Long Range Hunting Online Magazine


Go Back   Long Range Hunting Online Magazine > Rifles, Reloading, Optics, Equipment > Gunsmithing


Reply

Point of impact drift

 
LinkBack Thread Tools Display Modes
  #1  
Old 10-16-2013, 09:30 PM
Junior Member
 
Join Date: Oct 2013
Posts: 3
Point of impact drift

I have a 260, shooting 140 gr berger, shoots pretty good .5 moa at 500. The problem I am having is the poa is drifting to the right. At 500 3 to 4 in, 750 is 5 to 7 in and 1000 it gets out to about 20 in to the right. Everything seems to be level, am even going to try to cant my scope tomorrow. Just wondering if anyone has had this problem.
Reply With Quote
  #2  
Old 10-16-2013, 09:59 PM
Platinum Member
 
Join Date: Oct 2009
Location: Thunder Basin, WY
Posts: 1,788
Re: Point of impact drift

Shooting the 140 Hunting VLD in my 6.5-284. I purposely canted my scope so when I dial 22 minutes my poi is off to the left of Top Dead Center by ~ 3/4" at 100 yds. Since doing that, I haven't noticed any excessive spin drift at LR. Everything seems to dial in fine out to 1000 yds anyway. I believe we're shooting the same bullet.

Just for record, I didn't do this because of problems, did it to counteract normal spin drift and coriolis effects. I had heard that it's normal to get 9-12" of right drift @ 1000 yds depending on the bullet and the rifling twist and coriolis is supposed to be another 3" ??
__________________
Aim small = Miss small
SBruce


Last edited by SBruce; 10-16-2013 at 11:13 PM.
Reply With Quote
  #3  
Old 10-17-2013, 12:24 PM
Silver Member
 
Join Date: Apr 2007
Location: Fort Morgan, Colorado
Posts: 327
Re: Point of impact drift

With the 140 grain bullet from a 260Rem you are getting somewhat low muzzle velocity compared to some of the bigger cased 6.5 cartridges and therefore the bullets are slowing quicker also, so I think spin drift is more the culprit than scope cant. You didn't say what rifle/barrel you are using but if it is an off the shelf rifle the rate of twist may be too slow to keep the RPMs high enough at longer ranges. I bought a rifle for my daughter some years back and wanted a 260 but shied away from it because of the slow twist and opted for a 7mm08 instead. I like the cartridge but I think the rifles should be made with 1/8" ROT barrel. If you are using a custom barrel with a faster ROT disregard my ramblings.
__________________
Dennis
Reply With Quote
  #4  
Old 10-17-2013, 02:57 PM
Junior Member
 
Join Date: Oct 2013
Posts: 3
Re: Point of impact drift

My 260 has a kreiger 1/8 twist and have thought of spin drift but only considered 3 to 3.5 in. at 1000. Thank you for the info.
Reply With Quote
  #5  
Old 10-17-2013, 07:27 PM
Platinum Member
 
Join Date: Mar 2008
Location: SW Idaho
Posts: 1,148
Re: Point of impact drift

This is a good video that explains some of the things being discussed. I would agree that spin drift and/or a canted reticle is part of your issue. It may be Coriolis too depending on which direction you are shooting.

Demonstrating how Coriolis effects bullet drop at 1000 yards - YouTube

Scot E.
Reply With Quote
  #6  
Old 10-17-2013, 11:40 PM
Gold Member
 
Join Date: Sep 2005
Location: Central AZ
Posts: 906
Re: Point of impact drift

I think there may be a couple of things going on here at the same time. I don't believe spin drift could account for all 20" to the right of center @ 1000 yds. Possible that scope not being perpendicular could account for some as well.

Take a blank target and set it out at 100 yards Take a plumb line and draw a verticle line on the target. Now fire a couple shots at the line (POI should be touching or cluster the line. Now dial up 30 or 40 MOA (whatever your scope will allow), and shoot another couple of shots. You should have the same cluster around the verticle line, but the second cluster should be 10 inches higher than the first. If the second cluster favors one side of the line, then your scope is canted in the rings.
Reply With Quote
  #7  
Old 10-18-2013, 05:57 AM
Platinum Member
 
Join Date: Jan 2008
Location: Northeast
Posts: 2,273
Re: Point of impact drift

Quote:
Originally Posted by azsugarbear View Post
I think there may be a couple of things going on here at the same time. I don't believe spin drift could account for all 20" to the right of center @ 1000 yds. Possible that scope not being perpendicular could account for some as well.

Take a blank target and set it out at 100 yards Take a plumb line and draw a verticle line on the target. Now fire a couple shots at the line (POI should be touching or cluster the line. Now dial up 30 or 40 MOA (whatever your scope will allow), and shoot another couple of shots. You should have the same cluster around the verticle line, but the second cluster should be 10 inches higher than the first. If the second cluster favors one side of the line, then your scope is canted in the rings.
+1. Do you have a scope level mounted on your rifle?
__________________

"Let us speak courteously, deal fairly, and keep ourselves armed and ready"-T. Roosevelt
Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes


Current Poll
Do you archery hunt for elk?
YES - 29.22%
45 Votes
NO - 55.19%
85 Votes
Not yet, but I plan to. - 15.58%
24 Votes
Total Votes: 154
You may not vote on this poll.

All times are GMT -5. The time now is 10:00 PM.


Powered by vBulletin ©2000 - 2014, Jelsoft Enterprises Ltd.
Content Management Powered by vBadvanced CMPS
All content ©2010-2014 Long Range Hunting, LLC