Case weight?

[the big greasy]

I have developed a pet load with lapua brass, I bought another batch of lapua brass that weights 1.5 grains lighter. Will this effect my groups? Should I just load the same weights of powder as I did with the heavier brass? I hope so!

 

[DUSTY NOGGIN]

.005 - .010 extra trim length could weigh 1.5 grains , did ya trim em

youll have to shoot em all to match your chamber anyways , have fun shooting to finding out , while youre there put the speedometer on a few to see how they compare to your original fps

 

[Tripodmvr7]

The real criteria will be the case volume. After shooting some of the new cases you can then weigh 5 of each of the two batches and find their water volume in grains. Now you can get a better idea of how they match up.

 

[jdyoung]

The real criteria will be the case volume. After shooting some of the new cases you can then weigh 5 of each of the two batches and find their water volume in grains. Now you can get a better idea of how they match up.

Yup, I would determine case capacity by weighing them filled with water. Then do the same with some of your previous cartridges for comparison.
Here's a useful plug to help:
Primer Pocket Plugs (xxicsi.com)

 

[Tripodmvr7]

A primer plug points to the case having been resized. Fired cases without any resizing should be used as this gives true volume. That figure is also handy when using Quick Load to develop accurate ammo.

 

[Tiny Tim]

Dusty Noggin and Tripodmvr7 make excellent points. Case volume will be the true indicator, and both will have to be fire formed to your chamber to be accurate. I have 2 lots of Nosler 243 brass that weigh considerably different, but one lot is about .010 shorter than the other.

 

[the big greasy]

Ok I will do what you guys say, I'm hoping I don't have to load develope again

 

[Mikecr]

I hope you bought enough in lot so that this doesn't keep challenging you.
And I hope you chose a cartridge & load that will allow many reloads of the cases you have.
Constant case changes are as big a pain as constant powder & primer changes..

 

[cape cove]

@Mikecr When using Quickload ( I assume you use this) what case volume do you use in the program, the fired non resized case or the fired resized case. TKS

 

[Tripodmvr7]

You want the capacity of a fire formed case as that is the volume that is the closest to your chamber size - the case will now have the largest volume. If you size it you reduce that volume and it will give false calculations.

 

[BoomFlop]

My suggestion would be and should be to back off the charge and work back up just to be safe.

This should be done when changing any of your loads components (brass, primers, powder and bullets). Doesn't need to be a full load work up, simply backing off and coming back up and to verify accuracy/load.

Be safe,
Steve

 

[cape cove]

@Tripodmvr7 Check the link and scroll down to " optimizing quickload for the 6mm br cartridge" and check out "case capacity Almost at the bottom of the page. I believe it suggests to measure a sized case.

Quickload Reloading Software within AccurateShooter.com

 

[Tripodmvr7]

I copied this from that link. PV=nRT is the formula for ideal gases, but it would apply here as well. You would need the full volume where the reaction takes place = fire formed case.

2. Always measure ACTUAL case capacity. If your cartridge capacity is less than QuickLOAD assumes, you can get pressure problems with loads identified as safe. Remember different brands of brass may vary in case capacity by up to three grains (with the larger cartridges). Don't even think of applying QuickLOAD-generated recipes until you've measured the ACTUAL case capacity of your brass. Montana Marine concurs: "The biggest tip I would give is to measure the water capacity of your fire-formed cases, and enter that data into the equation. Before doing that, my chrono'd velocities were typically 20-40 fps slower than calculated. After entering in actual water capacity, results are generally within 10 fps of calculations. For example, the default water capacity of the 30-06 is 68.2gr. My fire-formed Remington cases hold 70.5gr, fire-formed Norma cases hold 73.7gr. That is a significant difference."

 

[Mikecr]

@Mikecr When using Quickload ( I assume you use this) what case volume do you use in the program, the fired non resized case or the fired resized case.

I fire form cases 3 times without body sizing for measure of H2Ocapacity. This is also a point where I decide about case trimming/trim length.
Late stage case preps & sorting for me.
I do not go into powder development until past this stage.
BUT:
I don't FL size cases -ever.
So my capacities hold as my brass lasts a very long time, with no further trimming or loosening of pockets.
It doesn't just happen that way. There are a lot of choices leading to that.

Where you do heavy body sizing, which leads to continued trimming, and case hardening that changes it's character, then effective capacity changes.
You're led to that by cartridge design, but if your chamber is loose as well then it's amplified to primer pocket loosening and cycles of case replacement.
This common scenario makes capacity matching a futile endeavor. I wouldn't bother with it there.
Instead, I would seek a forgiving enough powder load.

For QuickLoad I'm initially forced to guess the formed outcome. That's all we can do.
But I'm pretty good at it, consistently predicting within a grain H2O capacity of fully formed.
I measure all dimensions, including neck thickness, for 10 new cases in lot. I pluck the mean of them and measure it's H2O capacity.
Then I build this case in 'Cartridge Design' (sub program inside RCBS.Load), adjusting web height and thickness taper to match capacity.
Then I consider the chamber clearances and re-scale the case to [-1/2thou] of chamber, everywhere.
With that I get a predicted formed capacity that I initially use in QuickLoad.