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Muzzle Velocity Increasing with each fire

_Raining

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Minuteman
Feb 14, 2017
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I am only neck sizing and trimm after firing. The powder is Reloder 16, Lapua 6.5 Creedmoor Brass, FGMM Small Rifle Primers, 140gr Berger Hybrid Bullets.

The first 2 firings were 10 shots from 38.6 to 42.2 @ .4 gr increments.
The third firing was an OCW done with round robin of the powder charges 38.6 to 42.2 @ .2 gr increments (5 shots each).

Based on the data, the muzzle velocity increases with each firing of the brass. It seems to converge at higher powder charges. I first thought this might be due to the barrel heating up but the muzzle velocities of the round robin don't indicate any increase in velocity due to chamber heat. You would think that the avg MV for the 42.2gr charge on an OCW test would be lower than firing 90 rounds first (42.2gr being firings 90-100 instead of 20,40,60,80,100). The temperatures for the two days were similar 80-90 F for the third firing and 76 - 85 F for the second firing. I find it hard to believe that the temperature insensitive RL-16 powder is thrown off that much by less than 10 degrees, not to mention the first firing was lower then both and had temps of 80 - 89 F. My first thought was it took energy to expand the virgin brass to the chamber so that took away from MV, but after the second firing would it still be stretching significantly? Or is this a product of work hardening the brass (I do not have an annealer yet) causing increased neck tension?

Here is the google doc excel sheet with all my data and graphs, let me know if you think I am missing something and I'll see if I noted it down. From what I understand temperature is what effects muzzle velocity but I also have notes of density altitude.
https://docs.google.com/spreadsheets/d/1qmPC4-HVH1wtLMy8uW3QtkAOOy0yZa2jQKpPUqJFYpw/edit#gid=0

POI's for all three firings seem to follow the same pattern, so at least I have that:
http://imgur.com/lzvNErx


 
I don't think work hardening would create neck tension increases that would cause the greater muzzle velocities - I think you would notice increased stiffness if you reload on a single stage press. Off the top of my head I'd say perhaps if you started with a clean barrel and if you didn't clean that fouling the bore could explain an increase, but I would think your muzzle velocities would have stabilized by the time of your second reloading. It could also be that the brass is now fully fire formed and resulting in higher chamber pressure. Perhaps combination of both. Reload a sample set and reload a fourth group and see if MV has stabilized.

At least your groups look good and SD is low for the most part.
 
Brand new barrel with less than 200 rounds on it? If so is likely just the barrel speeding up as it breaks in.
 
Yep, what Sheldon said. If it's a new barrel it's just speeding up a bit
 
Yep, my Kreiger barreled 6.5 picked up nearly 150fps within the first 100 rounds (albeit quite a bit hotter round than the Creedmore with 65.5gr of Retumbo behind a 140gr Berger). New barrels are notorious for this, especially cut rifled barrels.

Sent from my SAMSUNG-SM-G891A using Tapatalk

 
I would have to look at my notes to be exact but I believe:
First day was 60 rounds of factory ammo.
Second Day was 30 rounds of reloads (cartridges wouldn't feed any longer till cleaned).
Third Day was 30 rounds of reloads (cartridges wouldn't feed any longer till cleaned).
Fourth Day was 100 rounds of factory ammo (wanted to make sure feeding issue wasn't rifle related).
Fifth Day was 40 rounds of reloads, no feeding issues but I will not be trying IMR4350 again.
Sixth day was 100 rounds of Virgin Brass at 38.6 to 42.2gr, 10 each.
Seventh day was 100 rounds of Once Fired Brass at 38.6 to 42.2gr, 10 each.
Eighth day was 100 rounds of Twice Fired Brass at 38.6 to 42.2gr, 5 each (.2 gr increments shot round robin).

So about 500/600 rounds through her. I cleaned before shooting the Sixth day but not vigorously. My next plan is to try the once fired brass from the IMR4350 set and see if those velocities match the once fired data I already have (They are from the same lot of brass).
 
Well, you've got some pretty meticulously collected data, kudos for that. If it's not the barrel speeding up or something like lack of cleaning leading to carbon buildup (your posts suggest those are not the cause) then I'm a bit at a loss. I think perhaps it's a combination of some of the things already mentioned above. Increased neck tension from work hardening brass and more fully formed cases (no energy going to brass expansion, if that even matters). Only other thing I can think of is if you're switching lot #'s of powder if you're working through multiple 1# jugs with different lot numbers.

As a side note, this is why I'm a big fan of setting up repeatable reloading processes. FL size every time so the case dimensions are the same every time. Anneal every time so the neck tension is the same every time. Buy components in bulk so I've got one lot of powder, bullets, primers to shoot out the barrel. . I want the third firing on my brass to shoot the same way as the 15th firing.

PS.... don't shoot your barrel out with all this testing. :)
 
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Barrels are disposable. An investment in knowledge pays the best interest.
 
For what it's worth, I ran into an issue like this with my SAC chambered. 284. Every successive loading of brass sped up until I had erratic velocities. I had too tight of neck, I couldn't even fit an expander mandrel into the mouth of a fired case. So turned the necks down and all problems went away..
 
When seeing speed increases think pressure. Something is causing it to increase. Neck tension increase due to hardening of necks, even increased fouling in the barrel might cause the pressure increase that translates to speed.

I like to anneal cases after every firing and also clean more often than "several hundred rounds". Not necessarily a 'bare metal' clean but maybe a wet patch followed by 2-3 dry patches, maybe every 50 to 100 rounds. For my shooting it seems to yield some real consistent performance without a ton of work. Annealing after every firing actually simplifies my brass management. Shoot a box, set the fired brass on the "to be annealed" shelf, use a second or third box of prepped brass, doing the same after firing, then just anneal and prep 100-150 pieces at a time. Right now I'm doing that with 250 pieces of 6.5 Creedmoor brass and it just means an annealing session every couple weeks.

 
Well, you've got some pretty meticulously collected data, kudos for that. If it's not the barrel speeding up or something like lack of cleaning leading to carbon buildup (your posts suggest those are not the cause) then I'm a bit at a loss. I think perhaps it's a combination of some of the things already mentioned above. Increased neck tension from work hardening brass and more fully formed cases (no energy going to brass expansion, if that even matters). Only other thing I can think of is if you're switching lot #'s of powder if you're working through multiple 1# jugs with different lot numbers.

As a side note, this is why I'm a big fan of setting up repeatable reloading processes. FL size every time so the case dimensions are the same every time. Anneal every time so the neck tension is the same every time. Buy components in bulk so I've got one lot of powder, bullets, primers to shoot out the barrel. . I want the third firing on my brass to shoot the same way as the 15th firing.

PS.... don't shoot your barrel out with all this testing. :)

I tried the 6.5 Guys 10 shot load development with great success. Took a total of 30 rds my first time but I bet I get there in 20 next time. Basically, they only look at velocity nodes and not groups. Tight velocity = good groups. Sure removes a lot of variables for error....excellent feedback, especially on annealing every time...