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COL, and brass capacity. Important Considerations.

 
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  #1  
Old 04-27-2009, 08:04 PM
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COL, and brass capacity. Important Considerations.

I thought I'd make a post regarding the differences that Cartridge Overall Length and Brass Capacity can make in reloading.

These variables aren't discussed very much (not as much as I believe they deserve) and I'm afraid that too few folks out here in 'etherland' understand the impact they can have on load performance. Other load/component differences like primer brand, powder lot etc.. are discussed all the time.

I'll use a specific example as a discussion point to highlight the impact that changes in the overall length and brass capacity can make. I will list the Quickload estimates as well as my actual chronograph values where available.

Disclaimer: Please do not accept any of the load data in this posting as being safe or recommended. Please educate yourself and adhere to all safe reloading principles and also work up to ANY load when you change ANY component.

The rifle and load that I will be using as an example is a 7mm Rem Magnum. The rifle is a Rem 700 with a whippy 24" barrel.

Brass: Norma
Primer: Rem 9 1/2
Powder: RL-22
Bullet: 160gr AccuBond
Average Velocity: 3015fps

Quickload lists the standard cartridge length for the 7mm Rem mag as 3.290", It also lists the Maximum water capacity for the 7mm Rem Mag case as 82gr of water.

Using the above information, Quickload says that it would require 66.3gr of RL-22 to achieve 3015 fps in the 24" barrel. (60007psi)

The load that I use in this rifle averages 3015 fps, but requires 70.2gr of RL-22 (nearly 4gr more powder!).

What is the big difference?

My Cartridge Overall Length is 3.421". And my brass holds 86.3gr of water (as opposed to the Quickload estimate of 82gr).

When I plug my load dimensions into Quickload, it estimates 3051 fps, only 36 fps difference than my actual average! (60650psi)

The big problem with sharing loads and not providing all the info (including case capacity and overall length) is that those changes can turn a safe load into a dangerous load.

For example, if I shared my load of 70.2gr and someone plugged it into their normal brass with standard overall length, what pressure do you guess they would see?

Quickload estimates that they would experience a pressure of 72727psi, which is more than 10,000psi higher than the industry limit for the 7mm Rem magnum! BTW, Quickload estimates the speed of that load from a 24" barrel would be a blistering 3198fps. Nearly 200fps faster than the safe load!

This is why all the variables should be taken into account when reloading, loads should be worked up from a known safe load AND they should be worked up over a known accurate chronograph.

I'm sure many of you are aware of this info, but I hadn't seen it posted and thought I'd put it up here for anyone that didn't realize the enormous changes that can occur with different brass and different seating depths.

Hope it helps someone,
AJ
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Old 04-27-2009, 09:00 PM
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Re: COL, and brass capacity. Important Considerations.

AJ,
Excellent info. Thank you!
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  #3  
Old 04-28-2009, 10:12 PM
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Quickload calibration to your rifle, your load

AJ

What do you find to be reliable and accurate correction factors to apply in Quickload in order to match actual performance in a given rifle?

I've been toying with start pressure, case weighing factor and the cross sectional bore area dimension.

I'd be interested in your thoughts/results.

Let me throw an example out for discussions sake, as thoughts on how to "calibrate" QL for a given rifle might be enlightening for other users of QL.

300 Remington Ultra Mag
Chamber is a Improved chamber, these are fireforming loads with out of the box factory brass.
The fireformed brass holds 118.8 to 119.2 of water to water overflow.
Remington SAAMI out of the box brass
113 grains of water to overflow (approx avg)
210 VLD
Retumbo
29.5" barrel from bolt face to crown
3.165 Ogive length
3.868 COAL
94.0 grains Retumbo
3175 fps shot string avg velocity


With the calculated case weight factor of .49 and a shot start pressure of 7200 psi....QL says I should see 3121 fps.

How do you get to 3175 fps?
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Old 04-28-2009, 11:01 PM
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Re: COL, and brass capacity. Important Considerations.

Those are the variables I've adjusted in the past. I think you have a good grasp of how to 'adjust' the QL simulation for your particular cartridge/load/rifle combo.

I've always felt that the weighting factor in QL for the 300RUM was incorrect.

The 338 Edge is based on the 300RUM and to make the 338 Edge numbers line up with QL, I've used .4 as a weighting factor, which would imply an even lower number should be used with the 300RUM, maybe .33 or so.

By using .33 and a 118gr water cap, QL gives a velocity of 3026fps which is 150 fps off your actual measurements. If you are really close to the lands, the pressure will go up and can be adjusted as you've discovered by adjusting the shot start pressure. QL says to add 7200 if against the lands. If you are real close to the lands (adding the 7200psi to the default for the VLD), you could be at 10,826 psi easily which would give you an estimated velocity of 3151fps, only 20fps from you actual measurements.


I have a few questions.

1) Can you trust your Chronograph? Has QL and your Chrono matched with other calibers?

2) Are you against the lands (or VERY close). ? If you are, you'll drive the pressure up and get a MUCH higher velocity that you need to adjust with the shot start pressure.

3) The temperature of the powder can also be adjusted by clicking on the box to the left of the powder name. Higher temps will drive the velocity up and cooler temps will lower the velocity. What was the temperature of the cartridges when you chrono'ed them?

I've never messed with the cross sectional bore area. Never had a problem getting really close with the other variables. I suppose if you had an odd groove configuration it would make sense.

Hope this helps, but I think you have a pretty good handle on the variables involved.

AJ
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  #5  
Old 04-29-2009, 07:38 AM
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Re: COL, and brass capacity. Important Considerations.

1) The chrono is a CED M2 with the IR Skyscreens. Due to intense sunlight, somedays even with the IR skyscreens, I'll fashion up 24"x36" piece of cardboard and make an impromptu hood to shade the sensors.

With the IR screens it's never "missed" a shot without the hood...it just makes me feel better I guess.

I've never compared it to another one though.

2) Best I can measure, I'm .015 from the lands with those velocities.

3) I forgot about this I did correct for this below.


After correcting for temperature (83 F), adjusting start pressure to 8500, and moving case weighing factor to .39, QL now predicted the velocites from my four different powder charges within 10 fps.

It also did well on some loads with H1000 that were previously fired....I'm thinking that might be the true "test" of the specific calibration technique....does it translate into accurate results when changes are made to the powder charge, or powder type itself.

You did say something interesting though....the 118 grains on the case capacity.

Wouldn't you use the SAAMI unfired case capacity for the unfired brass, despite it being fired in a Improved chamber?

After applying the above correction factors, and having QL "calibrated", now if place the case capacity up to 118 to 119 grains, it loses about 100 fps.

Thank you for the informative reply.
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  #6  
Old 04-29-2009, 08:20 AM
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Re: COL, and brass capacity. Important Considerations.

Looks like you have it figured out. I'd still use a lower weighting factor. .33 represents an Overbore Bottleneck cartridge and .5 represents a typical bottleneck cartridge. 0.4 works great for the Edge and the 300rum is a lot more Overbore than the Edge. I'd use at maximum a .35 for that factor. This gets you to your velocity with the 118gr capacity by using a shot initiation pressure of only 7000psi.

Good luck
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  #7  
Old 04-29-2009, 03:33 PM
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Posts: 279
Re: COL, and brass capacity. Important Considerations.

I have always had questions regarding seating depth and the max pressure. I seat .01 from the lands typically. How close do you have to be before you start to see pressures rise typically? I like to seat bullets out as far as possible to maximize the usable case capacity but dont want to increase pressures due to seating depth.
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