View Full Version : Changing point of impact
10-07-2012, 9:20 PM
I recently bought my first 7mm/08 and just started reloaded for it. After sighting in with 140gr factory loads, I worked up a load with 140gr Accubonds that shoots about 1.5 MOA. With my hand loads, Winchester Ballistic Silvertips and Hornady Superformance 139gr SSTs, point of impact and grouping is about the same.
I also worked up a load with 120gr Ballistic Tips. These are shooting under 1 MOA (about 0.8 actually), but the point of impact at 100 yards moved 10" up and 3" left. Has anyone ever experienced that severe a change in POI with that minor a load change? With my 7mm Rem mag, I get 140gr Accubonds and 175gr Partitions to shoot to the same POI. I am baffled as to how this is possible, but I have gone back and forth between the 120s and 140s several times and the difference is consistent - 10" high and 3" left with the 120s.
Any explanation would be welcome, also any advice on what to experiment with to try to bring these POIs closer together would be appreciated.
10-07-2012, 10:24 PM
10 inches? Thats a big POI change indeed. At 100 yards you might see a 2-3 inch increase but 10 is just crazy. Do you know the chrono speeds of each load? I guess if you are shooting a very thin barrel the harmonics could be just right to whip the barrel and change POI that much but I'm just guessing here.
10-07-2012, 11:06 PM
10 inches is way to far off...if ur under the 1moa the most u should see is around 2
10-08-2012, 5:00 AM
post the loads, is the 120 Gr a hotter or slower load, that could make a big difference, I have seen this in .243, at first I thought it was me till 2 different testing sessions same thing happened
10-08-2012, 5:09 AM
Obviously, it can happen because it did. Most I've seen with factory ammo is about a 4 inch difference. I'd check the speed with a chrono just to have that info. The 140's may just be way slower than the 120's. My guess.
10-08-2012, 8:42 PM
I didn't have the chrono with me, but will test that next time out. I'd be surprised if there is more than a 300fps difference between them. Both loads were CFE 223 (all the other rifle powder i had was too slow), 42grs in the 140 load and 47 in the 120s, all Federal brass with Federal match primers. The rifle is a Savage Lightweight, so the barrel is indeed quite thin. I wouldn't believe it had I not seen it happen, but I sure don't understand the physics of it.
10-08-2012, 9:00 PM
Sounds like you caught a high point on the barrel whip. Do you notice any vetical stringing? Or a diagonal stringing? Takes a while to KNOW you did it right- calling your shots- what you pulled... but a group should be pretty round. If there's stringing, it's usually a barrel whip. Or a lightweight barrel heating up over the string and moving on you.
The barrel vibrates VIOLENTLY when a shot is fired. The ideal load is to find a speed that is right in the middle- barrel neutral. Sounds like the 140's are neutral or a low point of barrel swing, and the 120's are on a high point. This can fling bullets pretty far off course- though 10" is a LOT!
If you have no signs of pressure- no heavy bolt lift, no flattened primers, and not at recommended max loads, I would start increasing powder charge.
Maybe - if you can safely get there- 1/2 to 1 grain more powder will hit another neutral node. Perhaps a little more...
No matter where they land on target... look for the tightest groups, and watch for any stringing as you build up those loads in .2 or .3 grain increments.
If you're not familiar with the subject, search "ladder test" should give you some better procedure/explanation.
Just saw- lightweight barrel. you didnt over heat it did you? let it cool off some?
10-09-2012, 9:13 PM
I don't think heating was the issue as I was only firing four rounds then letting it cool. The first time I took this rifle out, I did heat it up and saw some vertical stringing, but the POI was shifting downward as the barrel got hot.
The 120 load I was shooting is 2.5grs off max with this powder. I will try a little higher charge and see what happens. No pressure signs at all, but the recoil definitely felt different with the 120 loads. They grouped very well, shooting under an inch, whereas I never managed better than 1.5 with the 140s.
I will research the ladder test and see what happens. Thanks for the suggestion.
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