One of three rifles won't show consistent chronograph readings

[AZShooter]

I have an unusual thing happening with one rifle. It will only occasionally show velocity readings from my CED Millenium chronograph.

I can shoot my 308 and 7mm/375 across the chronograph as I did yesterday with very accurate readings. Switch to a 7 Rem mag and it only occasionally will show a proper reading. I will get a value like 250.6 or maybe 2045 instead of the real velocity of 2850.

Now for some details. I am using SS Remington takeoffs in 7 Rem mag for both the 7mm/375 and the 7 Rem mag. I did the rechambering myself. I set back the 7 rem mag barrel to fit a Ruger for a totally fresh chamber. Both chamberings were done with PT&G reamers. The 7 Rem mag reamer made two other 7 Rem mags that shot quite well doubt the reamer is an issue. Bore scope shows a throat with all lands the same length. If there is any issue there are some minor pitting in the rifling but my 308 has more and it shoots in the 3s.

Both 7mm rifles are shooting the 180 Berger hybrid. The 7mm/375 is shooting good groups while the 7 Rem mag is shooting horrible 2-3" groups.

I have switched powders with the problem rifle and it still won't read velocities but rarely and accuracy hasn't improved.

I have theory. I think the 1 in 9.25 twist (haven't verified this yet) of the 7 Rem mag is not enough to stabilize the bullet at this lower velocity. I think the bullet may be crossing the chronograph with a wobble throwing off the chronograph. I have pondered this and can't imagine a bullet leaving the bore and 10 ft away having enough wobble to do this. I have considered muzzle blast but the 7mm/375 is shooting another 8-10 gr of powder the same distance from the chrono with no ill effects.

I will be trying some 168 classics soon to see if the chrono issue and poor accuracy gets resolved.

Any confirmation on this? Ever happen to you? Alternative theories are welcome.

 

[LaHunter]

You may want to move the chrono farther away from the muzzle. I try to put mine 14 to 15 feet from the end of the muzzle of my 7 mm mag. It may have something to do with gasses and unburned powder passing across the chrono at 10 ft away. I think I read somewhere that with most magnums, the chrono needs to be 12 ft away from the muzzle.

Hope that helps.

 

[cowboy]

You may want to move the chrono farther away from the muzzle. I try to put mine 14 to 15 feet from the end of the muzzle of my 7 mm mag. It may have something to do with gasses and unburned powder passing across the chrono at 10 ft away. I think I read somewhere that with most magnums, the chrono needs to be 12 ft away from the muzzle.

Hope that helps.

Agreed - I try to be 18-20 feet away from muzzle

 

[AZShooter]

Thanks for the ideas gentlemen.

I forgot to mention I also shot my 300 Ultra mag yesterday along with the other three rifles. That rifle registered just fine and it is shooting 100 gr of RL-33 with the 230 gr berger out of a 24" barrel. Now that is muzzle blast!

I have never had issues with muzzle blast at 10 ft causing readings like this 7 mag. Since my first post I checked and the twist and it is exactly 1 in 9 1/4. Still think it is a bullet instability issue.

Hoping to do some more chronograph work tomorrow. Have some 168s loaded up for the test. Will report back.

 

[LaHunter]

I am no chrono expert by a long shot, but I would be surprised if it was an issue with bullet stability. I think you could throw a shoe across the sensors and it should give a reading if it is travelling within the fps range of the chrono.

If the barrel is in very bad shape, maybe some copper fouling could be getting 'blown' out of your barrel as a vapor or something like that and getting to the chrono.

Good Luck

 

[Deleted member 46119]

Move your chron farther away.

That load of R33 in the RUM is %100 burnt before projectile leaves the barrel.

The 7mm is not %100 burnt. Muzzle flash is messing with the chron's head.

 

[Deleted member 46119]

Get a MagnetoSpeed and all these problems go away.

 

[6.5shaggy]

I agree to moving the chrono farther from the muzzle. I have one rifle with a brake on it and the chrono will not read that rifle unless it is 15-20 feet from the muzzle.

 

[cowboy]

I have never had issues with muzzle blast at 10 ft causing readings like this 7 mag. Since my first post I checked and the twist and it is exactly 1 in 9 1/4. Still think it is a bullet instability issue.

FYI: I and my sons all shoot compound bows where we shoot our arrows through my chrony occasionally. One day we used a video camera while chronying our arrows. While playing it back in slow motion, those arrows launch out of a bow looking like a snake going though the grass for the first couple yards until they stabilize. Chrony read fine - every time. I wouldn't think your problem is bullet instability.

 

[AZShooter]

Thanks Cowboy! The arrow would tend to dispel my thoughts on a unstabilized bullet.

Thanks to everyone who contributed. If I don't get readings in the future I will move the chrono as far as the wires allow. Fully intend to get a MagnetoSpeed this year!


Won't be trying that poor shooting load again. It is clear today will go out and shoot the 7 rem mag with 168 classics and Retumbo. Hopefully this bullet will shoot. If it doesn't I can blame myself as I am the gunsmith. <grin>