Muzzle velocity not matching book values... Need help

[Punisher]

I had to work up a new load for my 7mm because I can't find bergers. I am shooting a 160 Nosler accubond with 63.0 gr of imr 7828.

I had to cut almost 100 fps off of the speed that I get from my Nosler manual to make my dope match reasonably "close " to field data. To hit spot on, I need to cut about 150 fps and drop the BC down .010.

*** is going on here?

My program has been right on the money for everything else. I realize that I'm now shooting a spritzer instead of a hollow point. But I refuse to believe that I'm only getting 2750 fps out of this load that should be closer to 3k!!!

I turned the atmospheric condition calculator function off and ran it again and got the same results.

The point of having the program and all this high tech gear is supposed to save me from having to shoot every range to get dope for that load.

I'm perplexed. What a blunderf***!

 

[gohring3006]

Are you going off of noslers advertised BC ? If so I'm not surprised that your having to drop the number. They are known to inflate there BCs. As for velocity there Numbers are from a particular test rifle and I have seen velocities vary. Check there manual it could have a longer barrel than your rifle. It can be a lot of variables when it comes to velocity....

 

[Punisher]

I do know that my barrel is the same length as the test rifle. Plus I have a muzzle brake.

I am not surprised about Nosler inflating their BC. But I use the same amount and type of powder on 180 gr bergers and get 100 fps more. Which is right on with my book values.

 

[bigngreen]

I would drop more BC for sure! The issue is not the program but the data your inputting into it, Berger BC's are very good, Nosler not so much so unless you have a Litz BC for a bullet your going to be using a chronograph and shooting a lot to get a solid trajectory.

 

[gohring3006]

I had to work up a new load for my 7mm because I can't find bergers. I am shooting a 160 Nosler accubond with 63.0 gr of imr 7828.

I had to cut almost 100 fps off of the speed that I get from my Nosler manual to make my dope match reasonably "close " to field data. To hit spot on, I need to cut about 150 fps and drop the BC down .010.

*** is going on here?

My program has been right on the money for everything else. I realize that I'm now shooting a spritzer instead of a hollow point. But I refuse to believe that I'm only getting 2750 fps out of this load that should be closer to 3k!!!

I turned the atmospheric condition calculator function off and ran it again and got the same results.

The point of having the program and all this high tech gear is supposed to save me from having to shoot every range to get dope for that load.

I'm perplexed. What a blunderf***!

Are you maxed out on pressure? If not I would go after the next node, I understand your frustration. I keep a bottle of ibuprofen in my range bag. I have been lucky so far to get everything close enough to work with my calculator but I'm going to have to get a chrony......

 

[Punisher]

I just did like 2 hard hours of Internet research about this problem. So I decided to look up some load data from Hogdon's website and guess what?

I am shooting 63.0 gr imr 7828 their max is 64.0 gr with a max velocity of 2885. About a full 100fps slower than my old Nosler manual. I don't have anxiety over being about 40 fps off the book value of velocity now! But I did still have to shave .010 off the BC and probably will take another .010 off at least, before I'm done.

I am with you guys about the chrony though! I want one, but I also want ... A whole bunch of other stuff too!

I would rather get on of the magnetic ones that Len offers! I think they are the easiest way to go. And probably as accurate as or better than the shadow box.