10mm barrel by the inch results

[shibumiseeker]

The lineup:

G29 3.8"
G20 4.6"
G20 longslide with LWD 6.6"
G20 with LWD 9"
Olympic Arms 10mm carbine upper, 16"

 

[shibumiseeker]

The powder: HS-6, 8.5gr.
The primer: Wolf LPP

Bullets: Hornady XTP 200gr, Ranier Plated 180gr.
Brass: multiple fired NNY with little crimp.

Methodology:
The guns were fired from a standing position with the Chrony F1 16' from the muzzle. Each bullet weight was shot three times through each gun. The results were averaged. This is not a very accurate statistical sample but enough for a first order approximation.

Results in ascending order of bbl length, weight <fps>:

180gr <0924> 200gr <0967>
180gr <0968> 200gr <1006>
180gr <1079> 200gr <1117>
180gr <1179> 200gr <1136>
180gr <1245> 200gr <1218>

From shortest to longest barrel:

180gr <321> 200gr <251>

Conclusions:

The 180gr behaved in a classic curve. The 200gr showed the most benefit from the shorter barrels with this powder.


If I get a wild hair I might do this again with other powders.

 

[rockhopper46038]

Interesting data. Hypothesis as to why the lighter bullet overtook the heavier bullet in fps at the longer barrel length?

 

[shibumiseeker]

HS-6 is a relatively slow burning powder compared to many used in 10mm. The only slower one I've used is BlueDot.

 

[Hohn]

Interesting data. Hypothesis as to why the lighter bullet overtook the heavier bullet in fps at the longer barrel length?

It's someting you will likely see across most powder speed ranges.

The heavier bullets accelerate more slowly. That means the point of peak pressure occurs with the bullet closer to the breech. That means that the rate of pressure drop down the remaining barrel length will be greater, and that the maximum useful barrel length for a heavier bullet will be shorter than for a lighter bullet.

With enough additional barrel length, the lighter bullet gains more advantage.

ETA: This is why heavier bullet weights are less sensitive to shorter barrel lengths (they lose less with shorter barrels and gain less with longer barrels).


Also, the degree of sensitivity is related to the peak pressure of the load. A really high pressure load like .357Sig will be more sensitive to short barrel length than a lower pressure load like .45ACP.

 

[rockhopper46038]

That was actually my first off the cuff thought - that the heavier bullet developed a pressure spike early to get moving; but I couldn't reconcile in my mind the rate of drop off. So I looked to see whether it appeared the powder fully burned early in the barrel, but the increasing velocities all the way to the muzzle seemed to indicate that was not the case. Then I thought maybe the increased surface area of the heavier projectile in the bore caused more friction than the lighter projectile, scrubbing off some velocity increase as the pressure dropped toward toward the bore. Finally I just decided to ask

 

[shibumiseeker]

I didn't care to hypothesize for the most part because the statistical sample is too small for me to be comfortable drawing inferences and relying too much on them. Also keeping in mind that one is a jacketed round and the other is a plated one.