Point of impact changes

[Bean Counter]

Anybody know if changing 3,000' in elevation is enough to knock off a whopping six minutes of angle to your zero? ... yeah: 24 clicks. 

[coachcw]

That's a ton !

[jasnt]

That's very extreme. What range are you zeroed. What preasure are you zeroed in and what was the difference in preasure?

[jasnt]

What caliber?

[Fl0und3rz]

Just running standard sample numbers at

http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi

for a 168 grn SMK at 3,000 fps, you only get about 0.4 MOA difference at 500 yards (that is, more drop at the lower elevation) and about 3 MOA difference at 1,000. 

I think you may have something bigger going on with the rifle than just an elevation (BP) change.

[Bean Counter]

.270 Winchester. And yes, same round and grain weight 

Not sure what you mean by 'pressure' 

[Bob33]

No.

Something else is causing that much shift.

[Bean Counter]

What range are you zeroed.

First range day I zeroed at 200 yards. Then I shot small steel targets from 300-550 yards all day long. Second range day I had to make some minor adjustments but was at least still on paper at 200 yards.

 

That's a ton !

Indeed. But I'm kind of glad it was so far off and not say 1.5 MOA with a hundred wounded lopes and deer across the Red Desert. I had lots of complete misses and it wasn't until I wounded a doe (and yes, we looked a LOT for her to finish her off) that I decided to finally blame the equipment rather than  buck fever and operator stupidity. Rezeroed and the bloodbath resumed.

[birddogdad]

No.

Something else is causing that much shift.

 

maybe 1 INCH of change (only for air) for a 10K foot elev change at 100yds... could be seen.. (all other variables equal)

[jaymark6655]

temperature difference and a really sensitive powder?  Maybe the swing in  chamber pressure is causing the bullet to leave the muzzle at a different point in its vibration or recoil?

Pressure changes with altitude, but temperature and humidity also change it so an increase in altitude of 3000 physical feet might be effectively a lot more if you sight in at sea level on a really cold day with no humidity and then climbed 3000 feet higher and shot on a really hot, humid day.  I can't imagine any change in weather that would cause that much of a shift though.

[CaNINE]

temperature difference and a really sensitive powder?  Maybe the swing in  chamber pressure is causing the bullet to leave the muzzle at a different point in its vibration or recoil?

Pressure changes with altitude, but temperature and humidity also change it so an increase in altitude of 3000 physical feet might be effectively a lot more if you sight in at sea level on a really cold day with no humidity and then climbed 3000 feet higher and shot on a really hot, humid day.  I can't imagine any change in weather that would cause that much of a shift though.

 

What powder are you using?  If you're not using a temp stable powder you could see that much shift.  What were the environmental conditions when you zeroed verses where you are shooting now?

[Bean Counter]

don't give me too much credit. I've never personally reloaded once in my life. I get what powder Federal gives me, and I'm just barely smart enough to buy the same caliber and grain weight consistently 

[300rum]

There can be differences Lot to Lot but I wouldn't expect that much difference.  Something went haywire more then likely your scope/mount. 

[Bean Counter]

I have Leupold high rings. Maybe that's it? But I didn't clang and bang it. I picked the weapon system up by the scope a few times as if it were a handle. Could that have done it?

[2labs]

temperature difference and a really sensitive powder?  Maybe the swing in  chamber pressure is causing the bullet to leave the muzzle at a different point in its vibration or recoil?

Pressure changes with altitude, but temperature and humidity also change it so an increase in altitude of 3000 physical feet might be effectively a lot more if you sight in at sea level on a really cold day with no humidity and then climbed 3000 feet higher and shot on a really hot, humid day.  I can't imagine any change in weather that would cause that much of a shift though.

 

What powder are you using?  If you're not using a temp stable powder you could see that much shift.  What were the environmental conditions when you zeroed verses where you are shooting now?

Not a chance. If this is true,rifles would make better clubs.

[300rum]

Your point of impact should not change from day to day.  I have scopes that I have not touched in a decade and they shoot exactly the same with the same lot of ammo, I don't have to make any adjustments.  I would be looking at the mount and scope. 

First range day I zeroed at 200 yards. Then I shot small steel targets from 300-550 yards all day long. Second range day I had to make some minor adjustments but was at least still on paper at 200 yards.

[Bob33]

My guess is the scope got bumped in transit. A 6 MOA shift is enormous and I cannot envision any environmental factors that would cause it.

[jaymark6655]

temperature difference and a really sensitive powder?  Maybe the swing in  chamber pressure is causing the bullet to leave the muzzle at a different point in its vibration or recoil?

Pressure changes with altitude, but temperature and humidity also change it so an increase in altitude of 3000 physical feet might be effectively a lot more if you sight in at sea level on a really cold day with no humidity and then climbed 3000 feet higher and shot on a really hot, humid day.  I can't imagine any change in weather that would cause that much of a shift though.

 

What powder are you using?  If you're not using a temp stable powder you could see that much shift.  What were the environmental conditions when you zeroed verses where you are shooting now?

Not a chance. If this is true,rifles would make better clubs.

I just heard a story about some scopes that once adjusted, you could sit there and watch the turrets turn back out or maybe it got bumped especially if they don't lock or something is loose somewhere.

[Bean Counter]

My guess is the scope got bumped in transit. A 6 MOA shift is enormous and I cannot envision any environmental factors that would cause it.

It was in a soft case and got buried under other crap for the long, LONG drive, and also shifted around a few times   

[Bob33]

My guess is the scope got bumped in transit. A 6 MOA shift is enormous and I cannot envision any environmental factors that would cause it.

It was in a soft case and got buried under other crap for the long, LONG drive, and also shifted around a few times   

As a standard practice, I always (try to, at least) fire a test group after travelling to ensure the POI has not shifted. Hit Shappens.

[jasnt]

Sounds like a scope base is loosening up to me. Should not be that much shift even with temp changes condition changes or barrel contact. Maybe scope isn't tracking right?

[JimmyHoffa]

My guess is the scope got bumped in transit. A 6 MOA shift is enormous and I cannot envision any environmental factors that would cause it.

Probably first guess too. Next might be a change in the stock or bind. How is the rifle bedded/floated?  Is the stock wood?

[Bean Counter]

As a standard practice, I always (try to, at least) fire a test group after travelling to ensure the POI has not shifted. Hit Shappens.

That's really all I had to do: the 25 yard battle zero. I shot two rounds at 25 yards and saw that I was 1.5" low at 25, ergo 6" low at 100. Ergo 24 clicks @ 1/4moa adjustments. Fired two more rounds and was spot on. Filled 5 out of 6 of my tags with ease at 200-400 yards after that.

Unfortunately this lesson cost me a really nice pronghorn on a premium quality unit. but I'm glad I didn't wound him being so far off. Its not the goats fault that the hunter is the idiot. I wound up plugging an average buck and will chalk it all up to a learning lesson 

[Bean Counter]

Probably first guess too. Next might be a change in the stock or bind. How is the rifle bedded/floated?  Is the stock wood?

Stock wood. I don't think its glass bedded or anything fancy like that.

[Bob33]

Well, you know BC that a few members on here do believe you may have a loose screw. 

[fisheral87]

I recently had two screws back out on me around the action on my Weatherby.

Also check the bedding, 7mmFan had a similar situation with his model 700 where the stock was expanding/contracting and throwing him off.

Al

[Bean Counter]

Well, you know BC that a few members on here do believe you may have a loose screw. 

  Post of the day right there! 

[jaymark6655]

One more theory:  Wet WA to dry WY with a wood stock, it shrank loosening your action screws.

[Bean Counter]

I'm currently residing in Arizona

[coachcw]

my bet is a loose base screw

[Stein]

I had my scope get adjusted a full turn last year somehow.  I completely missed a deer low at 104 yards.  I couldn't believe it until I took no less than 3 practice shots.  I still have no idea how it happened.

[jasnt]

my bet is a loose base screw

  this has happened to me on my 300wm and just happened to a friends 270

[CaNINE]

temperature difference and a really sensitive powder?  Maybe the swing in  chamber pressure is causing the bullet to leave the muzzle at a different point in its vibration or recoil?

Pressure changes with altitude, but temperature and humidity also change it so an increase in altitude of 3000 physical feet might be effectively a lot more if you sight in at sea level on a really cold day with no humidity and then climbed 3000 feet higher and shot on a really hot, humid day.  I can't imagine any change in weather that would cause that much of a shift though.

It happens.  I've had well over 6 MOA in variation with Reloader 33 in my RUM.  If you zero at 80 degrees at sea level then hunt at 3000 feet at say 40 degrees you may see a significant shift depending on the powder used.  This is why temp stable powders like H1000 and Ramshot Magnum are so popular.  I freeze my ammo overnight and then test to see if my zero will shift.  If it does the load is no good.
   
 

What powder are you using?  If you're not using a temp stable powder you could see that much shift.  What were the environmental conditions when you zeroed verses where you are shooting now?

Not a chance. If this is true,rifles would make better clubs.

[Curly]

Bean, did you end up getting a new scope for your .270?  If yes, what did you go with? 

[Curly]

You will probably want to double check all your scope mount screws before next year.

[Bean Counter]

Bean, did you end up getting a new scope for your .270?  If yes, what did you go with? 

Yep, sure did. Got a Zeiss HD5 3-15x 44mm. Has Rapid z800 recticle. Some might find it cluttered comPared to a basic duplex reticle i find it ingenious. Had to forego a buck deer tag in Wyoming this year to make it work with the budget but it was worth it. Smoked a few doe/fawns at 200-400 yards once I got it dialed in.

Enter the particulars of your load on their calculator at Zeiss.com. If I keep mine set at 12x I can align the BDC lines and they're within 10 yards for the bullet I shoot. It's so easy a bean counter can do it.