Differences in data for W760 loads

[steveNChunter]

Im about to start working up a load for one of my rifles chambered 6mm rem.

Im using 85 gr Nosler partitions and W760 powder.

I have three different load manuals that give three very different sets of data for W760 powder.

I have a Sierra 4th edition (circa 1995) that gives a max load of 44.5 gr @ 3200 fps with an 85 gr flat base

I have a new Lyman manual that gives a max load of 45.0 gr @ 3417 fps with an 85 gr HPBT

I have the load data from the Hogdon website that gives a max load of 45.8 gr @ 3329 fps with an 85 gr partition

All the loads are listed with a 24" test barrel.

I'm starting to wonder if W760 has been changed at some point over the last few years?

My powder is about 20 years old, but its been unopened, stored right, looks and smells like new. So if the powder has been changed, I'll definitely use the older manual. Does anyone know for sure whether or not the powder has been changed?

I'd also like to know which manual is the most accurate if I can hear what some y'alls other manuals say. It would be nice to know if I'm really getting 3400 fps or if I'm around 3200 (don't own a chronograph)

[Unclenick]

Do they all use the same case and primer? If not, pressures can be different. Copying a load means just that; in every detail. Usually we aren't identical in every detail. For example, the data may have been (usually is) developed with different lots of the powder, which can cause up to 6% difference in burn rate and give you different data even if all the other components are the same.

This and other factors are why you never believe maximum loads at face value. All you know for sure is that they were true for the gun and component lots the developer had available. This and pressure measuring error tolerances are why you alway go with the lowest starting load you can find and work up to see what your gun likes, watching for pressure signs and looking for a precision maximum (smallest group size). Published load data should be looked at as saying: "here's what we found was true for our gun using the listed components and seating depth".

For some chamberings data variation due to components is more important than in others, with .300 Win Mag being about the worst. There is about 7.5 grains of case water overflow capacity difference between Remington and Norma cases for that round, with other brands inbetween. That'll make up to about 3 grains powder charge difference to reach the same pressure just based on the brass brand choice alone.

Note that at the Hodgdon site you have to click the "Print" button and look at the resulting print preview to see what case and primer and barrel length and rifling twist were used.

[AllenJ]

^^^ What he said times 2. I've never seen two different reload sources agree on a load, let alone three. Start low and work up, it is the only way to be safe.

[steveNChunter]

I have seen differences before myself, just not quite as much as this, especially the 200+ fps velocity difference with the same length barrel. I was more worried about if W760 powder has changed over the years at all, and if new load data can still be trusted with 20 year old powder. To the best of my knowledge, W760 is the same powder it always has been. Same as H414 and they are both made at the same St. Marks Florida factory since 1969, and the powder made then is the same as what they sell today. Just want to make sure that is indeed the case.

And I'm working up from the starting load in my most conservative manual by the way, to be safe. But I may experiment gradually up toward the max in the other manuals as long as I dont start getting pressure signs. I want accuracy as my first priority, velocity second.

[WESHOOT2]

Uh, the different data was developed with different bullets (too, ay?).

[steveNChunter]

Yes they are all different bullets. Which would lead me to believe that the boat tail would allow the highest powder charge, but that's not the case. There must be more of a variance in the brands of brass and primers used than I realized. Or maybe differences in the lot #'s of powder. Guess thats why we have starting loads

[Unclenick]

Keep in mind that when SAAMI sent the same lots of reference loads around to different labs for testing, they got about 11% difference in average peak pressure in psi reported, and up to 23% disagreement in copper crusher average peak pressures reported. So some of this disagreement is due to limitations in pressure measuring precision, and some to SAAMI's 10 round sample size being smaller than most statisticians would want to draw a specific number from. Indeed, that 11% variation is, IIRC, close to the 95% confidence limit for comparing different 10 sample measurement averages, so it is expected to be seen with enough trials, even if one guy ran them all on the same equipment.