Chrongraph FPS VS Load data VS Chrony???

[Rule3]

I know there are a lot of variables between real life velocity readings and published load data but how much to allow for change in barrel length? I seem to remember reading somewhere it was almost 100 FPS (depending on caliber, powder, primer yada yada)

For example: I tested 2 strings of 10, 38 special. 158gr MBC 158 gr LSWC with 3.7 grs HP 38 out of a SW M 67 4" barrel. The gun is dead nuts accurate.

1st string AVG FPS 510, ES 25, SD 6.78
2nd string AVG FPS 514, ES 18, SD 6.16

So I can not complain about those numbers.

Hodgdons data says 834 FPS out of a 7.7 test barrel with 3.7 grs HP 38. I do not expect it to be exactly the same but how close should it be??
So over 300 FPS due to a 3.7 difference in Barrel length??


Lyman data shows 767 FPS With 3.6 grains out of a 4" test receiver
and 837 FPS with 4.0 grs

Is my Chrony Alpha that far off????

 

[rcmodel]

No, it is probably just fine.
Welcome to the real world of velocity testing!!

Lyman says in the manual they use a 4" barrel in a Universal receiver.
It uses a crusher slug (Copper Units of Pressure) to measure pressure deformation of the copper slug and convert it to C.U.P pressure readings.

Hodgdons 7.7" test barrel is a mighty odd length for a revolver to have.
So I would say they are using a pressure test barrel too, as many of thier results show C.U.P. pressure readings.

What does that have to do with anything, you might ask?

Well, for one thing, pressure test barrels are made to exacting SAAMI dimensions as far as exact bore diameter, rifling depth, smoothness, etc..
Your S&W isn't.

The other thing is, pressure test barrels used to measure in C.U.P. do not have chamber throats, forcing cones, or a barrel / cylinder gap to lose pressure.

They are a solid barrel, mounted in a cannon breach single-shot action.

rc

 

[Rule3]

I guess the main question then remains, what's the sense of Chrony the numbers if there is nothing really to compare it to?

They are just numbers then. Other than looking for consistency which is important to me, there is no indication of increase pressure due to higher velocities. So other than looking at primers or hard extraction it's just a crap shoot.

I am not going to do it, but someone could look at these numbers and so, "Oh look only 510 fps and the "book says 700+ so I can up the powder charge by x amount?"

Speer uses a 6" actual SW Model 14.

Their data is 783 FPS with 3.8 grs W231/HP38 so that has me believing aprox. 100 fps loss per 1" in barrel length if my readings are correct.

Does that sound reasonable??

 

[rcmodel]

Have you shot any factory loads over the crony?

If you were to shoot some factory standard pressure 158 grain lead loads and get around the same readings that should tell you something.

The other thing to check is your sky-screen spacing.
If they are not set exactly the distance apart called for, the crony readings will be off.

Too close together would read faster.
Too far apart would read slower.

rc

 

[Rule3]

I usually test it with 22 LR and it is pretty close to what the box says. On the Chrony Alpha Master, there is no adjustment for the screens, they can only be in one spot

Now that you mention it, I did fire a few shots from my carry gun with Rem GS 165 gr HP 40 Cal and they only registered in the high 700 fps, so something is a miss.

This is a replacement Chrony, the first one gave all bizarre readings. I doubt Midway will take it back but I think I need a better quality one.

Maybe it's just better not to know