What's the effect of using heavier brass?

[giannid]

So I've got a new 6 mm creedmoor gasser that I developed a load for during the brass shortage. I ended up resizing some old Hornady 6.5 creedmoor brass down to 6mm. Using 100 grain Hornady interlock, the hornady manual gives me a starting 35.6 and a max charge of 43.9 with H4350. I usually test in half grain increments so I shot 17 different loads, which had 3 different nodes. I ended up settling on 41.5 grains as it seemed to give me the best sd numbers and I was right at 3000 fps and shot a 3/4 inch group at 100 yards.

So I finally scored scored some Starline brass and want to start over load development with the new brass. I'd like to do it with saving as much components as I can. The old Hornady brass ranged from 149 grain to 155 grain per piece. The starline brass is definitely more consistent at 160 to 162 grains per piece. I haven't measured case capacity between the two but I imagine it's less with the starline because of the weight of the brass.

My question is, is it possible to get in the same node with the new brass? Really would like to stay in that 3000 fps mark as long as I get the accuracy and good SD numbers. I'm hoping to not have to go out and shoot 17 different loads for my ladder test. I know the barrel shoots good. Just trying to do load development using the least amount of components.

 

[Tokay444]

Since the outer dimensions of the brass are constrained by the chamber, the extra mass will equate to lower internal volume.
Its entirely plausible that you’ll be able to achieve your 3000fps goal, but with a lower charge weight.

 

[SavageShooter67]

Heavier brass relates to thicker brass. This means that capacity will be reduced slightly. Start about 10% under your normal load and work up.

 

[Park ranger]

In my Palma load, Lapua brass is 1/2 grain less powder than Winchester brass. Lapua being pretty heavy, Winchester some of the lightest 308.

 

[Rocketvapor]

With brass being about 8 times as heavy as most powders, an 8 grain difference in brass weight would look like about 1 grain difference in powder space. I'd start a grain (or a little more) less powder.
Fill a light case and a heavy case to the top with your powder. Weight the charges.

@ 41.5 grains starting 10% low would be 37.4 grains.
I'm gonna guess -1.5%, 40.8 grains as a new starting point would get you close to your node.

 

[giannid]

I actually filled up both primed cases with powder this evening. The Hornady holds 1.6 grains more powder than the Starline.

 

[Rocketvapor]

I'll replace this post with the one I made on the other forum
The 35.6gn is 19 percent off the max load of 43.9gn. That's a pretty large range. Was the max indicated as a compressed load?
Your 41.5gn is about 5% off the listed max.
The Hodgdon online data lists a similar 100gr bullet as
100 GR. HDY BTSP

Hodgdon

H4350

38.8

2,879

48,200 PSI

42.7

3,113

59,800 PSI

Your range of case weights, 149-155, and 160-162 gives you a difference of 5gn to 13gn between the two brands.
Pick something close to the average weight of each brand giving you a difference of maybe 8 or 9 grains.

While water capacity is the preferred capacity method, cases with the same geometry can be compared with powder almost as accurately.
For differences less than a grain, use water capacity but with 1.6 grains of powder difference you can get pretty darn close.
Looks like the brass to powder ratio is between 6 and 9 to 1.
Which case weights did you use and what were the powder weights you had to get 1.6 grains?

If you want to try the water method, add one drop of dish soap to about 1/2 cup of water. Mix and set aside for a while.
Take an average case with a reversed spent primer (record case weights used) from each brand carefully fill each case with an eye dropper (draw from below bubble level) to a positive (convex) meniscus and touch the meniscus with a tiny piece of tissue paper to level it to the top of the case. The tissue will wick a little water off.
I bet you come up darn close to the powder method.

I don't think you need to repeat the entire 17 load test with the new brass.
As I stated in the other forum, Try 1/4 grain increments a couple percent below and up close to the 41.5gn.
40.5, 40.75, 41.0, 41.25gn. Obviously STOP if things look higher than your previous velocities.
You will likely end up with an increased percent of fill. Watch that you don't get close to 100% with the heavier cases as that will change pressures.

 

[dannyjbiggs]

In my Palma load, Lapua brass is 1/2 grain less powder than Winchester brass. Lapua being pretty heavy, Winchester some of the lightest 308.

As I recall, Remington BR, .308 SR primer is even lighter...have some that come in at weight-sort of 153 and 154 grains. Hard to find anymore? Used it in Palma and F-TR.

 

[Park ranger]

As I recall, Remington BR, .308 SR primer is even lighter...have some that come in at weight-sort of 153 and 154 grains. Hard to find anymore? Used it in Palma and F-TR.

True.