fps and preasure

[zx10mike]

would the more experienced out there use fps ad a guide to pressure ?at 24.8 of rl10x with the 39 sbk my fps seems slower than any one else on the net?? 3300fps.with only a 20" barrel i expected slower but that seems a lot.i'm wondering if my batch of 10x is a bit on the slow side.i intend to keep going with care but wondered what you guys thought on the fps thing.using nosler brass and rem 7.5 primers

[TwentyBore]

The two don't correlate strongly enough to be useful. There are too many other variables - in chamber, bore, rifling, etc. - to let you draw any strong conclusion between the two other than "using the same gun, bullet, primer, and powder, load A is faster than load B, so A should have a higher pressure than B." However, you can't know how *much* pressure either has, or how *much more* B has than A.

And, if any of those variables change - bullet, bore, chamber, powder, whatever - then you can't really draw any more than a wild generalization between the two.

If your loads are significantly slower than others, my first advice is to make sure you're talking about the same barrel length, and then get a quality set of check weights and verify the charge weight that you think you are using. I've seen beam scales which are properly leveled and zeroed be off by 5% for any given weight, I've seen them (when dusty) fail to give the same measurement twice, and I've seen two different check weights, marked with the same weight, NOT come up equal on the same scale. There are lots of ways in which your actual charge weight can deviate from what you think it to be.

[zx10mike]

cheers twentybore yep 20" but i'm about 4-500 fps slower than many using the same charge.i' expect to be say 200 fps slower but not that much.i checked my scales and my digital ones do weight .2 different to my beam (both calibrated) so i guess the hottest i have tried is 24.8 so ill work forward some more.

[MZ5]

The two (pressure and velocity) are directly correlated; you can't have one without the other. That said, I agree with the various factors already identified which make it tough, and probably imprudent, to compare one rifle to another. From a pure safety standpoint, the prudent stopping point is the _lower_ of:
Max listed charge, or
Max listed velocity.

Are your velocities low with multiple load combos, or only this one?

FWIW, I can't touch published velocities with listed loads with any powder or bullet I've tried. Factory ammo is also significantly low in my rifle, my Darkker's, and a numbers of others we have direct experience with. I have good reason to believe this is because of how the rifles are throated, but I don't have strain gauge equipment nor appropriate hydraulics to calibrate such equipment, so I pretty well stick with published data and tolerate the low velocity.

YMMV

[zx10mike]

cheers for your help mz5 this was the starting load i'm going to try h322 and 26 grn varmint grenades this week to see how they go.will also try max and above (carefully)with the 39s

[MZ5]

Looking forward to reading the results!

[zx10mike]

Looking forward to reading the results!

might be next weeknow. high winds till then,not great for groups with light bullets.