Zeroed with old ammo

[tmax.reid]

Need some help. Got a new vortex scope for my patriot and took it out to zero today. My brother gave me an old box of 130 gr. Win Super X (we are talking at least 15 years old) so I figured I would get an initial zero with it and then test accuracy with some premium rounds.

I got decent MOA groups with the old ammo and switched over to 140 gr. Nosler Balistic tips that I like to use for deer. I still got good groups but my point of impact was about a foot low. My initial thought is that the old ammo has way lower velocity is causing the difference but I would think it the point of impact would be high if that was the case. Is logic wrong here or could this be something else?

 

[Scoop]

Can you chrono? That would be a big help.
Internal Ballistics: Basically the muzzle velocity.
External Ballistics: The bullet shape [affecting the ballistic coefficient] would affect your drop if the MV is close.
I'm guessing that the 140s are slower than the 130s. Since they spend more time travelling they drop more.

Give us numbers and somebody here will come up w/ a good answer. A pic of the bullets could help.

 

[tmax.reid]

Can't chrono unfortunately.

Nosler BT: 140 gr, 2900 fps muzzle vel, BC .456

Win Super X: 130 gr, 3060 fps muzzle vel, BC .372

Got this info from the Nosler and Winchester websites.

Thanks for the input!

 

[Scoop]

Playing w/ the ballistics calculator at http://gundata.org/ballistic-calculator/
I get a difference in drop related to the 5 millisecond flight time difference of only 0.12 inches.


RED .270 Winchester, Winchester Power-Point, 130gr BC=.372
000 Yds ... 3060 fps
100 Yds ... 2803fps 103 msec

BLUE .270 Winchester, Federal Nosler AccuBond, 140gr BC=.456
000 Yds ... 2900 fps
100 Yds ... 2697 fps 108 msec

- See more at: http://gundata.org/ballistic-calculator/#sthash.oYVOR3Sf.dpuf

There is much more involved in the physics of this. As you get to high velocities in air the drag is governed by higher exponents. But play around w/ ballistic calculators when you can't shoot or reload.

I am editing this right now. The table came out terrible. Give me a little time.

OK. I'm done.

 

[tmax.reid]

Well thanks Scoop. I didn't figure the difference in weight and muzzle velocity would make that much of a difference. For now im just going to chalk it up to my Patriot doesn't like those Noslers. Im definitely going to get a variety of factory loads to see if i can find anymore inconsistencies.

Thanks again for the info

 

[sullivan]

FWIW, my understanding is that your barrel vibrates like a whip. That is, there is a fixed end (the chamber/action) and a tip/muzzle, which whips up and down. It is more complex than that simplification but the muzzle could be Anywhere in its up-and-down-travel, when the bullet exits. Exactly where the muzzle is depends, for instance, on bullet weight. And where the muzzle points is where your bullet goes. If the Noslers grouped OK and you liked their performance on game, then it is a matter of adjusting your sight to a preferred point of impact.
Good Hunting

 

[tmax.reid]

That's a good point Sullivan. I actually don't have any experience on game with the nosler so this was my first go around. This is my first deer rifle and didn't get the chance to take a deer this past season. Hoping to take a hog here in a few weeks though

 

[MikeD]

Despitre being low was thge group decent?

Every gun likes a different load. It' possible that harmonics created with that load just aren't a match for your gun. Could be heat generated by the different loads. Lots of possible vasriables

I would try a few different loads and see what it does.

 

[Scoop]

Some full disclosure. In Post #4 above I made an error by a factor of 10 because I miscounted some zeros in my notes. I last edited it with the correct figure [0.12 Inches] a little after midnight on 3/30.

 

[Scoop]

Here are my notes. That's what happens when I get tired.

 

[terry2425]

Check and make sure your scope has not come loose or has moved. You'll find different rifles like certain types of bullets better than others. That's the fun of developing a load for a particular rifle and bullet weight. Case, powder and bullet can create different results, you just have to find the one that shoots the best.

I can't help you with why the Nosler shot so low. Personally I have very good results with the Nosler Balistic Tip bullets.

 

[Black_Sheep]

Thanks for linking to the ballistic calculator Scoop!

 

[Scoop]

You are welcome. The gundata calculator is one of several available: google ballistic computer Then pick the one that is easiest to use w/ the data you have. They may come out w/ different solutions if they use different models for aerodynamics. There is a lot of wiggle room in this science.

 

[tmax.reid]

Alright! Got an update. Went out to the range today after I got skunked by the wild turkeys. Got some new Winchester SuperX 150gr soft points, Federal Fusion 130 gr, and some Herters 130 gr soft points. Shot the first group with the Herters and was right at 7" low with just a touch more than MOA groups. Rezeroed and tried two groups of each of the other loads maintaining the same zero within a half inch. Then went back to those Noslers and right on the money! Best group with the Nosler was .72".

 

[Scoop]

OK! If you get a chance I'd love to see pix of some of those groups on paper.