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Old March 30, 2013, 06:50 PM   #1
All_Thumbs
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Question About Compressed Loads

I'm wondering, would you generally expect to lose muzzle velocity if you go from a "highly compressed" load to a "moderately compressed" load...by keeping the same powder charge, and the same bullet, but increasing the length a little bit?

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Edit:

Also, do some powders work "better" when compressed?
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Old March 30, 2013, 07:02 PM   #2
WIL TERRY
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NO.
NOW THINK AGAIN what you just asked...
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Old March 30, 2013, 07:15 PM   #3
chris in va
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I think it's a valid question. Why the sarcastic reply?
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Old March 30, 2013, 07:28 PM   #4
ScottRiqui
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I was thinking the same thing (that it sounds like a reasonable question). Also, I don't know why Wil is so emphatic that the answer is "no".

If you're increasing the available case capacity by not seating the bullet as deep, and keeping everything else the same, wouldn't you expect the pressure to be slightly less? Whether it would show up on a chronograph, I don't know.
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Old March 30, 2013, 07:52 PM   #5
All_Thumbs
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This question is the result of something I read on another forum, here.

In the 5th post down, someone makes the following comment, and being an inquisitive Newbie, I am trying to understand the implications.

"Note on compressed loads if that's what you choose:
Make 2 or 3 at whatever OAL you specify, measure carefully
with the caliper or micrometer and write it in Sharpie on the brass itself.
Let them sit overnight, then measure the next day and write that number.
Increasing OAL means you have compressed too far.
If no change in OAL or it grows only a tiny bit (.010 or .015 inch)
and then stabilizes, you're good to go."
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Old March 30, 2013, 08:12 PM   #6
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I too think the question is valid.

I for one have never worried about compressed loads. The manuals are full of compressed loads and compressed loads are common with overbore & magnum cartridges using slow buring powder.

One thing to consider is the brand of brass you are using. There can be considerable variance between some brands and even some lots of the same brand. Some brands will hold more powder than others.

IE in 7mm Mag using H870v I have a pet load that requires compression with R-P brass and no compression with WW brass.

For those who use black powder and reload black powder cartridges, compression is a way of life. Compression makes all the difference in the world when you are trying to maximize performance.
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Old March 30, 2013, 08:30 PM   #7
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seating closer to the lands generally increases pressure, which either results in more velocity or a rapid dis-assembly.

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Old March 30, 2013, 08:39 PM   #8
Brian Pfleuger
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Quote:
In the 5th post down, someone makes the following comment, and being an inquisitive Newbie, I am trying to understand the implications.

"Note on compressed loads if that's what you choose:
Make 2 or 3 at whatever OAL you specify, measure carefully
with the caliper or micrometer and write it in Sharpie on the brass itself.
Let them sit overnight, then measure the next day and write that number.
Increasing OAL means you have compressed too far.
If no change in OAL or it grows only a tiny bit (.010 or .015 inch)
and then stabilizes, you're good to go."
If the powder is pushing the bullet out of the case, you could end up with it jammed in the rifling, which would increase pressure dramatically. Also, severe compression can actually bulge the case, so you have to be careful there too.

I don't know that powders "work better" because they're compressed but there are other things that go along with compression that can be effected.

For instance, very slow pistol powders, like H110, have to be loaded very close to max loads at all times. They need the pressure to maintain burn.

CAUTION: The following section includes loading data beyond or not covered by currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assume any liability for any damage or injury resulting from use of this information.

In regards to how much it will effect pressure and velocity, I used QuickLoad to find a good compressed load in .243Win under a 95gr Nosler Partition at 2.650 OAL. It says that the max safe load with RL-25 would be a 110% compressed of 49.5gr (which borders on bulging the case), produce the SAAMI max pressure and 3,201fps from a 24" barrel. Pushing that out to the SAAMI max length of 2.710 only reduces the compression to 108%, drops pressure about 1,000 psi and speed just 27fps. However, if that compressed charge pushes the bullet out to 2.810, far enough to jam it in the rifling, the pressure increases to 10% over SAAMI max, at 67,000psi.

Now, if I go the other way, and start at UNcompressed and SAAMI max length, it thinks the 100% full charge would be 45.65gr and generate just 43,800psi and 2,914. If I shorten that round to 2.500 inches and keep the charge, I get 105% compression but it adds 5,000psi and gets 3,001fps.
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Old March 31, 2013, 12:04 AM   #9
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Most manuals will not show a maximum load that over flows their specified cartridge case. When I first started reloading, I use to favor the notion that the best combination was a bullet that seated exactly on top of the powder charge. Taking a "trick" from the black powder shooters, I use to sprinkle the powder through a cardboard paper towel tube on top of my funnel & I could usually get the powder level right to the powder crush point in all but the shortest bottle neck cartridges.

After all that work, my testing showed no difference and sometimes the highly compressed loads shot better!

I've never heard of loaded cartridges being so compressed they push the bullet out, but that can't be a good thing! LOL
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Old March 31, 2013, 12:40 AM   #10
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case fill and compression is a whole new thread in how they affect accuracy.

That is a can of worms that defies logic, as every once in a while some poor case fill combination turns out to be the most accurate.

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Old March 31, 2013, 07:42 AM   #11
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Quote:
If you're increasing the available case capacity by not seating the bullet as deep, and keeping everything else the same, wouldn't you expect the pressure to be slightly less?
In a bottle necked rifle round, No. Longer OAL means less bullet jump to the lands = Higher pressure not lower pressure. Shorter OAL gives the bullet a longer jump usually = less pressure.
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Old March 31, 2013, 08:38 AM   #12
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Looks like the OPs original question is answered.

I'm curious if the OP has done anything to shake the powder down into the case? I use the flat side of the vibrating shaft of an electric toothbrush and run it up and down the side of the case. You would be surprised how much the powder settles, especially stick powders. Loads that were once compressed in my 25-06 and 7mm RM now have a tighter packing density and I get no powder compression.
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Old March 31, 2013, 09:13 PM   #13
All_Thumbs
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Yes, I think my question's been answered. (Thanks to all who replied).

BTW I did try lightly tapping the case, but the powder didn't settle very much if at all. The electric toothbrush idea might be worth a try.

Anyway, I think I want to get that software that Brian mentioned...it seems it would be useful as a learning tool.
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Old March 31, 2013, 09:29 PM   #14
reynolds357
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Seat depth will change accuracy much more than it will effect velocity. I love compressed loads. I use a ultrasonic toothbrush to get the bullet crammed on top of the powder on a few of my loads.
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Old April 1, 2013, 07:27 AM   #15
SL1
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The answer to the original question is "It depends on A LOT of things."

As others have pointed-out already, changing cartridge over-all length changes BOTH the powder space under the bullet AND the distance from the bullet to the rifling, which affect pressure in opposite directions. Unclenick has previously posted (manytimes) a graph that showes the peak pressure changing as COL changes, with a minimum somewhere off the lands THAT DEPENDS ON THE RIFLE AS WELL AS THE LOAD (powder, charge weight, bullet, at least).

In addition, there is the effect of compression on how the powder ignites and burns. QuickLOAD cannot account for the differences in ignition or burn rate (or bullet jump, for that matter). So, only empirical data works for exploring compressed loads. We cans see the effects of compression on velocity using chronographs. But, the effects on peak pressure require pressure measuring equipment. One can't be reliably calculated from the other without a computer model that can properly take into account the effects of compression, and I don't think there are any such models.

On the other hand, Brian has recently acquired an RSI Pressure Trace, so he COULD do a study of how pressure changes with compression. How about it Brian?

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Old April 1, 2013, 07:29 AM   #16
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Quote:
Most manuals will not show a maximum load that over flows their specified cartridge case.
I just recently ran into that situation using IMR4350 in .308, and it wasn't even max load!
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