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Same load. Different day. 100 fps difference.
#6149122
01/22/16 07:18 PM
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Joined: Oct 2012
Posts: 29,611
Sneaky
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This happened awhile back. Been thinking about it. Can't come up with anything conclusive, except maybe a chronograph error.
I was using a brand new Magneto Speed chronograph. I worked up a load to 3450 fps. A few months later, that same load chronographed at 3350. All prep work was the same. Loaded the same way. Environmental conditions were similar, although it was a bit warmer the second time. The powder was H1000, and while I think it was from the same lot (same can, even) I can't say for sure. There was never a hint of pressure issues, either time. I can't think of anything that would cause that much velocity loss, so I'm guessing the chronograph was reading different. It was set up exactly the same, also.
I've since ditched that particular load, but I'm curious about what may have caused it in case I run in to that again. I'd hate to think it was a brand new Magneto Speed. That would hinder my confidence in the unit moving forward.
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Re: Same load. Different day. 100 fps difference.
[Re: Sneaky]
#6149324
01/22/16 09:15 PM
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Joined: Dec 2008
Posts: 41,173
J.G.
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What I've seen cause velocity changes:
Virgin brass vs. brass with a .002" bump to the shoulders. Significant ambient temperature change. Barrel that got 200-250 rounds down it. New lot of powder.
So if you rule out all that, I say chronograph error. Cuz I got nuthin else.
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Re: Same load. Different day. 100 fps difference.
[Re: Sneaky]
#6149339
01/22/16 09:25 PM
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Joined: Oct 2012
Posts: 29,611
Sneaky
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Brass was all fireformed and bumped back. It was warmer the second go round, which is when the velocity dropped. That's probably close on the round count, total, but no rounds were fired in between sessions. This is a possibility. I should have made a note of this. I was thinking it came from the same lot, but I'm not positive.
With more use, I'll probably be able to figure out if it's the chronograph or not.
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Re: Same load. Different day. 100 fps difference.
[Re: Sneaky]
#6149465
01/22/16 10:49 PM
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Joined: Dec 2008
Posts: 41,173
J.G.
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What is the chambering? The 250 mark, on the round count is where lots of them jump in speed. I presume .223 something? Chronographs are cool, but nothing will give you true velocity like 400 yards and beyond. And there's way more than that in Winters, Texas.
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Re: Same load. Different day. 100 fps difference.
[Re: Sneaky]
#6149511
01/22/16 11:32 PM
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Joined: Nov 2007
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RiverRider
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A couple more possibilities: a primer change? or maybe your scale was not performing the same for whatever reason...
A change in seating depth will can cause velocity to change also.
"Arguing with you always makes me thirsty." -Augustus McRae
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Re: Same load. Different day. 100 fps difference.
[Re: J.G.]
#6149710
01/23/16 01:47 AM
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Sneaky
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What is the chambering? The 250 mark, on the round count is where lots of them jump in speed. I presume .223 something? Chronographs are cool, but nothing will give you true velocity like 400 yards and beyond. And there's way more than that in Winters, Texas. It's the 7 STW. Jump up in speed? Or down? This went down, and overnight.
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Re: Same load. Different day. 100 fps difference.
[Re: RiverRider]
#6149716
01/23/16 01:49 AM
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Joined: Oct 2012
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Sneaky
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A couple more possibilities: a primer change? or maybe your scale was not performing the same for whatever reason...
A change in seating depth will can cause velocity to change also. Primer was the same. The scale is something I think could be a possibility. It's a GemPro 250. It seems like a good scale, and it gets good reviews, but maybe I didn't let it warm up properly, or something. Seating depth was the same.
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Re: Same load. Different day. 100 fps difference.
[Re: Sneaky]
#6150190
01/23/16 02:32 PM
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Joined: Dec 2008
Posts: 41,173
J.G.
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What is the chambering? The 250 mark, on the round count is where lots of them jump in speed. I presume .223 something? Chronographs are cool, but nothing will give you true velocity like 400 yards and beyond. And there's way more than that in Winters, Texas. It's the 7 STW. Jump up in speed? Or down? This went down, and overnight. Once they get 250 round count the speeds go up. That is why I am thinking chronograph error. Or powder lot change.
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Re: Same load. Different day. 100 fps difference.
[Re: Sneaky]
#6150264
01/23/16 03:38 PM
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Sneaky
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Re: Same load. Different day. 100 fps difference.
[Re: Sneaky]
#6150689
01/23/16 09:05 PM
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Joined: Aug 2011
Posts: 6,179
Tactical Cowboy
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I'm going to say chrono error. I've seen them read different speeds if they aren't set up properly or from differences in lighting.
The secret to a long life is to try not to shorten it.
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Re: Same load. Different day. 100 fps difference.
[Re: Sneaky]
#6150722
01/23/16 09:38 PM
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Sneaky
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The thing about the Magneto Speed is that it doesn't require light. I won't rule out operator error, but I had it set up the same way each time and in accordance with instructions.
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