Data Discrepancy 7PRC

[Powerstroke]

CO archery season is about done this year and time to move on. I didn't have much time to work on loads last fall for the 7PRC and now that I have time, I'm seeing some concerning max load variances. Has this been typical since the inception of this round? I was provided quick load data last fall for 175gr EH and decided to use N570. That data provided shows a max load of 75.1 grains. From that data I started working up with the time I had and settled on 74.5gr which gave me (I'll call it half inch) groups.
Last night I was on Viht's page and looking at new data. The max load shows to be 69.8 grains for the N570/same bullet. I've seen variances in data in the past, but this one seems excessive? Thanks for any input......

 

[SJB358]

I think a guy would drive himself crazy without a chronograph. Look at the variance with a 270 Win and 130 and H4831 for instance. Some max at 58 while some max out at 62. I think it’s literally a guide and you know what’s safe in your batch of components and rifle.

Good to see you around. Been awhile!

 

[RL338]

How does your velocity compare to their published data? Some brass manufactured today does hide pressure very well.

 

[Powerstroke]

I think a guy would drive himself crazy without a chronograph. Look at the variance with a 270 Win and 130 and H4831 for instance. Some max at 58 while some max out at 62. I think it’s literally a guide and you know what’s safe in your batch of components and rifle.

Good to see you around. Been awhile!

So true about the chronograph. I'm going to need to do a bit of updating in that department. Too difficult to go to the range now and set one up out in front of the benches. That Garmin has caught my eye. Hope to have good data to run with next weekend.

It's been too long - thanks again!

 

[Powerstroke]

How does your velocity compare to their published data? Some brass manufactured today does hide pressure very well.

Hope to know that this weekend, and will report back then.
Thanks for the info.

 

[TexasLife]

knowing case capacity in h2o will help as sometimes load data (example, nosler)) gives some relevance to it. more of a "with bullet seated" capacity.

 

[Powerstroke]

Update after a "brief" range trip.
1) New Garmin Xero
2) Last year's load provided through Q/L - Apparently Q/L uses a longer COAL than the Viht manual which accounted for the higher powered charge.
A stiff charge of N570 & 175gr EH yielded a 5 shot average of 3186.9fps. Slightly over the data, but also 24 vs 26 in tube. (Quite surprised at the result) Currently plan on staying with the N570 and probably reducing the load just slightly.

3) I did load 4 rounds with 24N41per Viht's manual and noted a significant drop, although right with what the book claims. 4 shot average of 2784 fps.

 

[Hunter26]

3186 is smoking, I'd definitely back that down. I've noticed VV's load data is on the more cautious side. Different lot numbers, different component combos, test barrel vs a factory gun, how chambers were cut and a host of other variances can change load data quite a bit.