Increasing Velocity Across 3-5 Shot Strings

[wirehead]

Good Folks of R/S,

I’m baffled and bamboozled… bewildered and befogged…

I’ve been working on load development for a Win Model 70 in 7mm Remington mag - rifle has a 26” sportier barrel. The load I’m working with is IMR 4350 and 150 gr Accubonds with latest load being 62.6 gr of I 4350. Seating depth is 0.025” off lands. The issue: I’m seeing a systematic increase in velocity with each subsequent shot when doing 3 or 5 shot tests. The average increase is ~20 FPS and each shot is higher velocity than the previous. I monitor time between shots and always wait 3 minutes but typically 4 to 5. I may also be seeing a consistent point of impact shift subsequent shots compared to the cold bore but I believe there are not yet enough data to concluded such. Average group size is 0.75 to 0.9“ at 100 yd.

I have done previous load tests with 140 gr TTSX with H100v and 162 ELD-X with H4831 though ES was horrible with the H100v and I was not burning powder efficiently with the ELD-x and H4831 loads, respectively.

My next step is to run a 3 or 5 shot group letting barrel cool to cold bore each time… I need these data to confirm MV and POI so to properly conclude load development (before going to MT in 3 weeks ).

Thoughts greatly welcome!

 

[LaHunter]

What is the ambient temp that you are shooting in?
Shaded / covered shooting position or out in the sun?
Your ammo could be heating up as it sits out in the warm temps, which could increase pressure / MV as it gets warmer.

 

[Wrench]

Shoot it at 700ish and see what the poi tells you.

There's a lot of tail chasing that gets done over chronograph data. Our sample sizes are so small that it's very hard to get accurate data.....but when we're talking about hitting a deer or elk lung, the tolerance can grow tremendously and still maintain lethal poi.

 

[EdP]

Are you perhaps taking more time to take the shot so that the cartridge is sitting in the chamber longer and heating up? I find 3 minutes works fine under all but the hotest summer conditons but if I don't get the shot off on the first try (breath). I pull it and try with a fresh one. La Hunter's point could also be the issue

 

[wirehead]

Thanks so much for all the thoughts and ideas!

Ambient temp has been in 60s and 70s with the shooting bench in the shade. My bullets set out on the bench in a cartridge tray. With the relatively good temp stability of IMR-4350 (~0.6 FPS / * F), that would be powder temps of 100 to 130 *F… I’ve not observed cartridge temps that high and would also be surprised if the chamber was hot enough to raise temps that much but I will absolutely study cartridge temp for the next shot string.

Could something tied back to barrel temp (eg thermal expansion of barrel and thus lower engraving pressure) be contributing?

 

[Rich M]

I get the same thing with a rifle or two. Cold bore is 50 fps slower. Critters still seem to fall over.

 

[wirehead]

I get the same thing with a rifle or two. Cold bore is 50 fps slower. Critters still seem to fall over.

But you’ve found cold bore MV to be consistent? I’m ok if it’s just a cold bore thing - I can deal with that!

 

[LaHunter]

How many rounds fired through this barrel total?
How often are you cleaning (round count between cleaning)?
Some barrels need several rounds fired through the barrel after cleaning for the MV to stabilize.

 

[wirehead]

How many rounds fired through this barrel total?
How often are you cleaning (round count between cleaning)?
Some barrels need several rounds fired through the barrel after cleaning for the MV to stabilize.

Rounds fired are on the order of 100+. I’m not in the “clean often” camp for rifle bores. I’ll run a patch or 3 with decoppering solvent once every 30 rounds. It has been ~15-20 rounds since last cleaning.

 

[Lawnboi]

3-5 shots is statistically irrelevant in what your doing. If the rifle is grouping well load 20-30 and see what they really do before changing anything.

With 100 rounds in your barrel may not even be broken in.

 

[Rich M]

But you’ve found cold bore MV to be consistent? I’m ok if it’s just a cold bore thing - I can deal with that!

Yes cold bore and then 2nd shot velocities are very consistent.

 

[Wrench]

3-5 shots is statistically irrelevant in what your doing. If the rifle is grouping well load 20-30 and see what they really do before changing anything.

With 100 rounds in your barrel may not even be broken in.

100%.

This may help explain.

 

[BjornF16]

100%.

This may help explain.

Is that Form?

 

[mt100gr.]

100%.

This may help explain.

 

[Tod osier]

Video is painful to watch the breakdown of 3 vs 5 vs 10 vs even 20 shots. That is a big yikes for me right there. Thanks for sharing that, I need some time for that to sink in.

 

[wirehead]

There is a subtle but important caveat in this… the strapping lad in the video is speaking strictly for data that are randomly distributed (following Gaussian PDF) with stable and well represented influence factors (influence factors being neck tension, charge weight, case capacity, seating depth, temperature, case neck lubricity, and a host of others). This is rarely the case, at least for myself - I and my setup are just not that good (yet). Additionally, there is a difference in the reliability of standard descriptive statistics (eg mean and standard deviation) for characterizing load performance and determining there is a systematic influence factor or trend that needs attention. Actually, the presence of a systematic change is statistical grounds for dismissing the application of standard deviation as the application of SD mathematically implies a Gaussian distribution. For my example, the probability that velocity increases for each subsequent shot as I am seeing is absurdly low - I’ve not taken the time to calculate the probability but I dont need to… something is going on. In the back of my mind, I’m thinking: am I getting inconsistent powder burn that could manifest as a big boom with higher temps or am I just on the wrong path? Am I doing something really stupid in my reloading procedure that could be unsafe or just result in more futile consumption of my shrinking supplies? Is my barrel junk-ish (would love for someone to tell me such so I can justify a rebarrel )… so thanks to everyone for the inputs!!!!

I would delight in applying proper engineering statistics in reloading and shooting… but Nosler and power OEMs are creating constraints and pain. I’m down to handful of 150 gr ABs and I’m having to ration myself. Not to mention the time and $ involved… So my thought is to use smaller sample sets to establish a “direction of goodness” for a given load. Once a “good“ load is established, I’ll shoot the heck out of it at varying distances and conditions to build a proper data set. My effort here is to conclude I’ve got a “good enough“ load prior to locking the load and investing time/components into extended shooting.

I’m going to roll with a primary hypothesis of hot pencil sporter barrel and keep shooting. Will shoot another string soon and share results! Again, thanks to all!

A few more thoughts:

I believe this is why maintaining a good shooting log is important. It allows a person to build data with increase confidence and understanding over time for a given rifle and load(s). I’ve long since been fascinated by the science of reloading - this is what has drawn me in.

Velocity ES is the biggest fallacy in shooting sports… it would take 100s of shots with consistent input conditions (good luck there) to truly generate an accurate estimation of absolute extreme velocity spread. Ever bought factory ammo from different lots and detected a shift? Ammo manufacturers go great lengths to ensure consistency with automated equipment… If their setup creates a shift, think about what we’re dealing with using manual equipment and, at least for most of us, grossly inadequate metrology equipment to properly characterize the inputs. Also the number of shots it would take to properly estimate standard deviation is more difficult than expressed in the video - again, he used a random number generator that assumes just that: pure randomness. Pure randomness is a theoretical concept and rarely seen in practice across the wide world of science and engineering. To accurately estimate SD, significant influence factors must be given a sufficient opportunity to vary across the shots fired to properly estimate an SD that is representative of all future shots taken, which I believe is the intent. For example, if the variation in case capacity due to variation in the setup of the FL sizing die was a significant contributor to SD (not saying it is but just an example), one would need to shoot a number shots reloaded with a FL die setup to properly represent the setup process influence factor.

What I have learned is that SD/ES are good indicators of gross problems like incomplete powder burn or poor control of neck tension… large SD due to incomplete powder burn - particularly when seen near max loads - is an indication of potentially dangerous situation.

What is most interesting to me in that video… I believe he just stated that ladder tests are not useful for finding velocity nodes. To that I 100% agree and see ladder tests mostly useful for finding charge weights that create pressure signs and determining if a given load is even in the realm of usefulness.

Again, I love this stuff… it is what draws me to reloading. Thank you for the discussion. I have long wanted to build a full process map to describe velocity and POI variation. Please keep the good (respectful) dialogue going!

 

[Wrench]

The author of the video has enough match wins that it's worth listening to.

 

[wirehead]

The author of the video has enough match wins that it's worth listening to.

I certainly did. I thought it was great.

 

[RS_Non-Res]

I have long wanted to build a full process map

This.
I have taken a reloading class and the basic process I learned there has been turned on it's head reading the Slide. There's so much additional process and testing detail discussed in this forum that it's pretty overwhelming trying to figure out a priority sequence.

 

[wirehead]

This.
I have taken a reloading class and the basic process I learned there has been turned on it's head reading the Slide. There's so much additional process and testing detail discussed in this forum that it's pretty overwhelming trying to figure out a priority sequence.

No kidding… I chuckle at the complete inadequacy in the amount of info provided by the Lyman reloading manual that I originally referenced when I started. In hindsight, I should’ve started with a .223 rather than high power big game hunting cartridges - realizing how bad i am at reloading would’ve been way less expensive.