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…

 

[elkaholic]

That is an interesting developement! I am assuming that you clean the barrel and don't have a serious fouling problem? I have heard of carbon building in the throat even when the rest of the bore is clean. A buildup in that area can definitely affect pressure. I would try putting a saturated patch of kroil or hoppes in the throat area and leaving it there for a day or two. Remove the patch and insert another patch wrapped around a bore brush, rotate several times and see what comes out? If you see a lot of black, continue the process until it is clean and try your load again.You don't sound like a greenhorn so you have probably checked this as well but over length case necks can also cause this problem. Another possibility could be a too thick case neck which would not allow enough clearance when firing. Usually you will notice difficult chambering when the condition reaches that stage. Good luck and keep us posted........Rich

 

[Robbin]

Thanks elkaholic,
I thoroughly clean every 30 to 40 rounds. Because of the tight neck, I measure ever loaded round before it goes in the box to take to the range. I don't check every cases length, as they are all the same type and number of firings and all cut to the same length at the start. I spot checked while annealing and all were well within the max allowed length. But any of those things would raise pressure, yet my sticky bolt has not returned since I lowered the powder charge.

Can you think of any way LESS powder could increase MV? It's a compressed load, even with the reduction of .4 grains, just not as much compression.

I think it has to be physical, like my new batch of bullets is ever so slightly smaller in diameter or like I've worn just enough of the throat to make a difference. Interesting that it is so sudden, with so few rounds between the last time I shot a full group thru the crony and this trip. I know Weatherby mags get their bump in velocity without increasing pressure by extending the lead and making the bullet jump, thus giving it a head start before it hits the lands. But the jump is LARGE, far more than I could have wore with 200 shots.
And I know the bullet is touching the lands when I close the breach, because I can feel it seating the bullet like it always does.

You mentioned cleaning and I have recently changed cleaning solutions. I assume it is possible that I wasn't getting it clean enough before, but I was using ammonia based cleaners. I've switched to stuff that has no ammonia and they say you can't hurt your barrel with it, so I leave it in longer. Other than the actual solvent, my cleaning routine is the same. But I plug the bore, and fill the barrel with the cleaner all the way into the chamber and leave it that way for 15 to 20 min. Before I just poured it thru, let it set wet for 5 min, the wet patched and wet brushed with a nylon brush. However, the max velocities have been consistent from when I first started shooting the rifle. Still, that could be the difference.....

Thanks for the input.

 

[elkaholic]

It IS strange that it happened so suddenly. I would also be kind of surprised if the throat was carboned up after only 200 rounds with your cleaning routine. (just grasping at possibilities) I had a strange thing happen myself last week with velocity increase only it regarded seating depth. I know that sometimes there are some strange dynamics going on that we don't fully understand concerning internal ballistics and there could be something to your compressed load theory? The annealing you did was the only change and that should have LOWERED pressure if it had any affect at all? I'll keep scratching my head