Muzzle velocity significantly dropping

[youssefchagoury]

Hello gentlemen,

Hope you’re all doing well and safe! So, we were hunting ibex in Tajikistan last week, and we had to use the outfitter’s rifle:
Rifle Brand: Ruger Hawkeye (Twist rate: 1 in 9.5)
Caliber: 7mm Remington Magnum
Bullet: Nosler Accubond 160 Grain (Trophy Grade)

While checking the rifle on the first day, I decided to chronograph the loads and get the average muzzle velocity using my Magneto Speed (Temperature was 28 degrees F). I know the velocity can drop in colder temperatures, especially when using unstable powder. However, this one was significantly dropping even between 1st, 2nd, and 3rd shots! I’m attaching a picture of the chronograph results.

1st shot was 987m/s
2nd shot 869m/s
3rd shot 784m/s

Has this happened with any of you before? What would you do in such a case? Knowing it’s very common to shoot at 400-600 yards in this hunting area (A 200 m/s OR a 656.168 ft/s difference will greatly affect the point of impact at such distances). Your help and experience would be highly appreciated!

 

[bruce moulds]

was the barrel clean prior to the 1st shot?
normally in that case, velocity increases with a few shots.
in fact in your case shot 3 is over 600 fps slower than shot1!
the reason will be interesting if discovered.
bruce.

 

[Areaonereal]

Factory ammo?..

 

[MMAL]

Can only be two reasons.

1) your powder is affected by cold weather and
2) your magnetos was affected by cold weather.

Both a little unusually but it happens. If using factory ammo even more unusually. I hear good things about magnetos but you never know. Was the magneto outside prior to setting up for shooting? If not maybe the cold weather was chilling the magneto as you shot? Can say for sure unless you can replicate it.

 

[SRvet]

With the magnitude of velocity reduction I would suspect a chronograph anomaly

 

[Ridgewalker]

I agree with the answers above. Electronics need a stable temperature As well as powders. If the cartridges were in a warm house, brought outside, and started cooling down, then the difference would increase as they cooled off. But that is a lot of difference in 3 rounds! At some point, it should equalize.
Maybe leave ammo and Chrony outside while you get a cup of coffee. This should help if it is a temperature issue. If it were me and I had no time to test, I’d go with the lowest velocity. JMO
Best of luck on that ibex!

 

[Whacker]

Agree with the above answers...if it was factory ammo I would suspect electronic equipment malfunction of some sort. One way to assist in determining...how did your rounds group on target? At 100 yards or over a 600 FPS difference is not going to give you a tight group. If you were shooting moa or under you have an electronic malfunction. I have had this happen before with one ended up being low batteries but target distance was 600 yards was preparing for an f-class competition with my .308. All shots were in the moa target but the chrono was different each shot by over 200 FPS...knew it had to be wrong for those shots to all hit that close. Changed batteries and the next few shots were all within 15fps of one another.

 

[dchamp]

Your first shot seems to be exceedingly fast and your last shot exceedingly slow. I agree with others that it is probably Chrono error.

I looked up the Nosler Custom Ammunition. They list the MV to be 2925fps / 892ms. Using that as a base I got on QuickLoad and set things up for that MV. I then adjusted the Powder Temperature down to 28f degrees. QuickLoad indicated a velocity drop down to 2862fps / 872ms. I found that to be interesting as your second shot was 869ms, pretty close.

Did you happen to shoot something, like a rock, at distance to see what if there was much difference in drop or group? I think that would have told you what the problem was.

 

[IvW]

I think your chrony don't like the cold.....what did the group look like? First shot should always be ignored if the rifle barrel was oiled and stored....take a fouling shot

 

[bruce moulds]

don't those old long cartridges look good.
i also like the old black powder match cartridges with long bullets seated about 1/8" in the long cases.
bruce.

 

[bruce moulds]

sorry the above post no 10 was meant to be elsewhere.

 

[Newboomer]

Sounds like cold weather and altitude. Electronics no like.

 

[youssefchagoury]

Many thanks, everyone for your valuable input! I have to agree it's most probably a chronograph anomaly due to cold weather as many of you suspected... Especially that the rifle was grouping well (A hair under 1 MOA), and it was snowing a lot (The chronograph got really wet).

 

[flatwater bill]

Sensor spacing? Moisture on the bayonette? Sensitivity set too high? I have never had a problem at the relatively warm temp of 28 degrees F. Tight connections, good 9v and CR123? Just random thoughts........thanks for posting .........FWB

 

[IvW]

Many thanks, everyone for your valuable input! I have to agree it's most probably a chronograph anomaly due to cold weather as many of you suspected... Especially that the rifle was grouping well (A hair under 1 MOA), and it was snowing a lot (The chronograph got really wet).

With a group like that just dump the chrony

 

[Hogpatrol]

Could these have been reloads? I've owned three iterations of that chrono and the only instances of questionable velocities were when the battery was borderline.

 

[Hogpatrol]

Too late to edit my previous post but 100 yard moa groups can turn in to shotgun patterns at longer ranges, even at 200.

 

[Mike Van Horn]

Looking at the picture, l would say wet connections on the chrono giving bad readings

 

[nztimb]

Too late to edit my previous post but 100 yard moa groups can turn in to shotgun patterns at longer ranges, even at 200.

And yet it was not so very long ago (or it dosent seem so) that an MOA group atr 100 yds was considered very good hunting accuracy.

 

[Longwalker]

May I suggest that you simply stalk closer? ;-)