Sudden 70fps drop in muzzle velocity after 150 rounds?

[Senor_Barney]

Tl;dr. My muzzle velocity went from ~2820 for first 150 rounds to ~2750 for next 30 rounds. Same charge 41.6gr H4350. Was it the weather, me cleaning barrel, or natural?

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I'm about to call it quits with reloading, sell this rifle, and move to Norway . Just really don't know what I'm doing...

I have an Accuracy International AX 2015 in 6.5cm with brand new 26 inch factory AI barrel. I. Reloading with H4350 and Berger 140gr Hybrids. I really just dont know what Im doing....read on

For the first 30 rounds, I shot some left over factory ammo I had on hand. I wasnt really caring then about accuracy or velocities. Just plinking for fun >>> Hornady 140gr ELDM. Velocity was consistently ~2830fps. Density Altitude 1,000 and temp 65F

For the next 30 rounds, I did a velocity ladder test of shooting three strings of 9 rounds from 40.9 to 42.3gr. which included three cold bore + sighting. Looked to be a "node" in the 41.5-41.7 and maybe again in 42.1-42.3. Density altitude 731ft and temp 59F.

For the next 60 rounds, I settled on 41.6gr H4350 and did seating tests with Berger 140gr Hybrids ranging from 20-85 thou off. Below is a target from one seasion...Velocity was consistently in the 2830 fps range. 350 Density altitude and temp 60F

....FINALLY...at 150 rounds down...I have the crazy idea to clean the barrel. This included 4-5 wet patch passes down (assume I did all proper bore guide, etc) until no more black and enough passes to dry it out. This is actually the first time ive cleaned a bolt gun barrel because usually I just am told to leave it....for whatever reason...I didnt.

Today I went back for another 30 rounds of seating test from 60-85 thou off. I upgraded to a Fx-120i and wanted to see if my velocities were more consistent having spent $800 for to upgrade from my $150 frankford arsenal scale.
For whatever reason, the velocities fell from consistently being in the 2820-2830 fps range with 41.6gr H4350 in my 26in Bartlein to an average of 2760fps. Density Altitude was negative 300 feet and Temp was 48F. Lower and colder than I've ever shot before.
This wasnt just thre first 3-5 rounds after cleaning or cold bore. This was over 30+ rounds.

Questions
1) Is it typical for a barrel velocity to slow after a barrel cleaning? If so, how do I avoid and will I get that velocity back?

2) Did that density altitude and temp have any impact on muzzle velocity? I can see how it would impact DOPE and ballistics at distance, but velocity at the muzzle I'm not sure how it can impact that.

3) I hear a bunch about barrel "stabilizing", But I've not seen it before to know how many rounds down the barrel it takes. With 3 barrels in my life, each has been stable from round 1 to round 250ish. Never seen a move up or down without a dramatic increase in temp (i.e. shooting in 88F vs. 62F). Is that really a thing...and is it more common for barrels to speed up or slow over time? Mine slowed from one session to anothet instantly...wasnt even gradual which is why Im curious if cleaning it or the weather was the culprit.

4) At what point does it make sense to even start load development? Assuming this "break in" barrel and "stabilizing" is real, do I seriously need to waste $300-400 of components just to get to a point where I can spend another $150 to do load development? Is that really what the advanced reloaders among us do? If so, I need to quit now.

 

[6.5SH]

A few details missing.

What do you use for a chronograph?
Did you ever use check weights with your Frankford scale? Have you compared any check weights between your scales?
Are these batches of reloads from the same lots of components?
Did you reload in wildly different periods of humidity?

As to the temperature differences stated it would likely be ~5 fps or less difference unless your ammo was heated well beyond ambient during your faster session.

 

[OREGUN]

I might have missed it, which Chrony?

 

[Jrb572]

Change brand of primers?

 

[OREGUN]

Likely not temps or DA related Or cleaning for that matter. New scale might be measuring differently, or different lot of powder? MAYBE different primer lot (unless you switched brands or from 400 to 450, maybe?).

if It is consistent, maybe it doesn’t matter much…you are right in the wheelhouse for h4350 and 140s at either speed…

 

[Senor_Barney]

A few details missing.

What do you use for a chronograph?
Did you ever use check weights with your Frankford scale? Have you compared any check weights between your scales?
Are these batches of reloads from the same lots of components?
Did you reload in wildly different periods of humidity?

As to the temperature differences stated it would likely be ~5 fps or less difference unless your ammo was heated well beyond ambient during your faster session.

Good questions that I should have noted...

Magnetospeed V3 mounted to pic rail via Wiser Precision mount

Last 30 rounds charged using Fx-120i and calibrated via included weight

Initial rounds charged with Frankford Arsenal were checked with included 50g + 50g weights

The poweder is all from the same 8lb jug of H4350...bullets as well...primers from same 1K brick. The brass was bought 1x fired off Hide and this is probably #3 fired with annealing done via Bench source annealer.

Its been pretty consistently cold here in northern california for the past few months. I would say the earlier rounds were loaded when it was cold and less humid...while the most recent 30 (the slow ones) were loaded when it was cold and high humidity outdoors, but always done in my garage so more or less stable.

 

[Senor_Barney]

Likely not temps or DA related Or cleaning for that matter. New scale might be measuring differently, or different lot of powder? MAYBE different primer lot (unless you switched brands or from 400 to 450, maybe?).

if It is consistent, maybe it doesn’t matter much…you are right in the wheelhouse for h4350 and 140s at either speed…

Got it.

In my AT it was 42.3gr H4350 would get me 2800fps out of 24in Factory barrel. In my AX it is 41.6gr in the 2750fps range in 26in barrel. If that is the consensus velocity for that charge weight then i wont sweat it and instead start to wonder if it was my Frankford Arsenal actually "overcharging" this whole time before. Unfortunately, I just sold it and don't have any way to confirm

 

[6.5SH]

Good questions that I should have noted...

Magnetospeed V3 mounted to pic rail via Wiser Precision mount

Last 30 rounds charged using Fx-120i and calibrated via included weight

Initial rounds charged with Frankford Arsenal were checked with included 50g + 50g weights

The poweder is all from the same 8lb jug of H4350...bullets as well...primers from same 1K brick. The brass was bought 1x fired off Hide and this is probably #3 fired with annealing done via Bench source annealer.

Its been pretty consistently cold here in northern california for the past few months. I would say the earlier rounds were loaded when it was cold and less humid...while the most recent 30 (the slow ones) were loaded when it was cold and high humidity outdoors, but always done in my garage so more or less stable.

Rules out alignment and lighting issues with the chrono, so that's good.

Try comparing your check weights on both scales for discrepancies.

The weather and loading in a non climate controlled environment is likely where this changed. If you know the dates the batches were loaded look for the historical humidity levels for your area.

Read this:

How does humidity affect powder?

You may have heard about a relationship between humidity and bullet velocity either in a book, on a reloading forum, or from a crusty benchrest shooter right after saying they get better ES/SD&#821…

chronoplotter.com

 

[Dot3]

Did you verify that what your frankfort arsenal scale "said" was 41.6g was the same on the fx-120i? Drop a charge on the new scale then pour it onto the old scale and see how far off they are.

ETA: Didn't see post #7 about selling the FA. I had a couple scales that didn't read the same. They were fairly repeatable independently, but they varied by a few 10ths from each other.

 

[Senor_Barney]

Did you verify that what your frankfort arsenal scale "said" was 41.6g was the same on the fx-120i? Drop a charge on the new scale then pour it onto the old scale and see how far off they are.

ETA: Didn't see post #7 about selling the FA. I had a couple scales that didn't read the same. They were fairly repeatable independently, but they varied by a few 10ths from each other.

I didnt, unfortunately.....should have.

However, unless the FA at 41.6 was "actually" reading 42.3 could I really be 70fps off if it was 0.2 grain? Is that a possibility you think?

 

[nn8734]

I didnt, unfortunately.....should have.

However, unless the FA at 41.6 was "actually" reading 42.3 could I really be 70fps off if it was 0.2 grain? Is that a possibility you think?

42.3-41.6= .7g.

Maybe I’m missing something? Just read this post not any of your others.

 

[alamo5000]

A few details missing.

What do you use for a chronograph?
Did you ever use check weights with your Frankford scale? Have you compared any check weights between your scales?
Are these batches of reloads from the same lots of components?
Did you reload in wildly different periods of humidity?

As to the temperature differences stated it would likely be ~5 fps or less difference unless your ammo was heated well beyond ambient during your faster session.

I would also add about the brass. The first firing vs 2nd could at least in theory cause a velocity loss.

 

[Senor_Barney]

42.3-41.6= .7g.

Maybe I’m missing something? Just read this post not any of your others.

My FA cost like $200 black friday. The FX costs like $700. I would be blown away that the FA, even at its price, would be 0.7gr off at a stated weight.

If it was...that would definitely explain my speed difference. Rounds 120-150 were loaded with the FA and avg 2820-2830. Loads 150-180 were with FX and averaged 2750ish.

Sudden crazy drop is baffling on such a "young" barrel.

 

[6.5SH]

I would also add about the brass. The first firing vs 2nd could at least in theory cause a velocity loss.

Thought about that as well, but he was starting off with used brass.

 

[nn8734]

My FA cost like $200 black friday. The FX costs like $700. I would be blown away that the FA, even at its price, would be 0.7gr off at a stated weight.

If it was...that would definitely explain my speed difference. Rounds 120-150 were loaded with the FA and avg 2820-2830. Loads 150-180 were with FX and averaged 2750ish.

Sudden crazy drop is baffling on such a "young" barrel.

I’m just using the numbers in that post, feel free to clarify or correct me if I missed something

 

[IHFarmer07]

Most likely your Frankford Arsenal scale was off, but there’s no way of knowing since you sold it.

Does it shoot the same, does it have the same poi and group size?

 

[alamo5000]

Thought about that as well, but he was starting off with used brass.

I must have missed that. So many words here.

In any case I would just carry on and treat the first part as the anomaly. It could be anything. A wrong setting on the Chrono, a heavy load...

I've seen people using electronic scales spill powder and not even know there were a bunch of kernels laying around throwing their measurements off.

 

[Dot3]

I didnt, unfortunately.....should have.

However, unless the FA at 41.6 was "actually" reading 42.3 could I really be 70fps off if it was 0.2 grain? Is that a possibility you think?

At .7, sure, it's possible. It seems the only thing that changed in the process was the scale. Think of it like a comparator if you will. You get a reference number and that's about it. I know, scales are different but same basic concept.

 

[Rocketmandb]

Its been pretty consistently cold here in northern california for the past few months. I would say the earlier rounds were loaded when it was cold and less humid...while the most recent 30 (the slow ones) were loaded when it was cold and high humidity outdoors, but always done in my garage so more or less stable.

It's been really humid in CA over the past couple months (atmospheric rivers and all) - last time I went out, my load was noticeably slower (50 fps or so). Powder had come from an open jug. I'm considering putting one of those humidor packs that keeps humidity constant at 62% into open powder containers (and pre-opening containers too) to keep things constant because we get such wild swings in humidity season to season.

Humidity probably doesn't account for everything you're seeing, but it is a sizable contributor.

Where are you in Nor Cal, btw?

 

[Senor_Barney]

Most likely your Frankford Arsenal scale was off, but there’s no way of knowing since you sold it.

Does it shoot the same, does it have the same poi and group size?

FWIW,
Top is 41.6gr H4350 weighed on the new FX after barrel cleaning. Only three shots...

Bottom is 41.6gr H4350 weighed in the old Intellidropper. Only four shots... similar accuracy.

Accuracy is as good/decent/poor as before, but the velocity falling off a cliff when all you ever hear is barrels "speeding up" as they break in is super puzzling

 

[nn8734]

FWIW,
Top is 41.6gr H4350 weighed on the new FX after barrel cleaning. Only three shots...

Bottom is 41.6gr H4350 weighed in the old Intellidropper. Only four shots... similar accuracy.

Accuracy is as good/decent/poor as before, but the velocity falling off a cliff when all you ever hear is barrels "speeding up" as they break in is super puzzling


View attachment 7780832

View attachment 7780829

Barrels speed up as the throat area smooths over from bullets passing through. Fouling build up also helps to a small extent.

If your prev scale was off then there’s no mystery as to why your mv dropped. I clean my bolt gun barrels every 200 rounds or so. After which ill reconfirm zero, log it then do drop validation (usually at 700+ meters in 100m increments for 6.5 cm)

Keep shooting and taking MV data, noting any significant mv changes. Revalidate your drop data on target at 100m (or 50m) increments out to 1000m or however far you typically shoot so you have a comfirmed drop chart for your AO using current mv data. Then revalidate throughout the year as seasons/temps change.

That’s more important than anything else.
————-
Edited to correct typos

 

[iceng]

It's been covered by the awesome members here.

Scales not calibrated to each other. Barrel running in plus the clean. That's it.

Does it still shoot ? Adjust your calc to the new velo, and roll with it. No big deal. Don't over think it.

 

[spife7980]

Also, sometimes things just change.
Go check it again and see what this third trip aligns with.

 

[Shoots 700's]

same lot of bullets?

 

[CK1.0]

Seating depth + weather could’ve caused that swing.

You said you had started with “20-65 thou off” early on, then later went to “60-85 thou off”…. that plus the other factors (weather + less carbon post cleaning) might have done it.

A longer bullet-jump can lower pressure, which will lower speeds.

That said, I don’t bother with any serious load development until after 150-200 rounds on my barrels. I know many use 100 as the usual accepted norm, but I’ve had barrels continue to speed up still between 100-200… so now I don’t let anything wonky bother me if it happens before 200rds.

 

[HousePlant]

Whoever told you not to clean a rifle barrel is wrong. And load development on a barrel that hasn't broken in yet is kinda iffy.

Agree with above, shoot 200 through it, clean it (including the chamber), then start load development. And don't worry about it... it's learning.

The highest grade factory match ammo can have a 70 fps swing from the same box... not the end of the world <600 yards to hit a target.

 

[Senor_Barney]

That said, I don’t bother with any serious load development until after 150-200 rounds on my barrels. I know many use 100 as the usual accepted norm, but I’ve had barrels continue to speed up still between 100-200… so now I don’t let anything wonky bother me if it happens before 200rds.

Helpful feedback.

I had no idea about this 150-200 round rule. It's almost as though I should go buy a bunch of low-cost, but not dangerously cheap components and shoot 150 rounds so I am not blowing through my match components. I'll start a "real" load dev all over now...and see if things speed up or what not.

 

[2aBaC̶a̶]

Chalk up the first load dev on this barrel as a learning experience and start over using the current numbers as a baseline.

 

[Gtscotty]

Most likely your Frankford Arsenal scale was off, but there’s no way of knowing since you sold it.

Does it shoot the same, does it have the same poi and group size?

Seating depth + weather could’ve caused that swing.

You said you had started with “20-65 thou off” early on, then later went to “60-85 thou off”…. that plus the other factors (weather + less carbon post cleaning) might have done it.

A longer bullet-jump can lower pressure, which will lower speeds.

That said, I don’t bother with any serious load development until after 150-200 rounds on my barrels. I know many use 100 as the usual accepted norm, but I’ve had barrels continue to speed up still between 100-200… so now I don’t let anything wonky bother me if it happens before 200rds.

These or a combo of them seem the most likely to me. It's a big, red flag that your change in velocity perfectly correlated with switching to a new scale. Also, if you chrono'd first with loads 20 off, and settled on loads 85 off, I would certainly expect lower velocities with the latter. Nothing to get too frustrated about, just move on with your new baseline. I'd personally probably load up some at 41.6gr and then a few slightly higher loads that match your old velocity to see which one shoots better.

 

[CK1.0]

Helpful feedback.

I had no idea about this 150-200 round rule. It's almost as though I should go buy a bunch of low-cost, but not dangerously cheap components and shoot 150 rounds so I am not blowing through my match components. I'll start a "real" load dev all over now...and see if things speed up or what not.

Yeah, if possible, use cheaper stuff for the initial 150-200rds, if there’s such a thing these days lol…

I used to use Match Burners, then after 200, I’d switch to DTAC’s… but I don’t think there’s really such a thing as a cheap 6mm bullet anymore. YMMV