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Posted: 5/12/2014 7:14:48 PM EDT
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I stopped at the range to check my zero the other day and I noticed something very odd. I only had a couple of boxes of ammo with me, and my intention was just to verify my zero had not changed, so I set up my targets and whipped out my 155gr "match" ammo, and it was good to go. I also had about 3/4 of a mag of South African 147's, so I shot it up too.
When I checked my targets I noticed that the SA was shooting over an inch higher at 25Y than the 155's. This was the first time I had shot both types of ammo in this gun [bushmaster ORC], and in my FAL [20"] and my M1A [18"] both of these rounds shoot about the same POI, but in this 16" AR pattern 308, there was a drastic difference. Any ideas why there might be such a difference between types? It wasn't hot, neither were in the sun. If I had had my chrono with me I would have checked them, but, like I said I was just verifying with the match ammo and didn't really realize I had the other rounds until I got to the range. |
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An inch isn't really drastic in some arenas, but can become a clean miss on others...
At short range, I wouldn't count on the spread of topics to stay within an inch unless tested. If this were a silhouette pistol or target pistol, the difference in ammo at 25 yards would be considered drastic, so the difference in impact would be expected. |
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The only possibility I can think of is the 147 gr. is a lighter bullet at a higher velocity, maybe loaded a bit hotter than the match ammo?
Still at 25 years there should be almost no difference in trajectory. Puzzling. I think you have to shoot it at 100 yards to get usable info.... |
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most likely the velocity and the fact that the 147's are leaving the barrel at a different part of the harmonic wave than the slower/heavier match rounds you were shooting combined with your specific twist rate.
25 yards is rather short to notice a difference out of a rifle but it's very common to get different POI with different ammo/loads |
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You have each touched on my point. 147 is not drastically lighter than 155, its not like I was shooting 220's.
I did shoot at 100, but like I said, I only had a few rounds and was only there confirming my zero, not zero-ing from scratch.. I confirmed my zero at 100, then had some extra rounds and some graprfruit targets [from the challenge YO] so I threw them out at 25 just messing around offhand. I noticed a couple of high shots that I didn't call, so I sat on the bench with a half a mag each and narrowed down that it was about an inch difference in POI at 25Y. |
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Quoted:
You have each touched on my point. 147 is not drastically lighter than 155, its not like I was shooting 220's. I did shoot at 100, but like I said, I only had a few rounds and was only there confirming my zero, not zero-ing from scratch.. I confirmed my zero at 100, then had some extra rounds and some graprfruit targets [from the challenge YO] so I threw them out at 25 just messing around offhand. I noticed a couple of high shots that I didn't call, so I sat on the bench with a half a mag each and narrowed down that it was about an inch difference in POI at 25Y. I would say that it doesn't matter what you consider drastic. 147 is not 155, and even the same grain rounds with a different profile or construction method can vary in POI. Including, just as you noticed, in some rifles but not in others. There are a lot of variables that factor into why ammo impacts where it does. Chances are, you are never going to be able to do anything about it, except learn where it hits, and change your aim point or the settings on your optic. |
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