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Lee Liquid Alox bullet Lube

 

Powder Position Sensitivity

Does the position of the powder in the case affect velocity/pressure?


 

Much has been written and discussed about the sensitivity of powder to its position in the case upon ignition. There are multiples of theories and opinions on this subject. I have done a little testing and have formed an opinion based on test results, not theory.

Executive summary:

"It depends."

Testing protocol:

I wanted a large case so there would be lots of room for the powder to move around.
Since this is mostly an issue with reduced loads and cast bullets I chose a cast bullet and a powder that some say is not very sensitive to position.

The largest case I have that would be suitable for this test was the .38-55 Winchester.

The Rifle was a Winchester 1885 Hi-Wall.

The rifle was mounted on sandbags front and rear.
The first screen was 12 feet from the muzzle.

For powder back, the base of the cartridge case was tapped lightly on the bench three times. The muzzle was elevated approximately 30 degrees and the cartridge inserted. The muzzle was then lowered to horizontal and the shot fired.

For powder forward, the tip of the bullet was tapped lightly on the bench three times. The muzzle was declined approximately 30 degrees and the cartridge inserted. The muzzle was then raised to horizontal and the shot fired.

For powder center, the cartridge case was held horizontal and shaken back and forth very gently. Then it was laid on the bench top and rolled two revolutions. The muzzle was set horizontal on the bags and cartridge inserted.

Each chronograph test was a string of ten shots shot through the screens at a target for group. Exactly the same as if the intent was accuracy testing.

The ten shots were fired five at bulls eye one followed by five more at bulls eye two. The entire process averaged about 20 minutes per 10 shot string.

Test 1

The Load:

.38-55.
Winchester Brass.
Lee 379-250 RF
Alloy ACWW
Sized .377
CCI 200 LR Primer
Unique 9.0 grains ( Weighed )
OAL  2.54"
Temperature 78 degrees
Chronograph PACT

This case holds 27 to 28 grains of Unique to the mouth and 20 to 21 to the base of the seated bullet.

This load was not chosen at random. It is my favorite plinking load with this cartridge

 

Results of Unique position sensitivity test:

Date 3/23/2011 
Powder
Position 
 High
Velocity
Low
Velocity
Average
of 10
Average
Deviation
Standard
Deviation
Extreme
Spread
5 shot group
100 Yd.
 
Forward  1227  1208  1214 5.4 8.0 19  1.04, 2.39
Center  1214  1195  1202  5.4  8.0  19  1.65, 1.15   
Back  1237  1196  1209  11.7  17  41  1.38, 1.24   
                 
                 

 

 

 


Note that the difference in average velocity of the three strings is only 12 fps.
That is LESS than the extreme spread in any of the three 10 shot strings.
Also note that the largest extreme spread occurred with the powder back.

My conclusion is that,  in this test, Unique shows no detectable sensitivity to where it is positioned in the case.

Test 2

The Load:

.38-55.
Starline Brass.
Lee 379-250 RF
Alloy ACWW
Sized .377
CCI 200 LR Primer
A-2015  23.0 grains ( Weighed )
OAL  2.54"
Temperature 70 degrees
Chronograph PACT

This case holds 47 to 48 grains of 2015 to the mouth and 27.9 to the base of the seated bullet.

Results of A-2015 position sensitivity test

Date 12-10-2011

Temperature 70 Degrees

 
Powder
Position 
 High
Velocity
Low
Velocity
Average
of 10
Average
Deviation
Standard
Deviation
Extreme
Spread
5 shot group
100 Yd.
 
Forward  1385  1261  1345  33  44  125  1.13,  2.21 
Center  1375  1267  1306  34 48  108 2.13,  3.25
Back  1447 1353  1408  27  37  94  1.88,  2.25   
                 
                 

 

 

 



In this test,  powder back shows a little less extreme spread, a little less deviation,  and a little more velocity.
I would say it shows a little sensitivity to powder position but not enough to affect accuracy or point of impact. All 30 of the above shots if combined into a single group would have hit in a 4" circle.

This is not a very accurate load. I have fired around 100 of them and the above groups are representative.


Test 3

.38 Special

Case capacity:
Bulls Eye Powder
Full 16.6gr
To base of 359477 11.3
To base of 148 gr WC crimp groove 8.6 (Lee C 358-148-WC )

Winchester Brass
C 358-148-WC
3.2 gr Bullseye thrown not weighed. - Hornady LNL AP (52% load density )
Sized .358
Alloy ACWW
CCI No. 500 Small Pistol Primers
OAL 1.26 ( to crimp groove ) 

 

These were not purpose loaded rounds. I loaded them a couple of months before as a batch of 300 "Plinkers". 

Powder forward.

The cartridge was held vertical with the bullet down.
Tapped six times on the bench top.
The revolver was held muzzle down and the cartridge placed in the indexed cylinder.
The muzzle was rapped on the bench three times.
The muzzle was carefully raised to horizontal and fired.

Powder back.

The cartridge was held vertical with the bullet up.
Tapped six times on the bench top.
The revolver was held muzzle up and the cartridge placed in the indexed cylinder.
The butt was rapped on the bench three times.
The muzzle was carefully lowered to horizontal and fired.

Date 12-08-2014
Temperature 55 Degrees

 
Powder
Position 
 High
Velocity
Low
Velocity
Average
of 10
Average
Deviation
Standard
Deviation
Extreme
Spread
5 shot group
100 Yd.
 
Forward  916 876 899 11.2 14.7 40 Not Fired
Center  Not Tested         Not Fired
Back  917 894 908 8 9.4 23 Not Fired  




 

Note that the difference in the 10 shot average velocity is 9 fps.
This is less than one standard deviation.

If there is any sensitivity to powder position here it would take at least 100 rounds of each to detect it.



 
 

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