Compressed Loads

N

Nathan

New member
Anybody who is putting together max loads for a big bore rifle, knows max loads are generally compressed...sometimes pretty heavily.

Then I read about how folks should avoid compressed loads.....something about the compression causing pressure spikes.

Then I read from Hodgdon that compressed loads are ok.

What gives? Any experience to shed?
 
Compressed loads within pressure-tested/manufacturers' combinations are fine -- even
desirable as the compression stabilizes the rocket fuel/column burn progression.

In fact when striking off into new territory w/ QuickLoad/Chronograph I filter for full-case/slight
compression, 90%-95% burn -- then highest velocity.
 
The max load for H870 in the book for a 7 Mag. will overflow the case. No problem in my rifle. I load 70 grains of ffg black powder in a 45/70 with a 530 grain bullet and that requires.200” compression. That has proven to be the most accurate load in my Sharps.
 
There are certain powders, Trail Boss in particular, and some flake powder that can get spikey when compressed. I don't believe I've ever heard of an issue with either sticks or spherical rifle propellants. Some commercial loads are compressed, like Federal GM308M or the old Hornady light magnum line.

For stick powders, using a long drop tube will often eliminate the need to compress them. Here is what a case full of IMR 4064 looks like. The last photo shows the effect of a 3-foot drop tube, so you can see that just having the grains oriented into better packing configuration can make quite a difference.

attachment.php
 
UncleNick's drop-tube effect can also be achieved by twirling the powder funnel against the base of the case -- the vibrations settle the powder grains down.

Anything that uniforms the distribution (both in individual cases and between different cases in the loading block) is helpful
 
...And we shouldn't forget the trick of setting the charged cases upright in a loading tray and then holding the tray against a vibrating surface like the top of a vibratory tumbler, or on an aluminum plate on some scrap carpet pile alongside a running fish tank aerator pump.
 
Personally, i look for a minimum 85% case fill with a max of 104%.
With large capacity cases (magnums) case fill below 85% can sometimes give you some weird pressure spikes.
With compressed loads i use a crimp. I haven't had too much issue staying at 104% and under.
With some loads compressed over 104%, without the crimp i've had the COAL actually increase over time. Not enough to shove the bullet out of the case neck, but enough to not fit the magazine and make chambering difficult.

When loading 160gr HPBT GameKings in 7mm-08, over a charge of RL19 you definitely can hear the grains of powder crunching while bullet seating.
 
I use my electric tooth brush as a vibrator to settle powder. I just leave the brush off and run the metal reciprocating drive stub up and down the shell case half a dozen times.
 
I prefer zero compression. Powder charge burns more uniformly and accuracy is better.
Click to expand...

How did you determine that?



UncleNick's drop-tube effect can also be achieved by twirling the powder funnel against the base of the case -- the vibrations settle the powder grains down.
Click to expand...

I’m not sure what you mean here...
 
compressed or non compressed I have gotten into the habit of once the powder has been poured in I leave the funnel in place and tap the case a couple of times against the top of the bench. Done it so much now it is a automatic thing, pour powder, tap twice, remove funnel ,insert bullet then seat
 
UncleNick's drop-tube effect can also be achieved by twirling the powder funnel against the base of the case -- the vibrations settle the powder grains down.
Click to expand...
I’m not sure what you mean here...
Click to expand...
If you spin the funnel lip against the base of the case, you'll actually send vibration into the powder/settle it.
(the best funnels for this have irregular/hexagonal flanges)
 
Well, I shot some really heavy max loads that were so compressed that I thought the bullet might pop out of the case on its own!..


No pressure spike, no issues, no bolt lift issues, no excessive case head marking.....but they are max because I cannot put any more powder in the case!

I don’t want to post the load because it would be irresponsible to post something over book. Unfortunately, this round is nearly a wildcat due to its lack of data available. It is available in factory ammo and I’m not exceeding those velocities by much.

All in the name of science, I guess.
 
Nathan,

Velocity spread increases if some charges leave a space at either end of the case body. 100% load density eliminates that problem.

If there is less load density, there's a powder charge positioning procedure defined by SAAMI to position the powder in the case to make velocity and pressure more consistent. From SAAMI specs on highpower rifle ammo:

A. Cartridges to be tested should be placed in a vertical position with primer-end down in a recessed holding block.
B. When the appropriate test barrel has been properly serviced and the chronograph reset, a cartridge should be lifted vertically from the block. It should be rotated slowly, end over end, in a vertical plane through 360° pausing momentarily when the powder is at the bullet end and again when the powder is at the primer end.
C. The cartridge is then rotated slowly, a minimum amount to enter the chamber, keeping the primer end in the lowest possible position until inserted gently and carefully into the chamber.
D. The cartridge should be seated in the chamber as far as practicable with the fingers. The bolt or breech mechanism should be closed gently in order not to disturb the position of the powder in the cartridge case. The object of this method of handling cartridges is to position the propellant powder at the primer end of the cartridge case by permitting it to fall gently against the primer while rotating the case.
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Often referred to as the "SAAMI twist. "
 
It depends on the powder, cartridge, rifle, and bullet. The rules aren't carved in stone.

Generally speaking a load that comes close to filling the case or is slightly compressed will be more accurate than one that leaves a lot of empty space in the case.
 
Nathan said:
Anybody who is putting together max loads for a big bore rifle, knows max loads are generally compressed...sometimes pretty heavily.

Then I read about how folks should avoid compressed loads.....something about the compression causing pressure spikes.

Then I read from Hodgdon that compressed loads are ok.

What gives? Any experience to shed?
Click to expand...
Compressed loads are the most stable, most consistent load in the big magnums and the overbore smaller stuff. Its kind of up for debate with the more conventional stuff. Compression also minimizes temp sensitivity of slow burning powder.
I compress some loads to the point of drop tubes and ultrasonic toothbrush.
 
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Nathan said:
Well, I shot some really heavy max loads that were so compressed that I thought the bullet might pop out of the case on its own!..


No pressure spike, no issues, no bolt lift issues, no excessive case head marking.....but they are max because I cannot put any more powder in the case!

I don’t want to post the load because it would be irresponsible to post something over book. Unfortunately, this round is nearly a wildcat due to its lack of data available. It is available in factory ammo and I’m not exceeding those velocities by much.

All in the name of science, I guess.
Click to expand...
I have loaded a lot of 7 rem mag stuff that had to be seated several times over a few weeks before they quit creeping out.
 
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