How Does OAL Length Affect FPS/PF ?

[bruce282]

I crono'd some 9mm loads out of my G34.

Rem 147gr FMJ

3.2gr TG

WSP Primer

Used brass

OAL 1.117~

If I increase the OAL to 1.147~ what effect will it have on my current 130 PF ?

Thanks

Edited July 28, 2008 by bruce282

[NogNog]

Considering you are using exactly the same powder and powder charge and bullet weight, lengthening your OAL will likely decrease pressure, velocity and PF of the round. How significant will be the change? It will be fully dependent on how long you will increase your OAL. In theory, the answer is YES.

Edited July 28, 2008 by NogNog

[SA Friday]

Longer oal can give you less chamber pressure and less velocity with the same amount of powder. I try to determine my OAL based on the mag and chamber with a new round before I built the loads. OAL's have much more to do with reliable feeding than it does for the pf, but it affects the pf. So, determine what will run reliably if possible and then work the load up to pf from there. I try to run my G34/17 reloads at about 1.125 OAL regardless of the bullet, and find that length pretty much works with all the different weights and profiles. Yo might be pushing your luck trying to go to 1.147 for reliable feeding.

BTW, the limiting factor for your rounds OAL will not be the chamber, but the mag in a G34/17.

[vluc]

I did the same when I started reloading. Found the OAL that the gun likes the best, and that fit ALL the time, worked the charge back from that.

SAF is right, if it will not fit in the mag correctly, it doesn't matter if it does in the chamber!

[steveknapp]

I can see why if you're loading hot loads or fast powders why longer OAL would be good to drop pressure. But my gun feeds long and short rounds equally well. Any reason other than lower pressure I'd want the longer OAL?

I've been loading to ~1.130 (reloading data was around 1.1-1.2). I tried some at 1.15 and 1.11, but not chronoed them yet.

[safenate]

Well, for short- the longest i've loaded MT Gold 115 JHP's was 1.17" and they didn't feed very well. I loaded up some Berry's 124 FMJs to 1.17" and they worked fine. i've most recently done some HS-6 at 1.11", 1.14" & 1.17". The loads are on ammoguide, but the velocities are respectively 1130, 1150, & 1158 from a G34 (stock barrel). Not much difference with the 6.5 gr load. I load my 115 MT Gold JHPs at 1.14" and will load my future MT 121s the same.

[ben b.]

FYI, and FWIW, I recently tested OAL as I worked up a load. 9mm, a 147 gr lead, with Solo 1000 powder. The complete lack of difference astounded me.

10 shots each.

OAL 1.150", 826 fps (sd= 9)

OAL 1.140", 826 fps (sd= 12)

OAL 1.130", 828 fps (sd= 11)

OAL 1.120", 826 fps (sd= 13.6)

Edited August 2, 2008 by ben b.

[Allgoodhits]

FYI, and FWIW, I recently tested OAL as I worked up a load. 9mm, a 147 gr lead, with Solo 1000 powder. The complete lack of difference astounded me.

10 shots each.

OAL 1.150", 826 fps (sd= 9)

OAL 1.140", 826 fps (sd= 12)

OAL 1.130", 828 fps (sd= 11)

OAL 1.120", 826 fps (sd= 13.6)

It would also astound me is that 3 of 4 different chrono tests even with the exact same load & gun gave the exact same average. Not saying it couldn't happen, but suspect to me. Also the sd was lowest with longest load. Were all or some of these slightly compressed loads? Very interesting findings.

MJ

Edited August 2, 2008 by Allgoodhits

[ben b.]

Could have been compressed. It was only 3.2 gr of powder. But I just checked with 3.4 gr (where my measure is set), and it looks like a 1.120" OAL with a long 147 gr TC bullet would at least touch and might compress that. I didn't figure out how to measure that easily, I just eyeballed it quickly.

When loading, all that I changed was the seating stem, so powder throw was the same. I checked and verified while testing that the chrony was recording separate groups of ten (not compiling all shots).

Another time I compared 1.135 .40 vs 1.125 and saw a only few pts fps difference, I think it was 3.

Edited August 2, 2008 by ben b.

[safenate]

i couldn't find a 147 lead, but guestimating a few things - i only saw a 19 fps difference over those lengths in Quickload. The shortest was also 92% full. FWIW, your velocities were about 50 less than those from Quickload also (from 4" barrel).

[ben b.]

...your velocities were about 50 less than those from Quickload also (from 4" barrel).

Every load I've chrono'd with this powder seems to be ~40-50 slower than other data.

I'm thinking I might have a slower batch. No biggie, if so, I'll just want to retest when this is gone. Only 7.75 lbs left!

[steveknapp]

Here's my OAL data in summary form, I shot 20 of each. They were from different "batches" and I always test/tweak the powder measure before each session. So there could be some *slight* variations in the powder...

[Chris]

It depends on the powder. Some powders like Clays are volume sensitive and will happily remove your top end if you have a bullet setback. (Used to happen all the time with guys loading 220gr .40cal bullets to the old major PF) Other powders are not volume (or temp) sensitive.

As far as what powders are/aren't, I have no idea. Just be careful with what you load and work up from the minimum published loads.

[al503]

FYI, and FWIW, I recently tested OAL as I worked up a load. 9mm, a 147 gr lead, with Solo 1000 powder. The complete lack of difference astounded me.

10 shots each.

OAL 1.150", 826 fps (sd= 9)

OAL 1.140", 826 fps (sd= 12)

OAL 1.130", 828 fps (sd= 11)

OAL 1.120", 826 fps (sd= 13.6)

Were you using the same headstamp or mixed?

[ben b.]

Were you using the same headstamp or mixed?

Yes, I should have reported that. I use mixed cases. The SDs are rather nice, I think.