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Where does peak pressure occur

 
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  #1  
Old 10-03-2007, 06:52 PM
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Where does peak pressure occur

In an effort to get a better understanding of internal ballistics I'd like to know where exactly does the pressure peak. In the nanoseconds after the firing pin strikes the primer I understand that the primer ignites the powder and as the powder burns pressure rises in the case until the bullet starts to move. Pressure then drops until the bullet hits the lands and stops(assuming bullet is seated off the lands). Follwed by more pressure buildup until the bullet starts to travel down the barrel.

So which is it, in the case before the bullet moves or when the bullet hits the lands or is it at some point as the bullet travels down the barrel? How would/does this change if you seat into the lands?

Thanks for any input

Chris
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Old 10-03-2007, 07:33 PM
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winmagman,

From messing around with the RSI Pressure trace gizmo, jumping the bullet or seating it into the lands makes no difference in peak pressure. Seating the bullet into the rifling seems to tend to increase the initial pressure giving a larger area under the entire curve which means more energy transferred to the bullet. Peak pressure is about in the same ball park for both conditions.

For a 150gr 30cal bullet w/a MV of 2900 the bullets leaves the barrel in about 1.2 milliseconds. Peak pressure occurs at about the 0.4 millisecond mark or about 1/3rd of 1.2 barrel time. Very roughly speaking this somewhere around the 8" down the barrel mark.

Once I get back to pressure testing I'll post some pressure traces. Right now my laptop used for traces isn't up to snuff software wise to transfer the traces into picture format.:o

Also pressure never appears to drop. It slows in its rise as thing begin to move but never drops. Also nanoseconds are way to small of a unit for this stuff. It just ain't that fast.

Also I have no idea of where in the barrel the bullet achieves its maximum velocity. I just assume its at the muzzle.:confused:
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Old 10-03-2007, 07:45 PM
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winmagman:

All I've got is QuickLoad but this is the "computer generated" display it gives for chamber pressure, barrel length and bullet velocity for my 7mm AM.

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Old 10-03-2007, 08:07 PM
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Dick,

Wholly Crap, a 19.2" Barrel on a 7 AM That pic in your sig is way out of proportion. You must be one of those little people as the gun looks full sized in your hands. Also a piece of foam over the muzzle will protect you from that sharp AL tip. It must stick out at least a little bit. :p

If you weren't such a nice guy you'd be wantin' to woop my butt about now

BTW, 65000 psi, as near as I can figger is where I shoot the 270 AM. That seems to be a sweet spot. On the sadder side I do know that that the ol' 721 action will handle 80k+, but that's another story.:o
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Old 10-03-2007, 08:14 PM
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Quote:
Also I have no idea of where in the barrel the bullet achieves its maximum velocity
I've often wondered about that myself, at some point you would think the friction of the bullet traveling down the barrel would start to slow it down, coarse you might need a 6 foot barrel before that happens.

Quote:
Also pressure never appears to drop. It slows in its rise as thing begin to move but never drops. Also nanoseconds are way to small of a unit for this stuff. It just ain't that fast.
See I learnt sumtin already... Seriously I appreciate the responses, don't know that I'm advance enough at this to put the information to good use, but it was an itch I had to scratch.

Chris
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Old 10-03-2007, 08:34 PM
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Quote:
Originally Posted by royinidaho View Post
Dick,

Wholly Crap, a 19.2" Barrel on a 7 AM That pic in your sig is way out of proportion. You must be one of those little people as the gun looks full sized in your hands. Also a piece of foam over the muzzle will protect you from that sharp AL tip. It must stick out at least a little bit. :p

If you weren't such a nice guy you'd be wantin' to woop my butt about now

BTW, 65000 psi, as near as I can figger is where I shoot the 270 AM. That seems to be a sweet spot. On the sadder side I do know that that the ol' 721 action will handle 80k+, but that's another story.:o
Those numbers in the boxes for barrel length, pressure and velocity change as you move the cursor along the curve lines. I just happened to have the cursor in a place that gave those numbers when I captured the picture. Don't think I'd want to be anywhere near that thing when it went off if it had a 19.2" barrel.
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Old 10-04-2007, 04:12 PM
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peak pressure occurs getting the bullet engraved into the rifling or ever so slightly afterwards.imagine how much force it takes to get the bullet into the rifling. that's what builds the pressure. the bullet doesn't begin to engage in the rifling until there's around 15K in the chamber. the pressure continues to build until complete engraving has occurred, then the bullet accelerates very rapidly. as the bullet goes down the barrel,the pressure keeps dropping until it exits the barrel where maximum velocity occurs.
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