Nearly 4000 fps 7mm Rem Mag???

[Barrett93]

My dad has been deer hunting with a Remington 700 ADL 26" barrel in 7mm Rem Mag since I was just a kid. He bought three boxes of Federal Premium 140 grain Nosler Partitions when he purchased the rifle. He's down to his last box and now that I have gotten into handloading he wanted to test them and try to get some handloads that closely mimic the Premiums as far as velocity and bullet. I set up the chronograph (ProChrono Digital) and shot my .220 Swift and 6mm Remington. The chrono was reading exactly how it should have been.

This is where things get interesting. We set up the 7mm Mag and shot the first of the Federal Premiums. It registered 3906 fps. Realizing this must be a fluke
(I've never had this chrono read way out of whack like that, it either reads or doesn't) he put another in the chamber and shot. 3918 fps... Now I am absolutely bumfuzzled. We shot four Federal Power-Shoks and they all registered right around 2900 fps. Then went back and shot two more Premiums that both registered around 3900 fps.

I simply do not believe it is my chronograph. I have used it a couple of times since that day and it's never thrown me any far fetched numbers while shooting ladder tests or ANY other time. But how in the world is Federal pushing a 140 grain Nosler that fast? Their own website only says that load would have a 3150 fps muzzle velocity. And how in the world am I supposed to even get close to mimicking that? Any insight would be very helpful!

UPDATE: I believe that the issue has been identified. The muzzle blast from the Premium ammunition caused an interference with the chrono. I don't own and belted or magnum cartridges so that's why I had never experienced the problem. I will take the chrono back to about 20' and try again. I normally shoot around 12' away.

 

[119er]

I did a quick look and it seems top velocities for that combo should be around 3250 fps, so I would wager that you're getting an error somehow.

I realize you shot two different loads of the same caliber that were roughly 1000 fps different. That alone is a red flag. How far is the chrono from the muzzle? I can only guess that maybe the Nosler load produces enough blast to interfere some how. I don't shoot either 6mm or .220 but I doubt they blast as much as the 7mm RM.

If my memory serves me correctly, the gases propelling the bullet from the muzzle are supersonic and actually can overtake the projectile for a very short distance past the muzzle. The chrono may be picking up those gases/shockwave and giving false information. That load may have had a relationship between these things that were close enough to not cause an error in the chrono's "logic"?

Just guessing here, but I'd bet that the ammo is closer to its advertised velocity. For reasons such as this and others I am seriously considering the Magnetospeed chrono.

 

[Barrett93]

I normally keep the chrono about 12-15' away. I've never actually stepped it off. I'm going to back it up to 20' and try again. I was talking to some guys on another page before I read your comment and we came to the conclusion that being too close was the issue. The rifles with the most muzzle blast that I own are 7mm-08 and 300 Savage. That's why I had never encountered that problem before. Thanks for the help!

 

[TimSr]

Holy crap! I get about 3500fps from my 7mm Mag with 100gr bullets!

I'm going to say move the Chroney out also, because I believe those readings are impossible and defy the laws of physics.

Then you can say something like "Wow, I can't believe bullet speed dropped off by 1000fps in only 20 feet!"

 

[readyeddy]

I'm still waiting for the numbers at 20'. It would be funny if they read around 3,850.

 

[TBJK]

Was the ammo setting out in the sun? Where the projectiles setback further than the other loads? What style of crony do you have? Magnum primers instead of standard primers?Could it be giving false readings due to clouds moving overhead? I've heard of some Crony's that would do that. Just throwing out other options.

 

[joneb]

I to would go to 20' away with the chrony, the 7mm Mag has a lot muzzle blast.

 

[RugerOldArmy]

One thing about Chrony(s)...there isn't any great way to calibrate them.

Maybe that will change.

Sun, clouds, weather, and screens impact the readings. For the ones that fold open (shooting chrony)...being fully open or not impacts their readings as well. Distance from the gun being fired matters too.

I highly doubt the 3900 fps readings.

Sorry if this is a buzzkill. Many of us have been there.

 

[Barrett93]

It's no buzzkill. I was scratching my head at what the problem could be. It will be a couple days before I get around to shooting it again. I will post on here what the new readings are.

 

[gamestalker]

Muzzle blast will definitely produce some interesting numbers. I manage to get in the neighborhood of 3700 fps with 100 gr. HP's, so there's no way a 140 gr. is gonna hit 3900 fps without blowing the rifle to pieces.

Move the chrony out to 15', or move it out until the numbers start looking typical for that particular load combination.

I have a snub .357 mag. with H110 and 158 gr. JHP's that will get 5000 fps when the chrony is up close and personal.

GS

 

[Dudedog]

OK gamestalker I know H110 pushes them fast but that would have to be a serious compressed load. 10 ton press to get the powder in maybe?

The chrono has to be reading muzzle blast or something flakey is going on. (Gravity field from a small black hole pulling the bullet downrange etc.)
You might be able to get 4000 with 7mm/50bmg wildcat but even looking at a 7RUM or say 30/378 W nowhere close to 4000.

 

[gamestalker]

The interesting thing about my extreme .357 mag. velocities, was the consistency of the muzzle blast readings. My Son was with me that day, he just started laughing, then asked, "just how much powder did you squeeze into those cases Dad? "

GS