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Could that light bump be a resonant pulse in the steel rather than a slight increase in the pressure? All the traces of secondary ignition that I have seen are closer to the primary pressure curve and usually with higher pressure.
 

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As Mike said, that's the standard explanation. It's a secondary pressure event, though not a secondary ignition or explosion effect (aka, S.E.E. or detonation, where the whole gun blows up). There seem to be two causes of the readings:

Actual pressure:

Jim Ristow at RSI has seen barrel ringing in a borescope from where the lump is expected to be. (Note that the strain gauge is at the breech end of the gun, so an event occurring further down the barrel takes time to send a wave back to the breech so it registers at the breech. The same thing happens with muzzle pressure after the bullet exits: it takes time for the drop to get back to the breech.)

Charlie Sisk had some pictures on THR long ago and that are now gone (his server was hosting them) in which he shows the muzzles blown clean off of 338 (Winchester or Lapua; I don't recall which) by a very light bullet with a slow powder. He was able to do this on demand and did so for several arms industry representatives who seemed to show no interest. It took 9 or 10 rounds but would happen reliably.

Other experiments have shown the lumps disappear when a faster powder or a heavier bullet is employed.

Other gun bursts and barrel bulging phenomena are done by powder mass alone being fired into an obstruction. I was tasked to find the cause of barrel bursts by one of the major gun makers about fifteen years ago, and that turned out to be the cause: powder firing into an under-bore-size projectile left a few inches down the bore. General Hatcher described investigating how safe it was to try to shoot stuck cleaning patches or other debris from a barrel by pulling the bullet from a cartridge and firing it into the gun like a blank, which was a practice followed by a number of shooters back between the World Wars. He cast a 4" plug of lead in the middle of an '03 bore and fired the "blank" into it and severely bulged the barrel. So powder mass alone can do it.

Faux Pressure:

Denton Bramwell, knowing that analog-to-digital converters that are single-ended (work between ground and the power supply voltage) as the Pressure Trace has can exhibit false bumps in fast transient events, got a double-ended (work between a plus and a minus supply with the ground between the two so you can see events in either direction) and with the same strain gauge found most of the secondary pressure spikes in his gun ceased to appear.

How you tell whether you have real pressure or faux pressure without a different instrument is tough to say. I would try going to a faster powder or heavier bullet to see if that eliminates them. If it does, you probably had the actual pressure.

Reference ammunition is hardly a definitive way to calibrate a commercial barrel. That ammunition is a reference for a SAAMI minimum barrel, and few if any commercial barrels are that - so the pressures will be different. Actual pressure in a commercial barrel will likely be lower, but really who knows for certain? IME the Oehler should be used as a relative pressure instrument, not an absolute one…
All load pressures are relative in the SAAMI system. Take a look at page 190 in the rifle standard to see how different SAAMI minimum chamber barrels measured the same reference lot on different copper crushers. The spread is 32,800 CUP to 41,200 CUP. The transducers show about half that variation, but it's still a lot. The whole point of reference ammo is that no calibration by tarage table for copper slugs or by hydraulic pressure for the piezo transducer, both of them static methods, can be trusted to measure accurately in a dynamic situation. The reference load pressure is considered a best guess, and by dividing your measured pressure by the reference load's rated pressure, you get a correction factor for your readings that basically says, here's how your ammo would read, on average, in SAAMI equipment owned by the makers involved in the tested.

Alaska Man said:
In any case, the action was originally a 270 with a maximum average pressure rating of 65000 PSI, so i believe the load is safe in this rifle.
Oh there's no real safety issue. You're still way below the 30-06 proof load range (80,000-89,000 psi) and the gun has to stand up to that. It's just about throat and bore wear acceleration, IMHO).


Alaska Man said:
I have run across several references over the years describing how new brass must be fireformed to the chamber dimensions and that takes a portion of the energy of the powder to do so.
Here's a thread with different people getting the opposite results. But I think this one where it speeded up is more interesting. One fellow mentions his barrel speeding up 150 ft/s as it aged, whether the brass was virgin or not. It would be some fun trying to get all the variables out of this experiment.
 
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"So how I assumed/thought/hoped/etc it would react, and how it did was rarely the same thing. FWIW"

Well, even the biggest computers can't figure out where hurricanes go. Who will be the next president. Etc.

And SEE's have been a hanging theory and discussion, for, A LONG TIME.

One of those: Who KNOWS ??

Still curious though.
 

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"So how I assumed/thought/hoped/etc it would react, and how it did was rarely the same thing. FWIW"

Well, even the biggest computers can't figure out where hurricanes go. Who will be the next president. Etc.

And SEE's have been a hanging theory and discussion, for, A LONG TIME.

One of those: Who KNOWS ??

Still curious though.
Indeed it is curious. My point with Super, was that it doesn't act like a typical powder in a "linear" fashion.
This is over simplified, so take it for the general premise.

A normal powder, say Win 748. Will hold the same basic burning curve from start to max loads; and generally a bit beyond (308). Increasing linearly along the road. If you jammed a bullet in a certain load the pressures increase as expected, but nothing fundamentally changes about the pressure curve.

Superformance behaves the opposite of that. It's curve doesn't change much until a certain pressure range, then it begins to shift (relatively) quickly, to a different curve. As soon as it gets cranky, which didn't always correspond with the cartridges operating pressures, the curve tended to shift again; and not always in a consistent manor.

Haven't used it in anything but the Creedmoor, so don't know that it's universal; but it is an interesting cat in that application.

Cheers
 
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Discussion Starter #25
I was able to work up some stats for the new versus olds cases. There does seem to be a small difference in pressure and velocity for this test. I will keep track of new versus fired cases and compile some statistics in the future.

99676


99669


New case trace
99670

99671

99672


I seem to have misplaced the stats for the following string, but the fired cases got about 20 FPS more velocity.
Fired Cases
99673
 

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The Shadow
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Is your throat really that short, or are you just seating to SAAMI?
 

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Discussion Starter #28
I tested the 200 grain ELD-X load for accuracy this morning. It's not bad, but I will want to adjust the load to get under 1" at 100 yards at a later date.
I will be taking this rifle, my Browning 7mmX.300 Weatherby, my Ruger .35 Whelen, and a Stevens side-by-side 20 gauge shotgun, along with my Ruger .454 revolver.

100 yards 1.75"
99678


300 yards 3." The third shot must have been off the target.
99679
 

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