OAL and PF

[Bakerjd]

If I go from 1.119OAL to 1.150OAL what effect would it have on power factor? 

[Alaskapopo]

You need a chrono too many variables to tell you 

[superdude]

As noted, you'll have to test your ammo.

 

That said, my notes show with a 115 gr bullet in a 9mm using 5.0 Silhouette in a 5" barrel going from 1.160 to 1.120 (0.040" reduction OAL) power factor went up 6. 

[Yeti]

You definitely need a chrono.  There isn't a linear relationship.

That being said, the same load with a shorter COL will typically have more PSI than the same load with a longer COL.

 

My experience going from 40S&W MajorPF loads for 2011 use at 1.185" COL to the same load for a G35 at 1.140" makes a +2-5% difference.  Shot in the same 5" 2011 barrel.

Minor PF changes in COL seem to vary much more widely. 

Edited July 24, 2022 by Yeti

[Bakerjd]

That's what I figured guys. Guess I'll do the old load a few up, test and once I get the load where I want it load all of them. 

[ysrracer]

Has anyone mentioned use a chrono, yet?

[Farmer]

I did a small test with a 10mm. Same load just changed length. 
1.270-1202 fps

1.250-1209 fps

1.240-1193 fps

You have to take into consideration that your not only changing the internal volume of the case but also the chamber, ie the jump to the lands. It was also interesting that the shortest had the lowest ES. 

[Bakerjd]

So I went from 1.119" to 1.130". PF over 10 rounds was 3PF higher. Lol. So really no change at all. 

[superdude]

According to your numbers, you lengthened the OAL. This made the bullets faster?

[Sarge]

11 hours ago, superdude said:

According to your numbers, you lengthened the OAL. This made the bullets faster?

Wondered the same thing. Must be a typo somewhere 

[Bakerjd]

Yes it did. Also confirmed at Ryan Rocks Charity Blast. At 1.119OAL I've consistently been at 130 all year. Multiple majors between Michigan, Ohio, and PA. At Ryan Rocks it was 135PF. 

[Farmer]

I think it has to do with the jump to the lands causes a psi jump at the peak burn, in other words, timing. If you look at my numbers the middle of the road is the fastest and by measuring the brass it also had higher psi. I’ve noticed this for a long time on rifle rounds. 

[deerslayer]

Was there a big difference in outside temperature when you chronographed the different OALs? 

[Bakerjd]

Uh so..... this was a faulty test. I switched guns between OAL changes. The tanfo is in fact about the same PF. The A01 however has a bit faster barrel. Thus is why I have had higher PF. 

 

So in the end going from 1.119 to 1.130 made negligible difference in PF. It has however seemed to fix my feeding issue in the tanfo.