Impact of OAL on velocity

[Maximinus Thrax]

Question regarding OAL in pistol calibers and its impact on velocity. First, I'll set the scenario so you can be on the same track that I am.

Goal: Load MO Bullet Co 124gr Lead Round nose projectiles using HS-6 and Titegroup for 9mm. It's the bullet/powder combo I have so here we go...

Problem: Lee Modern Reloading, Hodgdon.com, and Hornady don't have that EXACT combo. However, Lee Modern reloading 3rd edition on pg 510 has data for 125gr projectiles.

Theory: Use the data for 125gr projectiles,apply it to 124gr and keep the OAL longer (as long as it passes "plunk") and stay away from the min/max charges.

Results: loaded and fired at 1.140" and recorded velocities all above 1000 fps which, according to my understanding of power factor for 124gr, is my goal. Cool.

That said, I noticed some issues feeding so, staying above the min OAL I opted to drop it to 1.130 with the same powder charges. I expected pressure to change so I kept an eye on brass and pressure signs. However, velocity averages were lower by an average of 20-30 fps for every load except 1 where velocity increased by about the same range (20-30fps). Pressure signs were most evident on this particular load so won't be going there again.

The part that confuses me is this, given a drop of .010" OAL and approximately the same outdoor temp, why would I see higher pressure but a drop in velocity of an average of 20-30 fps? Lee Modern Reloading did not help me make sense of this so I turn to the collective minds here.


I'll be going back to a couple of the batches that showed no pressure signs and fed reliably for further testing so any further loads will be safe but I'm still perplexed as to why I saw the velocity data that I did.

 

[gregkl]

I can't help you out much here but I'm subscribing to learn.

What pistol are you using? I ran into issues with OAL when I owned a CZ. I had to load them so short that I eventually gave up and sold both of my CZ's. I want to be able to load one round for all my 9's which I can now.

IMO only, I wouldn't think that .010" would cause over pressure concerns if you are not loading to max. But I will be here until others smarter than me chime in.

 

[bigbang]

Did you weigh each powder drop?

How repeatable is your scale and crimp. 20 to 30fps isn't much.

Chronograph cld be +/- 5 to 10fps at that velocity.

Did you shoot both on the same day/temperature?

Sorry for all questions and no answers!

But yes, normally decreasing case capacity will increase pressure and velocity to a point.

.010" shouldn't put you over the edge pressure wise assuming middle of the road loads to begin with.

 

[shibumiseeker]

How OAL affects a round is highly dependent on several variables and doesn't well lend itself to a one-size-fits-all answer. The powder, primer, case length, case style, bullet weight, chamber, barrel tightness, OAL, ALL interact in a complex way to determine how the round fires.

That said, HS-6 and Titegroup are very well behaved powders tolerant of a wide range of factors. 20-30fps is not really statistically significant in the grand scheme of things.

Not that I would recommend going overpressure, but two things:

1. Pressure signs are NOTORIOUSLY unreliable. The only real way to know is with a pressure gauge.

2. Of all the pistol rounds, 9mm in a modern gun is one of the few that are remarkably tolerant of +p loadings.

 

[Maximinus Thrax]

I can't help you out much here but I'm subscribing to learn.

What pistol are you using? I ran into issues with OAL when I owned a CZ. I had to load them so short that I eventually gave up and sold both of my CZ's. I want to be able to load one round for all my 9's which I can now.

IMO only, I wouldn't think that .010" would cause over pressure concerns if you are not loading to max. But I will be here until others smarter than me chime in.

It's a full size Sig P320, it just felt a little "off" when cycling each round. They cycled, but it felt like the longer ones were a little tight and it took a little more force to get each round into battery. Hence, the drop in OAL. The shorter batches felt perfect.

Did you weigh each powder drop?

How repeatable is your scale and crimp. 20 to 30fps isn't much.

Chronograph cld be +/- 5 to 10fps at that velocity.

Did you shoot both on the same day/temperature?

Sorry for all questions and no answers!

But yes, normally decreasing case capacity will increase pressure and velocity to a point.

.010" shouldn't put you over the edge pressure wise assuming middle of the road loads to begin with.

I don't weigh each charge but my Hornady Lock-N-Load powder measure has been phenomenal. I very rarely get any variation using it and would look at my RCBS Rangemaster 750 for variation before my powder drop. I weighed a double charge prior to loading my test batch and go. The results were consistent enough that I'm confident it wasn't the charge. I crimp using a Lee factory Crimp die that, once adjusted, never moves.

Shooting was on different days but outside temps were comparable.

How OAL affects a round is highly dependent on several variables and doesn't well lend itself to a one-size-fits-all answer. The powder, primer, case length, case style, bullet weight, chamber, barrel tightness, OAL, ALL interact in a complex way to determine how the round fires.

That said, HS-6 and Titegroup are very well behaved powders tolerant of a wide range of factors. 20-30fps is not really statistically significant in the grand scheme of things.

Not that I would recommend going overpressure, but two things:

1. Pressure signs are NOTORIOUSLY unreliable. The only real way to know is with a pressure gauge.

2. Of all the pistol rounds, 9mm in a modern gun is one of the few that are remarkably tolerant of +p loadings.

20-30fps is not statistically significant at all and if it had been a velocity increase I wouldn't have been confused.. I'm just confused as to why it happened and see an opportunity to learn something.

1. Pressure signs are all we peons have so...I'll just avoid them.
2. Agreed, it's a very tolerant round and I trust my Sig to handle high pressure loads. I just have no reason to shoot them. If I meet my velocity goals then hitting the target just a little faster won't do me any good anyway.

 

[bigedp51]

The link below applies to all pistols and OAL, read Wobby's posts. A image is worth a thousand words and Wobbly's images are very good.


How to determine Max OAL for a CZ Pistol
https://czfirearms.us/index.php?topic=34225.0

 

[shibumiseeker]

20-30fps is not statistically significant at all and if it had been a velocity increase I wouldn't have been confused.. I'm just confused as to why it happened and see an opportunity to learn something.

1. Pressure signs are all we peons have so...I'll just avoid them.
2. Agreed, it's a very tolerant round and I trust my Sig to handle high pressure loads. I just have no reason to shoot them. If I meet my velocity goals then hitting the target just a little faster won't do me any good anyway.

I wasn't trying to be flip or blow off your concern with trying to learn. What I was trying to convey is that it's very complex and in order to really understand it takes a lot more than can be simply explained. I've spent much of my life studying ammunition loading and I do a lot of testing and I still feel like I barely know it. Each round is different so what may be true for one caliber may not be for another, even when they are similar. There are a lot of people on the net who prefer simple answers and will dogmatically cling to them and repeat it even when it doesn't really apply.

Same with my answer on pressure signs. When it's really obvious then it's really obvious, but some brass/primer/gun/caliber combinations will barely show any signs for something that is close to the maximum a gun can handle, while others will look overpressure even when within SAAMI specs. So while it is not a bad idea to pay attention to pressure signs, they are not the be-all, end-all some internet experts make them out to be. I have demonstrated this with new brass vs the same batch of brass that had work hardened by having 3-4 loadings on them. Using an actual pressure transducer (they actually are not that expensive if you do a lot of development) that showed that the load was within spec, the first loading showed to most "savvy" loaders were clearly overpressure (they weren't) while the ones loaded several times didn't look it at all.

Anyways, my first post wasn't intended to be a dig, and hopefully this is a little more information.

 

[gregkl]

work hardened by having 3-4 loadings .

Not to thread drift, but I'm interested in this. I load my 9 mm until I see something that doesn't look right. I may have loaded a casing a dozen times. I don't keep count. I shoot it, pick it up, add it to my tumbler, add it to my clean can, then my sized can, then my primed can and so on. I catch stuff after tumbling, when I'm resizing and when I'm priming so I do cull casings.

Do I need change up what I am doing?

 

[shibumiseeker]

Not to thread drift, but I'm interested in this. I load my 9 mm until I see something that doesn't look right. I may have loaded a casing a dozen times. I don't keep count. I shoot it, pick it up, add it to my tumbler, add it to my clean can, then my sized can, then my primed can and so on. I catch stuff after tumbling, when I'm resizing and when I'm priming so I do cull casings.

Do I need change up what I am doing?

Nope. Load them until they crack or won't hold neck tension. The latter there may be something you want to check if you don't already. Depending on what type of bullet you are using it can be made worse with work hardened brass.

In one of my brass life tests I was getting 30-50 loadings out of pistol brass (I did .40, 10mm, and .45acp) with the failure mode typically cracking, but with some failing to hold the bullet securely after seating.

 

[Maximinus Thrax]

I wasn't trying to be flip or blow off your concern with trying to learn. What I was trying to convey is that it's very complex and in order to really understand it takes a lot more than can be simply explained. I've spent much of my life studying ammunition loading and I do a lot of testing and I still feel like I barely know it. Each round is different so what may be true for one caliber may not be for another, even when they are similar. There are a lot of people on the net who prefer simple answers and will dogmatically cling to them and repeat it even when it doesn't really apply.

Same with my answer on pressure signs. When it's really obvious then it's really obvious, but some brass/primer/gun/caliber combinations will barely show any signs for something that is close to the maximum a gun can handle, while others will look overpressure even when within SAAMI specs. So while it is not a bad idea to pay attention to pressure signs, they are not the be-all, end-all some internet experts make them out to be. I have demonstrated this with new brass vs the same batch of brass that had work hardened by having 3-4 loadings on them. Using an actual pressure transducer (they actually are not that expensive if you do a lot of development) that showed that the load was within spec, the first loading showed to most "savvy" loaders were clearly overpressure (they weren't) while the ones loaded several times didn't look it at all.

Anyways, my first post wasn't intended to be a dig, and hopefully this is a little more information.

Your post wasn't taken as a dig at all. Thank you for the info you've provided so far. It's exactly why I brought my question here.

I could have easily stopped where I was. I have a couple of batches that provide some options for me to consider but I just can't leave a mystery like that alone. I appreciate your efforts in trying to help me make sense of it.

 

[Maximinus Thrax]

The link below applies to all pistols and OAL, read Wobby's posts. A image is worth a thousand words and Wobbly's images are very good.


How to determine Max OAL for a CZ Pistol
https://czfirearms.us/index.php?topic=34225.0

How in the world did you manage to dig that up? Awesome resource. I learned something tonight, thanks!

 

[71silverbullet]

Could the case wall thickness been different in the one round?

 

[craigkim]

I can't help you out much here but I'm subscribing to learn.

What pistol are you using? I ran into issues with OAL when I owned a CZ. I had to load them so short that I eventually gave up and sold both of my CZ's. I want to be able to load one round for all my 9's which I can now.

IMO only, I wouldn't think that .010" would cause over pressure concerns if you are not loading to max. But I will be here until others smarter than me chime in.

Ive had 3 CZ 9mms, still own an Accushadow Black, but all of them give/gave me fits reloading. For my accushadow, I have to load 115g Hornady FMJs down to about 1.100 oal to get them to cycle. The gun shoots 124g S&B well, but I can't replicate those results on my press. I talked to the guy at Cajun Gun Works about it several years back and he says, "Why do you want to load longer?" Obviously, the theory is that the shorter the distance from the bullet to the lands, the more accurate the round will be, but he felt like this was negligible in a 9mm handgun. As an added benefit, he says, you need less powder in a shorter OAL. At some point, I intend to have the chamber lengthened on the accushadow, as that thing is a laser when it functions.

I have used Titegroup for 45 more than 9mm, but it is a faster burning powder. I assume that powders of various burn rates will be affected differently by varying the OAL? I used to get a lot of blackening of my cases and low pressure signs from titegroup in 45.

Also, when comparing OALs, make sure you are using a comparator and not simply measuring from the case head to the tip of the projectile, which is unreliable.

 

[DadSmith]

I always do a plunk test then back off about 0.003 and load up 5 round batchs until I find the load it likes best. My CZ P10F out of the 3 9mm I own has the shortest oal so all 9mm get loaded to that length so they will cycle in all 3.

 

[Maximinus Thrax]

Could the case wall thickness been different in the one round?

If it were one round, maybe. These were 5 round test batches and the numbers are averaged from each batch.