Muzzle velocity and Power factor

[Stmark73]

Hi to all forum members,
I recently acquired a magnetospeed chronograph which measures velocity an inch or so from the barrel, compared to the traditional chronograph which measures velocity 10 -15 ft away.
My question for the group is, do I use the actual PF calculated from the muzzle, let say 130PF and be done with it. Or go 135PF to compensate for the distance difference that would be used during a chrono at a match?

[1SOW]

I didn't major in physics or math,  but I doubt very much the bullet is slowing much at all another 10' from the muzzle.
I wouldn't load to 125 PF , but at a reliable 130 PF you should fine.  My bet would be 1 or less PF loss.

Find a shooting friend with a common chrono and you can verify the speed lost at10'.
I and likely others would be interested in your findings.

[CPTKILLER]

Physics 101

F=MA (Force = Mass X Acceleration)

Yes velocity matters.

Also look at the velocity curves from most manufacturers.

[Wobbly]

I'm with 1SOW. Get a reliable 130PF going and you'll be OK when you get to the match. Ten feet is simply not going to drop the velocity that much.

[Stmark73]

Thank you for all the replies.
I don't have an access to a regular chronograph as of now. I will report back as soon as I get an access to one.

[Wobbly]

Did you see this thread....

http://www.czfirearms.us/index.php?topic=97302.0

[tdogg]

I have a Caldwell unit and as part of the data input you specify distance to chrono.  It then does a true muzzle velocity calculation (I'm assuming using elevation, pressure, and ballistic coefficient data entered/captured during string but it may be dumb and just uses standard atmosphere values).

With my latest chrono session shooting plated 165 grain RNFP and assuming a BC of about 0.15, I get a about 8 fps drop with my chrono at 10 ft from the muzzle.  This equates to a drop in power factor of about 1.3 PF (172.8 with MV, 171.5 at Chrono).  So your way conservative assuming a drop of 5 PF with a traditional chrono at 10 ft.

I would just make sure your loads chrono with the magnetospeed at 130-131 PF (or somewhere above this at an accuracy node) if your interested in making minor PF at a match.



Cheers,
Toby

[tdogg]

What I don't know is if the match PF calculator uses the Chrono velocity or does the Muzzle velocity calculation to determine PF?  I have never been to a match that has a chrono stage.  I will know more this weekend as I'm shooting the Northern AZ Classic Level 2 Match.  It has a chrono stage and hence the reason for the data shown ( I needed to confirm I would make major).

Cheers,
Toby

[1SOW]

I would suspect a 165gr bullet at the same speed as a 130PF 9mm would lose more velocity than the 9mm due to  the BC of the smaller diameter 9mm bullet.

[tdogg]

You're correct, the BC of the 9mm is 0.159 vs the 40 at 0.135.  So the short answer is it really doesn't matter that much unless you are trying to just squeak by above the PF floor.

Cheers,
Toby

[IDescribe]

I looked at ballistic charts, and for 9mm, it's between 0.5 and 1.0 PF less at 15 feet.

Find your most accurate load between PF 132 and 137 and shoot it.

[1SOW]

Did I win a new hat?

[IDescribe]

Are you finally letting us give you a new hat? 

[eastman]

Did I win a new hat?

didn't you already win a new hat?

[jameslovesjammie]

Did I win a new hat?

didn't you already win a new hat?

That's what I was gonna say!!!