sudden velocity increase mystery

[robbin]

Got an interesting problem. 2 months ago I shot a ten shot string, got a solid MV average, which confirmed my previous shooting and ordered a custom dial. I’ve had the dial a month, but haven’t had time to shoot. My step son showed up for a weekend shoot, and his CED crony consistently showed that rifle over 100 FPS faster. I assumed it was just a different crony, but at 800 yards, the dial was very high. I went home, got my crony and it’s average velocity was within 5fps of my step sons. The rifle has 200 rounds thru it, so it should be about broke in. It shot very well, 4.5 inch 3 shot groups at 800 with an 8 pound encore, so I’m tickled pink! But where did the solid 100FPS+ velocity increase come from.

The brass is neck sized only, 4th firing. The only difference is I annealed it before this firing.
Primers are the same out of the same batch. CCI 450
Powder is RL 17 same 5 pound jug. Every round measured to the .1 charge.
NEW batch of Berger 140 VLDs (6.5X47 lapua, neck turn .290 neck) MOLY coated
Bullets loaded long, final seating done by when chambered on both guns.

Shooting in Florida, it’s warmer now than it was 2 months ago, but not by a large difference. Upper 80s vs upper 70s.
We used a sun shade this time so the gun and ammo were NOT sitting in the sun.

NOW HERE IS THE KICKER, I lowered my favorite load by .4 grains because I’ve gotten uncomfortable with the sticky bolt or in this case sticky opening of the frame.

My bench gun shoots the EXACT same round (both guns built with the same reamer) it showed an increase of about 8 fps, which I attribute to the higher temps.

So where did the 105FPS of MV come from WITHOUT getting my sticky bolt back. I’m shooting 105FPS faster with less powder and no sticky bolt that I was getting when the gun was shooting 100FPS slower? If it came from temps, why didn’t my bench gun that shoots the exact same round show the increase? So two guns, shooting the exact same round, I don’t even separate the brass, or the loaded rounds, I use them in both guns.
One shows an 8FPS increase one shows 105FPS increase. If the higher temps made the powder burn hotter, it would have increased my pressure and my sticky bolt would have returned with a vengeance….

Interesting stuff, I’d like to hear your ideas…

 

[zullo74]

ALIENS!

 

[robbin]

I don't know about Aliens, but it sure is strange.... ;D

 

[bheadboy]

temperture

 

[robbin]

I thought that might be the problem, but I shot two guns with the exact same round. One showed an increase of 8 FPS, the other, 102 FPS. I figured the 8 FPS was due to temperture change. The other gun was 10 times as much. Don't think that could be temperture.

 

[ThunderDownUnder]

I think I might be able to answer this question of yours. You have annealed your cases and you allow the chamber to do the final seating of the bullet. Correct?

If thats correct then what I think may have happened is that the annealed case necks now have a tighter hold of the bullet and this has caused the bullet to now be jammed further into the lands. Tighter necks = increased jam = increased pressure = increased velocity with less powder.

I have experienced this same phenomena with my 6.5x47L when I dropped the load off 0.3gr of powder but still picked up 50fps because I jammed the bullet 0.030" more than it was before.

Ian

 

[TonyR]

Charles Ballard describes a similar experience with a 7mm Broughton barrel in this article,

http://www.6mmbr.com/gunweek088.html

 

[cliffy]

A 10 degree increase in ambient temperature can account for 100 fps increase in velocity alone, not to mention barometric pressure changes and relative humidity variations. This is why factory loads cannot compensate for such changes, so handloading to specific conditions cannot be beaten. I load differently for Michigan than for Texas conditions. Cliffy

 

[robbin]

Thanks Tony, great article and sounds exactly like what I've encoutered.
Thanks again,
Robbin

 

[robbin]

I think I might be able to answer this question of yours. You have annealed your cases and you allow the chamber to do the final seating of the bullet. Correct?

If thats correct then what I think may have happened is that the annealed case necks now have a tighter hold of the bullet and this has caused the bullet to now be jammed further into the lands. Tighter necks = increased jam = increased pressure = increased velocity with less powder.

I have experienced this same phenomena with my 6.5x47L when I dropped the load off 0.3gr of powder but still picked up 50fps because I jammed the bullet 0.030" more than it was before.

Ian

I don't think that is it, because I have another gun that uses the exact same round. I use the ammo in either gun, as they were both built with the same reamer. Both are loaded long, and seat the bullet when the action is closed. One has 800 rounds thru it and this one has 200. The one with 800 had an increase of 8 fps, which I attributted to the higher temps.
Thanks for the input...

 

[robbin]

A 10 degree increase in ambient temperature can account for 100 fps increase in velocity alone, not to mention barometric pressure changes and relative humidity variations. This is why factory loads cannot compensate for such changes, so handloading to specific conditions cannot be beaten. I load differently for Michigan than for Texas conditions. Cliffy

Hi Cliffy,
Your comment is true, but I've got a control group for this experiment. Another rifle that shoots the exact same round, I use same ammo in either gun. If it was temps, the increase should have effected both rifles...
Thanks,

 

[robbin]

Charles Ballard describes a similar experience with a 7mm Broughton barrel in this article,

http://www.6mmbr.com/gunweek088.html

I re-read the article, and he had an increase is PRESSURE and VELOCITY.
I've got an increase in Velocity with a DECREASE in pressure..
I'm with the first poster ... ALIENS....

 

[zullo74]

I told you! ;D

 

[robbin]

Spend an hour cleaning your barrel with bronze brushes and some bore paste then chronograph it again.
Lynn

I'm pretty sure my rifles are clean. I don't try to overclean, but I clean often. But if the barrel is fouling, wouldn't pressure INCREASE? The really weird thing about this is
I've got an increase in Velocity with a DECREASE in pressure..

 

[TonyR]

Definitely Aliens!

 

[normmatzen]

I think THUNDERDOWNUNDER is right.

You may have two guns, but I'll bet the throat is different in both for a different "JAM" amount.

 

[hardrock6]

Maybe the moly

 

[camac]

NOW HERE IS THE KICKER, I lowered my favorite load by .4 grains because I’ve gotten uncomfortable with the sticky bolt or in this case sticky opening of the frame.

I am a little confused - Was there an increase in pressure or decrease?? This first post seemed like an increase in pressure - allah the barrel fouling comments - copper, moly or carbon build up on a barrel that probably needed more breaking in (hence Lynns recommendation - which I would still go for personally). If it is a velocity increase with lower pressure it is a bit harder to explain?? Are you sure pressure was lower??

 

[juliomorris]

I recently had the same issue with a 243 shilen barrel, Rl 25 and 115 gr DTAC's. The velocity jumped from 3050 to 3200fps with no pressure signs. At first i guessed it was the temp but i decided to clean the barrel just to illimenate it as the culprit. On the next outting the velocity returned to normal.

 

[zullo74]

I recently had the same issue with a 243 Shilen barrel, RL 25 and 115 gr DTAC's. The velocity jumped from 3050 to 3200fps with no pressure signs. At first i guessed it was the temp but i decided to clean the barrel just to eliminate it as the culprit. On the next outing the velocity returned to normal.

Don't you normally clean your barrel after a shooting outing?