What do you guys make of this?


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SCPigpen
May 24, 2010, 09:04 PM
What follows is some data collected while shooting various load and rifle combinations.
Data was colected using a Master Chrony about 15feet from the muzzle. Temp was about 90 degrees and sunny. We noticed extreme velocity speads with a couple of loads in particular.(see below)
At first I thought I had screwed the pooch on a couple of rounds but then my brother got similar velocity spreads on some of his loads. His loads were 5.56 from an AR using 20 somthing grs of WC846.

I don't have the 5.56 data but here is what I have in my little book:

M91/30
7.62x54R
147gr FMJ BT .311
Privi Brass
47.5gr H4895
Win. LR Primers
AOL:3.10"

1. 2648
2. 2607
3. 3286
4. 3247
5. 2502

ES. 787.3
SD. 377.7

I also had some 8mm I shot over the chrony:

M48
8mm Mauser
220gr Honady Spire Point
49grs H4350
Win. LR Primer
RP Brass
AOL:3.05"

1. 2464
2. 2454
3. 2463
4. 2455
5. 2466

ES. 12.0
SD. 5.47

Some from the G-1
7.62x51
147gr FMJ BT
42gr WC846
TZ Brass
Win LR Primer
AOL: 2.75"

1. 3088
2. 3137
3. 3099
4. 3102
5. 3103

ES. 48.84
SD. 18.41


All of the ammo was stored in the trunks/bed of our vehicles and sat in the sun for an undetermined length of time. Only thing we could surmise was that the heat/sun is the cause of these spreads. It's always good to get other opinions.

So, what do you think?

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Ding
May 24, 2010, 09:12 PM
An old sniper trick to get an extra bit of range is to set the magazine/ammo in the sun for a short time to increase the velocity.I wouldn't do it for very long though if at all.So yes it does pop the FPS up.

Kernel
May 24, 2010, 11:29 PM
The standard deviation on the 8x57 and 7.62x55 is tiny. Nothing wrong there, though 5 shots strings is kind of a small sample. From a statistics stand point, 10 shot strings would give much better results.

The SD on the 7.62x54R is excessively large. So large it's got to be a mistake, mix up, or something else is at fault. I don't think temp alone would account for it. I've seen temp studies done by powder manufacturers and the variation due to temp extremes is no where near what your data shows.

Perhaps this is due to a chrony error. How fresh was your battery? The velocity differences are so large you should of clearly been able to feel and hear the difference when the shot were taken. Could you tell the difference? If not, then it was very likely a chrony error.

blackwalnut
May 24, 2010, 11:33 PM
Hey SCPigpen I think it is very possible that the ambient temperature (90 degrees ) was giving you those velocity spikes which would be the result of higher pressures. I was always to keep my ammo out of the sun and in a case or box. Some powder are sensitive to temperature extremes at either end. I have had the experience. Shooting in very cold weather (nothern winters) can also give you a drop in velocity and sometimes it reallly shows up in your point of impact changing. Did your point of impact change also in this case? Where there signs of pressure build up like sticky bolt lift or flattened primers or cratering? Check your data too. Incidentally I reload for 8X57 too and i get good accuracy. I have a Remington 700 Classic and a CZ manufactured Model 98k made in 1950. One more thing Hodgdon came out with there Extreme line of Powders designed not to react to temperature extremes or changes. I use BLC-2 in my Remington. The other uses IMR because that is what is loaded. Good Luck.

243winxb
May 25, 2010, 09:02 AM
Privi Brass in the M91/30 may be the problem if of mixed lots. Other wise what Kernal said.

GooseGestapo
May 25, 2010, 09:38 AM
Either a change in the lighting during the shot, or the chrony was/is too close the the muzzle and you're getting an errant reading due to the muzzle blast.

I get the same thing when chronographing either my .257WbyMag and/or .300RemUltMag.

Set to chrono to something like 30' from the muzzle, and refire the 7.62x54 data.

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