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Hello Guys. I recently acquired a second 6.8, this time a bolt gun, its a rebarrelled CZ527. Ive been working up loads using 120 SSTs, and shooting some factory ammo. Factory SSTs gave 2550 fps with groups averaging right on 1 inch.

Down here in New Zealand we cant get AA powders, and choice in other brands can be limited so I have chosen imr 8208 - its what I could get!

Brass is Hornady, I have weight sorted it and identified two distinctly different "lots" one at 129gn/35.6 h2o and a newer lot, 110gn/36.4 h2o. I have chosen to do my load development in the 129gn brass. Col for my magazine is 2.3", that gives me some jump.


I worked up, 27.2 8208 gave 2385 fps, went to 28.3 and got 2476 - 20" barrel.

Made up some test loads at 27.8 and 28.3. Both shot very nicely in the 0.4" to 0.7" range at 109 yards - I LIKE this CZ!

But the ES was a bit higher than I was expecting, 53fps for the 27.3 and 37 fps for the 28.3

My question - using this combination of components, how can I reduce my ES whilst maintaining accuracy? And is it realistic to expect very low ES with this cartidge/combo.

Thanks
 

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I'm not an expert here, but you might try a few experiments with different primers to include some match grade primers. I'd start there then if still "looking" I'd consider playing with seating depth and neck tension.
 

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What type of primers and what is the temperature? Next is the velocity climbing with each shot?

It could be several things
anneal the necks
I get more consistent velocity using non-magnum primers
crimp but the cases MUST be trimmed to the same length
If velocity increases with each shot it is the barrel heating up the powder.

On the crazier side when using compressed loads I have had the powder push the bullets out (not crimped)over a 24hr period making different OALs when fired
 

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Interesting regarding the Hornady cases. Do the different weights have different head stamps?
Yes , the ones with 6.8 are Hornady made the ones with 6,8 are S&B . That is the easiest way to tell without weighing them . My Hornady made by S&B weigh the same as S&B brass . Naturally I use the Hornady made for play and S&B made for hunting .

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If velocity increases with each shot it is the barrel heating up the powder.
Ah, yeah, that's a really obvious one that I didn't even think of. I usually note my "cold bore" shots and throw out that data point. Of course, if you are a hunter, that's all you care about so you'd want to do the opposite and ensure your barrel is cooling back to ambient temperature between shots.
 

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With 8208 XBR you can use more powder and still be below max chamber pressure. You might find reduced spread that way with a more efficient load (filled case volume). You would also benefit from a drop tube. I use a 16" drop tube made from a 2117 Alu arrow shaft and RCBS funnel.
 

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Ah, yeah, that's a really obvious one that I didn't even think of. I usually note my "cold bore" shots and throw out that data point. Of course, if you are a hunter, that's all you care about so you'd want to do the opposite and ensure your barrel is cooling back to ambient temperature between shots.
For deer hunting Iwould rather have a barrel that shoots 10- 1/2" first shot cold barrel groups and then 3" for the rest than one that shoots 2" groups with the first shot just somewhere in the group .

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Chasing velocity and developing a load with low es/sd is extremely important when your intended use is long range engagement, let’s say beyond 600 yards.
Developing a load for hunting with a cartridge that’s not intended for long range use (based on extreme spread) can be an exercise in futility. The smallest groups at 100 yards don’t always have the best (lowest) extreme spread. A load that makes tiny groups out to 300 yards with an extreme spread of 50 fps is more than adequate, when the intended use is hunting inside of 300 yards.
There’s plenty of data out there to support this theory. That being said, component selection, preparation, your reloading practices and ability to consistently repeat the process can reduce extreme spread.
 
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