Thread: Confusing pressure signs

Started OCW development today using new brass. Starting charge of 35gn 2208 showed ejector marks and tightish bolt lift. This is quite a lot lower than published loads so was confused. Persevered and did a .2gn ladder up to 37.2gn. Ejector stamps and bolt lift disappeared between 36-37gn. So how are my pressure signs going away but charge weight and velocity increasing?

6.5x47, 130ELDM, tikka action bartlein barrel.

Did you check the cases were sized correctly?
Was all the lube removed from the cases?
Does the bolt have a burr around the ejector?

It's not doing something tricky at low charge weights and burning faster with more physical space between powder granules? Kind of like a lower explosive limit.

That's purely a guess! Along the lines of it was blowing the dust out of the barrel so subsequently shots had less resistance. Ergo lower pressure.

Might be your low enough to get into the dangerous reduced loads territory, where you get random pressure spikes. I can't explain exactly what is happening with certain reduced loads that create pressure spikes, but one explanation I have heard is that on ignition the projectile gets pushed into the throat but then stops on contact with the rifling due to low pressure, you then have a fixed volume for the rest of the burn to happen and this increases the burn rate artificially (powders burn at different rates depending on pressure) which results in a fast pressure spike until it can push the projectile down the barrel. I believe it's essentially the same thing as seen when a projectile is loaded long and pushed hard against the lands.

Basically it's way easier to get a bullet into the rifling with a bit of a run-up. If you are starting from stationary and already against the rifling, it's going to take more pressure to get the projectile started than if it hits with a little speed.

Take everything above with a grain of salt. This idea is based purely on shit I have read elsewhere on the net, so could be worth what you paid for it.

New cases.

Lube on brass or chamber, too much headspace, detonation from too low powder charge. Keep going with powder charges until pressure signs appear at top end then you know where the max is....reduce from there.

What sort of velocities are you getting?
With your starting charge only 3% below ADI book max I very much doubt you have an issue with low powder charge, but velocity will give an indication of whether your loads are low or not.

Measure the length of those fired cases. It is possible that the shoulder isn't "quite right" on the new brass for your chamber the firing pin forces the case forward off the bolt face fires it then the case blows back into the bolt face causing the marks you see.

I got neck donuts with Lapua 6.5x47 brass on the first resize, I think it was die related but maybe you got something like that going on.

It has always been my understanding that the "low pressure signs" you are experiencing and typical high pressure signs are, although both caused by the same thing, are happening for very different reasons.
When a load is excessively light, there isn't enough pressure developed to expand the case to grip against the chamber wall, therefore the case is forced back against the bolt face and causing what appear to be high pressure signs.

The majority of new 6.5x47 Lapua brass have given me stiff extraction with loads a number of grains below the eventual maximum. This went away after applying one of two fixes; either annealing the necks, or hitting them hard with a max load (fireforming to chamber dimensions). My notes say it took two firings for some of the cases to settle down and behave normally. It seems a peculiar characteristic of 6.5x47. Bottom line is that stiff extraction can't necessarily be used to determine maximum straight off.

Cheers guys. It’s Peterson brass not lapua. OAL is short too. Velocity’s are between 26-2850 but 27” barrel so that’s not too surprising. I’m going to fireform all 200 pieces with 36gn then redo load development.

Does a 6.5mm projectile fall through the neck of a fired case (not resized case) easily?