7MM WSM velocity verses powder charge question.

[lisagrantb]

I'm getting low velocities for a given amount of powder (according to load manuals); here are a few loads I've tried.
IMR7828 65g 2884fps, H4831SC 64g 2811FPS, Reloader25 66g 2786FPS, Reloader17 61g 2980fps all using Federal Mag. Match primers.
This is a new rifle I'm breaking in, it's a Defiance Mutant long action with a 27 inch Brux 1:9 twists throated for the Berger 180 grain vld. Case OAL is 3.15" using Berger's 180 grain hybrid vld bullets. I've tried 4 different powders and they are all giving the same results. As I get close to maximum load the velocity is lower than what I would expect but I'm not showing any pressure signs at all. Primers look good and no resistance at all when you lift the bolt and I've verified the chronograph against two others.
I think the long throat is giving me more capacity compared to one that has half the bullet inside the case and it's just going to take more powder to get the same results. One thing that's odd is you would think the slowest powder (RE25) would produce higher velocities than the faster powders in this circumstance.
Has anybody seen this before?

 

[FURMAN]

I have a 7mm WSM built on a long action Stiller with a 25in. Brux. I had to get up to 66gr H4831SC or 67gr H1000 to get speeds in the 2900-3000. You will, of course, need to work up.

 

[LouBoyd]

I'm getting low velocities for a given amount of powder (according to load manuals); here are a few loads I've tried.
IMR7828 65g 2884fps, H4831SC 64g 2811FPS, Reloader25 66g 2786FPS, Reloader17 61g 2980fps all using Federal Mag. Match primers.
This is a new rifle I'm breaking in, it's a Defiance Mutant long action with a 27 inch Brux 1:9 twists throated for the Berger 180 grain vld. Case OAL is 3.15" using Berger's 180 grain hybrid vld bullets. I've tried 4 different powders and they are all giving the same results. As I get close to maximum load the velocity is lower than what I would expect but I'm not showing any pressure signs at all. Primers look good and no resistance at all when you lift the bolt and I've verified the chronograph against two others.
I think the long throat is giving me more capacity compared to one that has half the bullet inside the case and it's just going to take more powder to get the same results. One thing that's odd is you would think the slowest powder (RE25) would produce higher velocities than the faster powders in this circumstance.
Has anybody seen this before?

No, since I don'have a rifle chambered in 7mm WSM. However I do have other rifles which use longer than SAAMI length COAL and I use Quickload regularly to predict their performance. Its usually within 50 fps for 3000 fps cartridges.

Here are Quickload simulatiions assuming a 27 inch barrel with a 180 Berger seated to 3.15" COAL. All other inputs are Quickload defaults.

65 grain IMR 7828 : 2949 fps @ 58271 psi - a reasonable load - I'd use it .
64 grain H4831sc: 2879 fps @ 58329 psi - a reasonable load
66 grain RL 25: 2891 fps @ 54424 psi - 67 grains would be more reasonable. 2929 fps.
61 grain RL 17: 2982 fps @ 69432 - OVER SAAMI MAX PRESSURE

The results you're getting aren't unreasonable. Without being there it's hard to know what might be wrong, if anything. Have you measured the actual case capacity of your fired brass? Quickload assumes 81.1 grains H2O. Quickload also assumes the bullet is seated close to but not on the lands as the default.

Disclaimer: Quicklload is only as good as the info given and assumptions made. Always work up loads carefully and watch for pressure signs.

 

[lisagrantb]

Thanks guys, haven't checked case volume but measurements of fired cases look good but I will check and I'm .015" off the lands. I will work up slowly and see what happens. I would be happy with 2950ish with no pressure signs, I don't like to push the envelop, especially with a potential barrel burner like the 7wsm.

One guy sugested I slug the barrel to make sure it's not oversized.

 

[lisagrantb]

Well I worked my way up from 66 grains to 70 of RL25 and got to 2975FPS and still no signs of pressure at all. The load density for 70 grains of RE25 is about 95%, I think I will play around with this load to see if it will deliver.