the case growth when full length sized.

Quote:
I'm getting the best groups I've ever had at 200. Still need a bit warmer load though.
With the extra slower powders, oft times max loads don't shoot as accurate as those powders a bit faster. And rarely does the powder producing the most muzzle velocity at max pressure specs shoot bullets as accurate as a slightly faster powder shooting bullets out 50 to 75 fps slower. The velocity vs. accuracy compromise issue may have settled in your domain.
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  #90  
Old 03-16-2013, 09:08 AM
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Re: 300 RUM load development questions

[QUOTE=Bart B;779715]Your chamber headspace is probably somewhere between the 2.4742" and 2.4842" dimensions shown in the following:

http://www.saami.org/PubResources/CC...a%20Magnum.pdf


Cases typically get longer (head to mouth) after each shoot and reesize cycle; it's the nature of the beast. How much depends on the diameter differences between case and chamber; the difference in your chamber and die diameters effect this; a greater difference means more case lengthening with each cycle. Some of the change is caused by how much the fired case shoulder's set back each time; minimal setback reduces the case growth when full length sized.


The base to mouth lengthening is not my concern. I trim to same length after sizing. My measurements are base to whatever point on the shoulder my Sinclair tool indexes to. Dont know if its actually datum line or some other point on the shoulder. I wouldn't think it would matter as long as I use same tool in same way each time. It gives consistent measurements. I measure before firing, and after to see what is happening. I havent taken real good notes, so my #'s may not be exact, but this is whats happening. My cases grow about .005 after first firing. I bump back 1-2 thou. Another firing, measure and see that it is now longer base to datum line than it was after first firing. The base to datum line is where I see it growing. Is it just taking more than one firing to reach chamber headspace length, and no big deal?
Probably amounts to nothing more than shorter brass life?
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  #91  
Old 03-16-2013, 11:04 AM
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Re: 300 RUM load development questions

Your measurements are about right. Case headspace grows a bit after firing a new case, then it grows a tiny bit more with each firing if the shoulder's not set back far enough. And it varies with brass makup and hardness.

As long as the bolt closes easily on cases you full length size and you set the shoulder back about .002" each time, your are doing all the right stuff.
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