I got a Chronograph original for my air guns. Last fall I took it out to measure my new rifle, and while I was at it my .357Mag and .357Max loads. Everything was pretty much as expected. Std Dev in all three was in the 22 to 15fps range.

This week I measured my .45ACP and .40S&W loads and I am somewhat concerned. I am getting Std Dev in the range of 250fps!!!

Is that normal? Do semi-autos round vary that much? My .45ACP load is 230gr lead round nose over 5.5grs of Unique. A pretty standard load. My loading technique can't be that far off since my .357 and .338Fed loads are right on.

To double check my Chrono, I fired my .357Mag and the readings were right on with a SD of 22. So I don't think it was the Chrono.

Any insight as to why the wide deviation for my .40 and .45s?

Alf


Alf

Depends on many factors, but bullet pull is one which with a pistol will throw the numbers all over. Too heavy a crimp sometimes causes a poor hold on the bullet. Primers which may be erratic could do it... just investigate your loads further.

The muzzle blast wave ahead of the subsonic bullets can do strange things to chronograph readings. The chronograph readings are definitely suspect.

Ken O

I too would suspect the muzzle blast as being a factor. Ensure that the Chrono is out away from the pistol by the appropriate distance so as to minimize the blast effect.

Thanks for the advice. It makes sense since I was closer to the screens this time than in the past. I'll not panic and try again with the screens further away.

When you're on the range and trying to coordinate with other shooters, sometimes you loose sight of the basics.....like don't get too close to the screens.

At least I didn't hit my screens......yet.

If it won't behave, see the "Stopped Charging Chrony" thread.

An SD of 250fps in the 45acp would be almost impossible unless your powder charges vraied by 1gr+. I supsect if your screens were closer than 15ft, you were getting false readings. That is the closest I'll set the screens, I prefer 18-20ft.

quote:

Originally posted by KenOehler:
The muzzle blast wave ahead of the subsonic bullets can do strange things to chronograph readings. The chronograph readings are definitely suspect.

Ken O

You just heard it from the horse's mouth.

Thanks FRED338. I've pretty much determined I was just too close and didn't realize it. Only the last outing produced such wild results. I'm pretty anal about measuring my powder charges and checking the powder measure every fifty rounds.

RICOCHET: I posted something in that thread. Borrowing a line from the movie "D-Day", there are two types of chronographs, those that have been shot and those that will be shot.

From the instructions of our laboratory screens,

"If the bullet can be subsonic, thou shalt use a blast baffle twixt muzzle and first screen.

An ancient commandment found chiseled on a test tunnel wall."

Distance from muzzle to first screen won't always do it.