6.5 PRC velocity loss.

[montanablackdog]

I know this is a rabbit hole. But here goes.
1st round of loading new brass, 147 grain Hornady ELDM, 210 primers, 2.85 COL, 57.5 grains H1000 yielded 2920fps, Next load 58.5 grains H1000 2970fps. The gun was zeroed for the lighter load.
Ok first "reload" on once fired brass, RCBS full length dies, wire brushed the primer pockets, did not clean inside of necks. Only lubed out side of cases. Wiped clean after sizing did not trim cases yet. Everything else load wise was tge same except for resizing.
Went to range today to test grouping between loads and both shooting an inch low from original zero. But still grouping half inch MOA ish. So because it was low I decided to check the velocity. 200 fps loss on both loads on a Caldwell chronie. I would have questioned the chronograph had the zero not dropped on paper first. The head scratcher is both loads are still tight grouping. Side note air temp was consistent with original zero day. So it has to be something in the resizing of the brass. Its not my first rodeo on a press. I have it set up the way I always do, its camming over ect.... My only thought is either my scale got off ( I always calibrate though) or neck tension is way off. Only cross check I can think of is to run a test on another couple of new brass loads and a couple once fired loads to cross check. Pics of grouping attached
First is the higher 58.5 grain load

 

[Seabeeken]

My first thoughts would be perhaps the powder absorbed moisture/humidity since your original loads. This would make the same amount of powder heavier resulting in a charge that is actually less powder.

 

[montanablackdog]

Top

 

[FURMAN]

I wouldn't trust an optical chronograph any farther than I could throw one.

 

[montanablackdog]

I wouldn't trust an optical chronograph any farther than I could throw one.

Yeah I get that. But I did drop off almost an inch from my original zero. Thats why I checked it with the chronograph.

 

[FURMAN]

Yeah I get that. But I did drop off almost an inch from my original zero. Thats why I checked it with the chronograph.

If it were me I'd zero it and do drop validation at distance. I can almost guarantee you did not lose 200 FPS.

 

[Another Casual]

There can be variations as the brass grows to fit your chamber the internal capacity also increases, but 200 FPS seems like a lot. I'd check your scale, Chrono, and verify at distance before I start worrying.

 

[MagnumManiac]

Normally, when going to once fired brass from new brass, the same charge weight INCREASES the velocity.
An increase in velocity can change POI, even though you zeroed at whatever distance, that change can be in ANY direction.
As rfurnan24 indicates, you need to verify drops and not just go off of chrono readings.
A slight angle on an optical chrono can change the reading by a few hundred fps.
I run 2, they never match each other perfectly, the difference when set up right is about 25fps, if it's more than this, I know that it's not set up level to the bullet path.

Cheers.

 

[montanablackdog]

Normally, when going to once fired brass from new brass, the same charge weight INCREASES the velocity.
An increase in velocity can change POI, even though you zeroed at whatever distance, that change can be in ANY direction.
As rfurnan24 indicates, you need to verify drops and not just go off of chrono readings.
A slight angle on an optical chrono can change the reading by a few hundred fps.
I run 2, they never match each other perfectly, the difference when set up right is about 25fps, if it's more than this, I know that it's not set up level to the bullet path.

Cheers.

I understand. But I dropped my 100 yard zero by almost an inch. Its why I went to the chronograph. Thats a big drop.

 

[MagnumManiac]

I understand. But I dropped my 100 yard zero by almost an inch. Its why I went to the chronograph. Thats a big drop.

In my F-class rifle in 6.5x47, if I change from several fired cases just to once fired cases, I get a shift in POI. I then have to either re-zero, or just do my tests with a different POI.
This is not unusual. If I zero with a 123g Scenar and switch to a 123g Scenar-L there is also a POI difference. Charges just happen to be identical.

Cheers.

 

[montanablackdog]

in

In my F-class rifle in 6.5x47, if I change from several fired cases just to once fired cases, I get a shift in POI. I then have to either re-zero, or just do my tests with a different POI.
This is not unusual. If I zero with a 123g Scenar and switch to a 123g Scenar-L there is also a POI difference. Charges just happen to be identical.

Cheers.

Yeah Im gonna run a re-test with once fired and new brass to see. Ive also got a unopened pound of powder to cross check on the scale with the one Im using. But I live in a 20% humidity climate so I really doubt powder gained moisture.

 

[MagnumManiac]

I don't believe powder gains moisture anyway, I do believe that atmospheric conditions change the specific gravity on the day that you are loading.
Unfortunately for me, my circumstances dictate that I load in the garage, it is not climate controlled and when I check my powder measure settings that are in CC's to a weight, the weight changes, however the volume does not and I know my powder hasn't been exposed to humidity.
Until I get my Labradar, I guess running 2 chrono's will have to do.

Cheers.

 

[Seabeeken]

I don't believe powder gains moisture anyway, I do believe that atmospheric conditions change the specific gravity on the day that you are loading.
Unfortunately for me, my circumstances dictate that I load in the garage, it is not climate controlled and when I check my powder measure settings that are in CC's to a weight, the weight changes, however the volume does not and I know my powder hasn't been exposed to humidity.
Until I get my Labradar, I guess running 2 chrono's will have to do.

Cheers.

There is a chapter in the Norma reloading manual on the subject. Brian Litz adresses the issue also

 

[MagnumManiac]

There is a chapter in the Norma reloading manual on the subject. Brian Litz adresses the issue also

Then if water is taken up by the powder, why do IMR keep a jar of IMR 4198 in WATER and take some out, dry it and then test it against new batches in calorimeter bomb tests?
This would render the powder useless.
The moisture content that is written about in smokeless powders is not WATER content, it is solvent content which is displaced over time as it dries out.
I worked in the plant that makes Hodgdon powder and spoke at length with the guys doing the formula. They said that smokeless single base powder has LESS THAN 1% WATER IN IT OVER THE ENTIRE BATCH MADE. Double base powders are even less than .5%. If this is tonnes of powder, then how much water is in it?

Cheers.