Accounting for a flatter trajectory

[BLFields]

Rifle: 300 WSM. Bullet: 190 gr. Berger VLD

Using virgin brass, the bullet .005 off the lands and runout at about .005, I had an established, repeatable trajectory out to 1000 yards.

Getting ready for a Palma match,F-Class) I used once fired brass, set the bullets off the lands .035" and noticed that runout had been reduced to about .001". Based on cooler weather conditions, I figured I'd need a little more elevation. Wrong. The trajectory was flatter than normal. At the 1000 yard line, the trajectory was 1.5 MOA flatter than normal, when it should have been a +0.5 MOA due to the cooler conditions.

I finally decided that the improved runout accounted for the flatter trajectory, but am having a hard time believing it. If the trajectory had improved a half an MOA, well maybe. But, a two MOA improvement seems out of line.

Have any of you had a similar experience?

 

[jb1000br]

Hi Blaine -- what is the terrain over the length of the range like?

flat? Bumpy? raised firing lines?

Headwind, tailwind?

Welcome, BTW

Jason

 

[BLFields]

Hi Jason,

The terrain is fairly flat with slight raised areas for shooting the various ranges. The wind conditions were between 0 - 3 mph at 45 degrees, so there was a small headwind component.

This is the same range at which I had shot many times before with the virgin brass loads and had established the come-ups on a number of occasions in varying weather conditions, including both small head and tail winds. As I said, the only real differences were the once fired brass, a greater bullet jump and better concentricity.

Yesterday I shot at 800, 900 and 1000 yards and all of the ranges required significantly less elevation than normal. The 1000 yard elevation was the shocker when I dialed in my normal elevation,corrected for the lower temperature) and I was printing at 12 o'clock in the 6 ring. I had to drop 2 MOA and even then had to favor low to get center hits.

Since no one is jumping in ratifying the fact that one of these factors could, indeed, account for the flatter trajectory, I've got to assume that my zero is screwed up somehow and the problem lies with the scope. That would actually make me feel better because ballistically I'm at a loss to explain what happened.

 

[DesertLefty]

Blaine,

Two thoughts come to mind:

,1) I'd look more carefully at the difference between virgin brass and fired brass. Of the three factors that you mentioned, this might be the most significant. Have you ever tested this variable?

,2) What powder are you using? My experience is that the effect of temperature on combustion rates is much greater than the effect of temperature on air density. Ballistics programs only consider the latter effect. I've never seen anyone try to quantify the effect of ammunition temperature on trajectory. Any chance that your ammunition temperature was higher? Where was it stored overnight? Between stages?

Nice to see you here at 6mmBR,

Richard Holmes "Desert Lefty"

BTW - At our 1000-yard tactical match on 12/4/05, Blaine fired the second highest score ever recorded,328-12X). Just a whisker off the course record of 331-10X.

 

[jb1000br]

OK, i see a few possibilities here:

LIFT from the raised firing points and the partial headwind component.

Diff POI's at 100yds with virgin v 1x fired brass affecting percieved comeups.

shifted zero resulting in same

JB

 

[BLFields]

Richard, thanks for your input.

1. I've never noticed any significant changes before using virgin v. fired brass. I do attribute the improved concentricity to the fact that the chamber on my rifle is well reamed and produces good, uniform fired brass. I've loaded up some more rounds and may head back to the range next Monday to do some additional testing.

2. The program I use accounts for the change in air temperature by changing both the air density and muzzle velocity. Actually, the user can specify any powder temperature and specify a powder temperature response profile which will then result in a change to the muzzle velocity. I used H4831SC in the rounds I shot Saturday. Although I haven't yet gotten a data sheet from Hodgdon, based on advertising data from their website, for this powder I use about 1 fps per degree change in muzzle velocity. So, when I was computing the proper elevation, the program was using the proper air density and a slightly lowered muzzle velocity.

Jason asked about the topography at the range. Have you ever seen a condition where the air was being pushed up sufficiently to push the trajectories up 2 MOA at 1000 yards? I never have, but you shoot there much more frequently than I. It is a very logical explanation although I didn't see any evidence of that condition at the time.

DMoran, interesting thought although I'd be extremely disappointed if Berger bullets exhibited these kinds of fluctuations. But, as it turns out the bullets came from the same lot.

 

[BLFields]

Jason,

I posted a response to Richard before I saw yours and as I said, yours seems logical although I failed to notice evidence of an updraft during the shoot.

Let me ask you this: can going from .005" to .001" runout improve the flight characteristics sufficiently to explain the results? My initial thought was that the initial drag developed by the bullet in the first 100 to 150 yards as it stabilized might account for the effect . . . less runout, quicker stabilization, less early drag, better trajectory.

 

[jb1000br]

Blaine -- i have never thought about correlating runout to drop...i dont think it is a real connection, but have no more than my opinion behind that.

Another thought...had you chronoed the ammo after seating the bullets 30thou deeper? with that much diff in seating, i bet you boosted your velocity a bit...

JB

 

[sparker]

Blaine, I wonder if stability may not be a factor here. If the twist rate was marginal in other conditions and it became quite stable under the conditions you were shooting in, an increase in BC would occur and the bullets would impact higher.

Just for thought,I was shooting prone at 800 yds yesterday. I changed my sling tension and my next shot impacted a full minute higher. I made the elevation correction and the next shot went in the x ring. Gun handling can explain many anomalies.

Scott Parker
Bakersfield CA

 

[BLFields]

I went back into my handloading records to see if I could tell what the muzzle velocity would be with a .035" jump, but the data is not clear. If the muzzle velocity increased by 30 fps, the results could be explained that way. So, I'll check muzzle velocity at the same time I recheck the zero. I hadn't considered a change to MV due to the deeper bullet seating. Good detective work. Thanks.

Blaine Fields