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ai308

1st shot in a string velocity

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Hi guys,

I've an issue with the measured velocities I'm getting from my AX308 out of a 26" 1/8" .260rem barrel.

I've done some load development and just recognized some weird behaviour when firing a string of 5 rounds out of the magazine:

The 1st shot of each of the series had a higher velocity (about 20 - 30 fps) than the other 4.

When I finished a 5 shot string, I reloaded the magazine with another 5 rounds, prepared the LabRadar and started the next series. The behaviour is repeatable.

 

Here''s an example of one of the series :   ( 43.1 gr RS62 behind a 139gr Lapua Scenar )

1) 2876

2) 2833

3) 2834

4) 2845

5) 2858

 

My 1st thought was about the time the 1st round of the new series is located in the chamber. It could take longer time for me to acquire a comfortable shooting position again and during that time, the powder temp increases.

But I've checked it: hurried up for the 1st round and slowed down for the 3rd. Same behaviour. Also paid attention to consistent pressure on the bipod.

As said, I've loaded from the Magazine. Maybe I should try with the single shot adapter?

I've no idea, what else could cause this.

 

My .308win also behaves in a similar way: Whenever I pause shooting for some minutes, the next shot will be around 20-30fps faster.

 

Does someone have an idea?

 

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I've just completed my first ES test with a magneto

14 shots with a 200 ftper sec spread !!!

First shot was the culprit at 150 ft/sec slower than the average (which was 2654)

In good 'statistical fashion' I deleted the first cold bore shot (note SLOWER) and the ES dropped to 60

I'll do it all again shortly but was wondering whether there is an explanation (other than me) for the slower first shot

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Ai 

if you cannot get to the bottom of it shoot all your test loads in a round robin method after a couple of foulest to settle things down- don’t use a magazine, single feed.

Bowjijihn- can I assume the barrel is clean ie has some reidual lube I it ie les resistance so lower pressure =!lower velocity, if so what does discounting the first 2 or three shots donto your es as IMHO 60 FPS is abut in the high side?

T

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I think the barrel was OK 

12 shots in all - not 14 - first one was deleted - it was 25** something

Shots fired are as follow

2648

2640

2635

2645

2656

2669

2688

2669

2644

2670

2640

Sorry - ES was 53 with SD 16.8

Deleting the first 3 shots barely changes the SD - 16.3

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18 hours ago, ai308 said:

Hi guys,

I've an issue with the measured velocities I'm getting from my AX308 out of a 26" 1/8" .260rem barrel.

I've done some load development and just recognized some weird behaviour when firing a string of 5 rounds out of the magazine:

The 1st shot of each of the series had a higher velocity (about 20 - 30 fps) than the other 4.

When I finished a 5 shot string, I reloaded the magazine with another 5 rounds, prepared the LabRadar and started the next series. The behaviour is repeatable.

 

Here''s an example of one of the series :   ( 43.1 gr RS62 behind a 139gr Lapua Scenar )

1) 2876

2) 2833

3) 2834

4) 2845

5) 2858

 

My 1st thought was about the time the 1st round of the new series is located in the chamber. It could take longer time for me to acquire a comfortable shooting position again and during that time, the powder temp increases.

But I've checked it: hurried up for the 1st round and slowed down for the 3rd. Same behaviour. Also paid attention to consistent pressure on the bipod.

As said, I've loaded from the Magazine. Maybe I should try with the single shot adapter?

I've no idea, what else could cause this.

 

My .308win also behaves in a similar way: Whenever I pause shooting for some minutes, the next shot will be around 20-30fps faster.

 

Does someone have an idea?

 

I keep looking at this sampling and can't see a huge problem with it. The first shot is closer in speed to the fifth shot than the fifth is to the 2nd and 3rd shots. (18 fps compared to 25 and 24 fps respectively) To me, the 2nd and 3rd shots are the ones you should suspect as being off the mark, regardless of the first round being faster.

Of course, that's just an observation of one, five shot string. A much larger sample would be telling. What "trigger" are you using for your LabRadar? Rifle or Doppler?~Andrew

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On 8/2/2018 at 7:39 AM, ai308 said:

Hi guys,

I've an issue with the measured velocities I'm getting from my AX308 out of a 26" 1/8" .260rem barrel.

I've done some load development and just recognized some weird behaviour when firing a string of 5 rounds out of the magazine:

The 1st shot of each of the series had a higher velocity (about 20 - 30 fps) than the other 4.

When I finished a 5 shot string, I reloaded the magazine with another 5 rounds, prepared the LabRadar and started the next series. The behaviour is repeatable.

 

Here''s an example of one of the series :   ( 43.1 gr RS62 behind a 139gr Lapua Scenar )

1) 2876

2) 2833

3) 2834

4) 2845

5) 2858

 

My 1st thought was about the time the 1st round of the new series is located in the chamber. It could take longer time for me to acquire a comfortable shooting position again and during that time, the powder temp increases.

But I've checked it: hurried up for the 1st round and slowed down for the 3rd. Same behaviour. Also paid attention to consistent pressure on the bipod.

As said, I've loaded from the Magazine. Maybe I should try with the single shot adapter?

I've no idea, what else could cause this.

 

My .308win also behaves in a similar way: Whenever I pause shooting for some minutes, the next shot will be around 20-30fps faster.

 

Does someone have an idea?

 

Over how many five shot strings did it display the same trend? 

An ES of 43fps, that is entirely possible depending on many variables, does the Labradar show a similar pattern with any other rifle you have?

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Keep in mind ES only tells you the difference in velocity between two shots. It tells you nothing else. Regards JCS

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10 hours ago, Andrew said:

I keep looking at this sampling and can't see a huge problem with it. The first shot is closer in speed to the fifth shot than the fifth is to the 2nd and 3rd shots. (18 fps compared to 25 and 24 fps respectively) To me, the 2nd and 3rd shots are the ones you should suspect as being off the mark, regardless of the first round being faster.

Of course, that's just an observation of one, five shot string. A much larger sample would be telling. What "trigger" are you using for your LabRadar? Rifle or Doppler?~Andrew

Rifle. 

I did publish just one 5 shot string as an example. Have shot 10 of them in sum, each with 0.1gr increase of RS62.

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3 hours ago, Big Al said:

Over how many five shot strings did it display the same trend? 

An ES of 43fps, that is entirely possible depending on many variables, does the Labradar show a similar pattern with any other rifle you have?

I did 10  5 shot strings, all showing the same behavior.

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22 hours ago, terryh said:

Ai 

if you cannot get to the bottom of it shoot all your test loads in a round robin method after a couple of foulest to settle things down- don’t use a magazine, single feed.

Bowjijihn- can I assume the barrel is clean ie has some reidual lube I it ie les resistance so lower pressure =!lower velocity, if so what does discounting the first 2 or three shots donto your es as IMHO 60 FPS is abut in the high side?

T

I' will try single feed tomorrow.

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23 hours ago, No i deer said:

Cold bore shot

If it would happen only for the 1st shot, it would be an explanation. But it happens for the 1st shot of every 5 shot string.

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That does sound odd

only fire the 2nd 3rd 4th and 5th shots 😁

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From my experience from testing is the first shot is the fastest shot of the string.i would be interested on knowing if the point of impact is in the same place everytime you shoot your cold bore shots.its something I've not tried.this photo was the last time i checked the zero with my 284 with 162gr amaxs at 100yds.i couldn't see the point of firing anymore shots.i think the first shot the higher one

20180803_190714.jpg

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9 hours ago, jcampbellsmith said:

Keep in mind ES only tells you the difference in velocity between two shots. It tells you nothing else. Regards JCS

JCS

?? Can you expand on this as I’ve always viewed an SD using only say a 5 shot sample as pretty meaningless (statistically speaking), so view both as an indicator of a good load if it can be repeated 

Terry

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I use the ES for 2 reasons.firstly to see how my reloading is going and secondly for figures to enter in my bal cal. 

Standard deviation figures just make it  look better then the actually are.why use SD when Es's are tighter tolerances which is what we're after.

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On 8/3/2018 at 5:59 AM, ai308 said:

 

 

9 hours ago, No i deer said:

I use the ES for 2 reasons.firstly to see how my reloading is going and secondly for figures to enter in my bal cal. 

Standard deviation figures just make it  look better then the actually are.why use SD when Es's are tighter tolerances which is what we're after.

SD is the more accurate tool for statistical evaluation when a decent sized sample it used. What  SD does is justly minimize the outlier on a large sample. If you fired 100 rounds over a chronograph and 99 of them were within 10 fps but one, for whatever reason, was 60 fps out, which would be the most accurate number to judge the quality of your ammo? The ES of 60?  Or the SD of (perhaps) 5 fps.  Admittedly, SD is only of real when a large sample is used but then, that is true for all statics. The greater the sample, the greater the accuracy.~Andrew

 

PS: Sorry for the added box above. Couldn't get rid of it from an earlier, abandoned response

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If i put 5 shots through my chrony and get single figures ES's that's good enough for me.my 6.5x47 almost never makes double figures.accuracy is superb at 100yds  but I cant seem to nail a stunning COF at 1000yds snaps yet and I can get siting shots markers can overlap 🤔.I wish I could get my 7mm saum to shoot single figure Es's.pushing the cartridge to it's limits isn't the best way forward for single figure Es's.

The OCW is but I've not worked that out yet.it may just be a case of 1gr less or so powder 

Test at 100yds prove stunning.shoot out to 1000 to good effect.going from low double to single figures will I see a difference.

dunno......!

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On 8/3/2018 at 2:02 PM, ai308 said:

I' will try single feed tomorrow.

Unfortunately, the range was closed.

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On 8/3/2018 at 2:50 PM, No i deer said:

That does sound odd

only fire the 2nd 3rd 4th and 5th shots 😁

Yes, I had the same idea. 😁

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I guess it's one of the downsides to owning a Accuracy International 🎣.

We're probaly going to need a bigger boat on this one 🚣‍♂️

It sounds like the chronograph to me bud.its probaly a female one that's why is temperamental 😁.no way would your first shot be 150fps faster than the others.it must be down to the great run of hot weather upsetting these chronos. 

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Verify the instrumentation

Borrow a magneto speed.

Fire with both magnetospeed and labradar at the same time.

See if the trend persists

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How will that verify instrumentation? If different, which one will be correct?

One suggestion might be to set the LabRadar to doppler triggering. See if that changes anything. To the OP: Is the rifle suppressed?~Andrew

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Andrew 

concur that SD is a better indicator but I do not think a 5 shot string is a large enough sample, think you need to do something like 20 to make it ‘believable’ ie you’d have confidence in your SD value?

if you look at it this way, the SD is sort of predicting what your load will do, so if you plan on say using the load for 1000 rounds is a 5 shot sample a good indicator?

least that’s my take on it, someone out there’s bound to a statistician :) and chip in?

T

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4 hours ago, Andrew said:

How will that verify instrumentation? If different, which one will be correct?

One suggestion might be to set the LabRadar to doppler triggering. See if that changes anything. To the OP: Is the rifle suppressed?~Andrew

Because if the trend of a fast first shot shows up on both instruments then it is a reasonable assumption that this it is a real phenomenon even if the absolute velocity values on the two instruments are different.

If the trend on the first shot does not show up on both then this increases the probability that the trend is itself suspect. Given this is an unusual phenomenon it would then be reasonable to be a bit suspicious of the data from the Labradar.

Occams razor.

It could just be a statistical anomaly of course in which case shooting a heap of groups would see it disappear

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