Case wt-vs-Powder charge

[jpretle]

This question is in regards to the 243 case.

From a much used and safe load, would a 10g reduction in case wt. warrant a 1g increase in powder charge?
On the other side, would a 10g increase in case wt. warrant a cautionary 1g decrease in powder charge?
I am thinking someone here, has been down this road before.

 

[mikeinct]

The really heavy for size..say a 223, if that is 8 to 10 grains over your avg..It can cause big troubles...I've only run into that problem once in 40 years of reloading..The point being, Weight sort your lot of cases..Develop your load from the best grouping of same weight & brand cases..

 

[MagnumManiac]

I can’t say that a 10gr increase/decrease in brass weight would interpolate into a 1gr increase/decrease because case capacity and expansion ratios differ between cartridges and bore diameters.

I know for absolute certainty that a charge in Federal 300WM brass CANNOT run the same weight as a charge in Norma brass, there is a 10gr of WATER difference in capacity, Norma cases hold 10gr more water than Federal cases, max loads run around 4gr of powder difference.
Norma brass weight runs around 215gr, Federal brass runs above 240gr or even higher.

Cheers.

 

[dmoran]

@jpretle
My experience with 243 brass is that the case weights and case capacities have poor correlation. And strongly suggest to check the case capacities of the cases your in question of, not taking nothing for granted when it comes to charge levels and case capacities.

 

[cw308]

When loading 308 year back with Rem. & Win. brass , a shooting friend that doesn't reload gave me his FC brass . When loading my brass the powder would be at the base of the case neck , with the FC it came almost to the top of the case . I thought all cases were the same , I was new an didn't know better. I use the thicker brass , FC , HSM & ADI . I don't load hot , using IMR 4064 the powder charge is at the base of the case , using a Sierra 168gr.MK the powder fills the case just right . Groups are very good.

 

[rammac]

As stated, case weight doesn't directly correlate to case volume. A 10gr change in case weight is enough to check for velocity change. If it has changed AND your groups have changed then adjust the powder charge as necessary. If velocity has changed (and still under reloading manual max) but the groups are still acceptable then don't worry about it.

 

[jpretle]

Thanks all for the responses. I went from cases of 160g to cases of 170g case wt. I was thinking dropping 1g of powder would keep me below an over pressure load, and I still think so. Not looking to match velocities of the previous loads, but looking to stay below an over pressure load. I thought some one here would have already broken this ground.

 

[ireload2]

Brass is 8X the specific gravity of powder if that helps you any....
The volume of 8 grains of brass is about the same as the volume required for 1 grain of typical powder.

 

[jpretle]

Thanks ireload2, I thought I was on the right track.

 

[Rsadams]

When loading 308 year back with Rem. & Win. brass , a shooting friend that doesn't reload gave me his FC brass . When loading my brass the powder would be at the base of the case neck , with the FC it came almost to the top of the case . I thought all cases were the same , I was new an didn't know better. I use the thicker brass , FC , HSM & ADI . I don't load hot , using IMR 4064 the powder charge is at the base of the case , using a Sierra 168gr.MK the powder fills the case just right . Groups are very good.

I have done the exact same thing , going from rem brass to FC gmm brass I picked up alot of speed... Your observations are correct... I didn't run into over pressure not changing the powder load.. 168SMK and 42gr of IMR-4064... Shot even better with the faster speeds...

 

[cw308]

Rsadams
The ranges in my area are max. 200 yards , Im grouping well with this low charge 40.8 the thicker brass cuts down on any free space after seating. 42.0 grains is the recommended load for accuracy , What rifle are you shooting . Mine started out as a Rem 700 LTR , after 4000+ rounds I changed to a Rock Creek M24 5R 11.27 twist 21" barrel changed the HS Precision stock it came with to the HS stock on the 700P was alittle bigger . Blueprinted when the barrel was changed also Jewell trigger set at 10 ounces , it's my one an only I just love shooting it .

 

[Bart B.]

There are some who used Oehlers chronograph & pressure system testing this stuff that concluded a 7/10ths grain powder reduction for a 10 grain increase in charge weight was a good rule of finger to keep peas pressure the same.

One of the few velocity tests I did with the 308 Win using case weights 150 to 180 grains showed that rule was close enough to keep velocity average in a 15 fps spread shooting IMR4064 under 168's. Good enough for me.