Unexplained shift in velocity

[lawrence97]

I am shooting a 284 shehane and 183 sierras. My powder charge and seating depth range have been developed at 1000.
Powder has been measured to the kernel(calibrated a&d scale), brass weight sorted, brass annealed, cases sized equally, cases trimmed,bullets sorted by weight/ogive, br2 primers and seated with a hydro press. I've gone over the typical things such as scope, bedding, rings, base, rear bag, front rest, etc and nothing is loose. The barrel is a 4 groove krieger with roughly 320 rds thru it.

The shift in vertical I am seeing at 1000 yards coincides with my increase in velocity. I know I am within the powder charge node and a good seating depth because groups are good and vertical is good if I separate the two distinct velocities I am measuring with my labradar.

The issue I had today started with 5 rds using s&b primers and 54.9gns of h4831SC. My velocity was in the 2820-2830 range (higher velocity than what previous testing yielded)and vertical at 1000 was less than 6". In previous testing my charge yielded 2740fps. I adjust my scope to move the group from the 7-8ring on a 1000 yard fclass target down to the X ring.

After the first 5 rds with S&B primers I shifted to my usual br2's. The rounds were now running 2750 fps and impacting 20" low on target. I adjust scope again and end up hammering the 10 and X ring with less than 6" of vertical. Not sure how a primer would cause an 80-90fps swing in velocity because I have used these primers before and found that they usually don't vary more than 10fps if any.

Suddenly, my velocity shifted up to the 2810-2830 range and the rounds formed a nice group between the 7-8 ring. The shift in velocity measured by the labradar coincides with my drop chart. 80-90 fps meant roughly 20" high on target!

I am using fully prepped lapua brass that I have successfully used in the past and I have not been able to detect any mechanical or Mother Nature factors that could cause this. The rifle shot excellent at 2750 or 2830 but when will it make up its mind and stick to one or the other?

If any of you have any ideas or suggestions I am open to your words of wisdom.... My next test is to not clean the barrel and try 15rds with s&b primers and another 15rds with br2's to see if it hovers between two distinct velocities.

Thanks.

 

[XTR]

You shot S&B's got higher velocity
You shot BR2s got lower velocity

Results from groups and chrono agree.

I'm going with primers here.

 

[ShootDots]

When you changed primers, did you also change lots of powder? I am with you on the primer deal>>>nearly 100f.p.s. variance is too high for primers to make that much difference. However, H4831SC is well known for huge differences between lots of powder. Other than that I don't know..

 

[lawrence97]

I did not change lots of powder, it's still the same 8lb jug. I had already tried the S&B primers before and I was in the 2750 range also.

XTR, I got the same shift in velocity with the BR2's. The Br2's started at 2750 and then switched channels to the 2830 range.

Anything else I might be overlooking?

 

[XTR]

Sounds like you shifted powder lots. Last time I shifted lots of H4350 I lost about 75FPS. If it's not powder lot I'm kind of stumped unless there is some big temperature differential.

 

[lawrence97]

No mixed powder lots. Same 8lb jug. I am definitely puzzled since it is 2 very distinct velocity ranges and if it sticks to one or the other it shoots tight vertical.

 

[riverwolf1]

Air Density could cause a shift this high! What was the temp &humidity like did you have changes in either?

 

[ppaul]

Just a guess, but have you stripped and cleaned your bolt lately and a new firing pin spring?

 

[lawrence97]

All rounds were shot at same temp., same time. It was a 25 shot string.

I have had the bolt assembly apart cleaned off and it is within spec. Not much use on the assembly. It is a bat mb with bix n Andy trigger.

 

[MislMan]

Maybe just the barrel settling in. With only 320 rounds down the tube the velocity of the diiferent loads (primers) have picked up speed. Might have something to do with jam/jump the effect on the burning primer/powder charge. The two different loads seem to shoot similar velocity within their respective assembled group.

 

[lawrence97]

All rounds were using the same load. There weren't two different loads. I only used 5 rds with S&B primers as sighters because in previous testing they run the same as BR2's.
In running just br2's after the 5 sighters, the br2's also jumped into the higher velocity range with no rhyme or reason.

 

[SheepDog]

You are getting a 2 MOA shift with a coinciding shift in velocity. That kind of rules out scope or action and points directly at your loads. A drastic change in environment might be there if the velocity increase was not but your bullets are flying correctly for the velocity they are traveling. I think it's time to look closely at your reloading procedures. Some part of what you are doing is adding velocity changes during the process of reloading.

 

[Lbart]

Lawrence97,
Try this; load your 54.9 grs charge and another 5 with 55.9. Shoot them both. You may find your answer.

I say this since it looks like you are using a digital scale. I love them and used a Sartorius which reads to 0.000 grains with a Redding measure to dump charges and a Dandy Trickler to top off the charge.

When I first got it I would dump a charge of H-4831sc and trickle up but after transferring it to the case…the thought would come to mind, “was that 60.222 or 61.222? I tended to get locked in on the last digits and forget about the first 2. I checked a few times and I was over or under a full grain. Fortunately Adam MacDonald came to the rescue with his Auto-Trickler which automatically trickles up to the correct amount every time. No more doubts.

 

[lawrence97]

Larry, thanks for chiming in!
Yes, when I used to trickle and measure on my a&D scale I did have an issue or two.
Now I am dropping powder with a Prometheus. My Prometheus has been set to drop 54.9. An undercharge with it is 54.89 and an overcharge is 54.92..... it doesn't get much better. Since my last Range session was only 27 rds, I decided to check all charges dispensed from the Prometheus and it was deadnuts.

Sheepdog, in my area (AZ) temps can shift from 20-30 degrees from sun up to mid day. I do understand a major change in environment will cause a shift in velocity as well as barrel heat.
But, all rounds were fired within a 25-30 min window early in the morning at a normal match pace.

 

[SheepDog]

The heat of the day here in the summer in the Walla Walla valley does the same thing. I am a bit puzzled because I would expect the velocity to increase with the barrel temperature but it is going up and down with barrel temperatures that are likely rising all the time. It points to inconsistent ignition but that is unlikely. Is tere any way that there could be some contamination with lubricants inside your cases? Just thinking in print here.

 

[lawrence97]

Lubricants on bullets or cases has been brought up but my cases were annealed, sized, stainless tumbled, dried in a food dehydrator and used several days later. The cases were loaded the day prior and weren't around any contaminants. The bullets were pulled straight out of the box and weight sorted.
The heat of the day would affect me if I had shot early in the morning and then gone back out at noon. My temps in the morning are high 80's and by 1pm temps are as high as 115°. The load was developed a couple of weeks ago in the same morning temp.

 

[seymour fish]

Lubricants on bullets or cases has been brought up but my cases were annealed, sized, stainless tumbled, dried in a food dehydrator and used several days later. The cases were loaded the day prior and weren't around any contaminants. The bullets were pulled straight out of the box and weight sorted.
The heat of the day would affect me if I had shot early in the morning and then gone back out at noon. My temps in the morning are high 80's and by 1pm temps are as high as 115°. The load was developed a couple of weeks ago in the same morning temp.

Lawrence, have you segregated the higher velocity cases for close inspection re: case length, case water volume, and any retained foreign matter/objects ?Have you double-checked seating depth consistency ? Have you used this powder in any other rifle over the same timeframe ? Any chance some magnum primers slipped into the mix ? Who else has access to your reloading room/powder storage ? Does the firing pin fall sound exactly the same on dry-firing ? Seymour

 

[Keith Glasscock]

Here are a couple of guesses that I have.

Carbon. Any chance you developed a carbon ring or carboned up the throat?

It wouldn't make a lot of sense with the velocity initially dropping back down, but I've seen enough strange things to not try to over think things when troubleshooting.

Second thought is bullets. Have you checked the diameters of the remaining bullets from that box? Wouldn't be the first time that someone found bullets from two different sets of dies in the same box.

 

[lawrence97]

Lawrence, have you segregated the higher velocity cases for close inspection re: case length, case water volume, and any retained foreign matter/objects ?Have you double-checked seating depth consistency ? Have you used this powder in any other rifle over the same timeframe ? Any chance some magnum primers slipped into the mix ? Who else has access to your reloading room/powder storage ? Does the firing pin fall sound exactly the same on dry-firing ? Seymour

I do have the cases segregated and the first thing I did after the Range session was to weigh and measure the cases. They were all the same! As for seating depth, the bullets were weight sorted and seated on a hydro press with a Wilson seater. All bullets seated were within .0005 of each other and very consistent. The 8lb lot of powder has only been used for this barrel. I only have two different primers in the reloading area: S&B (gold) and br2's. No one has access to reloading room. The firing pin fall sounds solid.
The ignition is consistent because the ES between rounds is within 20 fps. When i fired my sighters with s&b primers ES was under 10 and the only issue was the higher velocity(2810 Plus). When the br2's were fired, ES was also good but in the 2750 range which eventually fell into the 2820 range.

 

[lawrence97]

Here are a couple of guesses that I have.

Carbon. Any chance you developed a carbon ring or carboned up the throat?

It wouldn't make a lot of sense with the velocity initially dropping back down, but I've seen enough strange things to not try to over think things when troubleshooting.

Second thought is bullets. Have you checked the diameters of the remaining bullets from that box? Wouldn't be the first time that someone found bullets from two different sets of dies in the same box.

Keith, I'm gonna check things out with my borescope. I have had a carbon ring before and many barrels ago! I try to clean after matches or at least between 60-100 rds. When I did have carbon, velocity stayed high and never dropped until an aggressive cleaning was performed.

I have not checked bullet diameter but I did weight sort before loading. I also came to the conclusion that base to ogive was also consistent because I did not have much variance in base to ogive after seating. I had no more than .0005 between all rds. Used.

I would also think that a larger diameter bullet would show a higher pressure reading on the hydro press but the press showed all rounds to be within 10-15 psi. The rounds that had significantly higher or lower psi showed NO correlation on target at 1000 yards.

From most that have chimed in and those brains I Have picked elsewhere... it seems as if something large would be needed to get that much of a shift in velocity. Powder charge would have to be almost 1 full grain higher! Some disturbance in the barrel,etc.