Strange situation, any ideas?

[Ian M]

Was shooting long today with a fellow who wanted to learn some basics. We established drops at 5,6 and 700 yards for his .308 Win. Rem. M-700 VS. Load is 46 gr. Varget, 165 gr. Nosler Ballistic Tips, not sure about case or primer. Scope was a 6.5-20 Leupold set on 20.

Drops were 500 - 12 moa, 600 - 16 moa, 700 - 19 3/4 moa.

Rifle shot well, had some three-shot groups around .5 moa. Five above zero C, winds were 6-8 mph from 11.30. His ammo was reloaded, not sure how many times for the brass used initially. He ran out of the stuff that had been reloaded, switched to reloads with new unfired brass. Shot at 100 yards and he hit into a previous group.

Downrange drops changed to:
500 - 10moa, 600 - 15.5 moa, 700 - 19 3/4 moa.

Why did the 500 yard zero change two minutes? Checked and rechecked - this is what the rifle and load did. How can the zero go out two minutes at 500 and virtually back to what we had originally at 6 and 700? Only variable was the unfired brass. Accuracy remained impressive, he had some very good 3-shot clusters at each range.

Pretty confident this took place as described but not sure why.

 

[ExpiredMember2019]

Could it have been something wrong with the scope? Maybe there's an internal problem with the elevation adjustment at one certain point but not at the others?

I don't know....just a thought.

Doug

 

[royinidaho]

Been there done that, gave up trying to figure it out. /ubbthreads/images/graemlins/crazy.gif

As Michael says: "Because any other sport just would not be frustrating enough!!"

Actually, my problem was terrain. A swale about 3/4 downrange, when the wind was from the right direction and up towards 10 mph, I got a rise in POI but the POI was consistently high from there on out.

Your experience violates someone's law about something.

 

[sniper2]

pressure...

 

[Ian M]

Doug,
The scope tracked really well, returned to zero perfectly. I really doubt the problem was the scope, it was properly installed and mounts tight. Definitely defies some law of ballistics, not sure which one or how many but has me beat.

 

[speedbump]

Ian, if the wind was from 11:30 (nearly head-on) as you said, he might've shot his fireformed 3-shot group in a gust, causing it to require more elevation. It doesn't take long to put three shots downrange, even less for a wind condition to change. Did ya'll note a stronger gust about that time? It is curious since the others were pretty close to the original ...... /ubbthreads/images/graemlins/confused.gif

 

[Ian M]

No sir, wind was dropping 0ff slowly at the time. Matter of fact we shot the drops twice to confirm it was happening. Darned if I can figure that one.

 

[Mysticplayer]

Mostly, I would say changes in wind. Spending alot of time practising for F class shooting, you can really see the affect wind, light/mirage and THERMALS have on POI. I have had groups move 2MOA at 300m just on condition change.

The 308 gets moved around alot so doesn't take much of a change for a big change in POI at LR. A let up in head wind WILL affect POI.

I would suggest a repeat of the shooting with the 'new' reload and see what you get. Calm conditions with overcast skies, early in the morning would be ideal.

You can get thermals anytime of year and with the terrain you shoot over, I would expect them to be quite common and very large. A small bubble can be 100yds across (some only a few ft) and last for a seconds to minute or two. Some bubbles can be 1/2 mile across.

If there is a glider club or private airfield in your area, talk to them about the weather patterns/ground effects. The upward velocity of these thermals can be very impressive. Think moving a Boeing 747 a few ft or more (called air turbulence) while it is traveling 450mph. Or making commercial planes crash on take off/landing (commonly refered to as wind shear).

Unfortunately, we have no way to watch for this occurance (no vertical wind flags, at least not yet).

Something to add to the mix of confusion.

Jerry

 

[goodgrouper]

More info needed to get the answer.

Was the new brass same brand/lot as old brass?
Were the bullets fired also from the same lot for both new and fireformed brass?
Did the new brass have the same neck tension as the old?
Is the chamber on the rifle sloppy and thereby allowing the fireformed cases to measure larger capacity?

 

[Ian M]

Grouper,
Good questions, cannot answer all tho.
Brass was different lots for sure, believe Win.
Bullets different lots also.
Previously shot brass was full sized, nothing on new brass, just inserted a bullet.
No idea about chamber measurement but expect this had something to do with it, pressures. But weird that the zeros came back into order at 6 & 7.
Might have been some thermals but this was late in the day and wind was dying out.
Got better things to worry about. New rifle. No time to shoot it. Not fair.

 

[Mysticplayer]

Thermals are most 'active' when there are large swings in ground vs air temp. An approaching cool air mass over a hot field will cause bubbles. As would the sun poking out from behind clouds quickly heating the ground.

Also, rapid changes in wind speed or direction ARE indicators of thermals lifting off or downdrafts filling in the air that just left.

Days with lightish winds will have these conditions more pronounced then a day with steady winds from a certain direction.

Oh by the way, thermals will move across large spans of terrain/prairie 'sucking' up air in their path. Think very mild tornado with a really fat butt/base. The backside of this is equally strong downdrafts.

Grab a book on R/C glider flying and they will give you lots of info and tips about thermals.

Jerry