Matching different bullet trajectories

[wadegarrett]

I figured this forum would have the most knowledge about trajectories and can point me in the right direction. I love stretching out the distance with the bow and figured it would be just as fun shooting long with my rifle. Please let me know if there is another thread that talks about this that I missed.

I have a Vortex with BDC reticle with 1.5, 4.5 and 7.5 MOA compensation marks. I cruised around to find rounds with ballistics that would land me with pretty round numbers at each mark (within my shooting ability error). When zeroed at 100yds, I get around 200, 300, 400, etc with the hashes with my hunting rounds.

I shoot premium rounds for hunting so I started wondering if I could get similar ballistics out of lower end bullets for shooting practice. I don't reload so I thought finding a box of rounds at half the price that shoot about the same would be good enough for practice. I found a lighter round with a worse BC that followed the same trajectory and figured I could check the sight and go. However, when I sighted in the cheaper rounds, I was consistently shooting 4 MOA high at 100yds. For reference, my hunting round is 180gr Accubond @ 0.507 BC, 2700fps and my "target" round is 150gr Corelokt @0.314, 2910fps. I understand there is a difference in velocity (and drop) so I checked a ballistic calculator by zeroing both rounds at 0yds, and they both drop within 0.5 MOA of each other at that distance. When zeroed at 100yds, these two round have very similar ballistics according to the manufacturer out to about 400yds.

Here are my questions: How can the zero be off by so much (4MOA) at 100yds? If it was a barrel twist thing, would I expect to see some L/R changes too? If I adjust 4MOA on the turret between rounds (ie re-zero), would they then shoot about the same? Are the differences in round quality that noticeable at close range? I'm mostly curious about how all this stuff works!

Thanks in advance for your help,

WG

 

[Cahunter805]

Different loads usually have different harmonics and is exactly what you are experiencing. If you want to use the cheap rounds for practice just zero them and go shoot/test your hash marks. Keep in mind before season you will need to rezero for your hunting load and be sure to verify your hash marks with them also.

 

[AZ_Hunter_2000]

Cahunter805 is spot on.

You may want to look at using a ballistic app and just dial. The problem with hashmarks with a SFP is the yardage changes depending on magnification. Max zoom is not always practical nor desirable. Here is an example (assuming SFP Vortex BDC reticle, 16x is your max zoom, achieving published velocities on your Accubond, not taking into account environmentals).
* At 16x zoom, the 3rd hash mark is 408 yards. Already 2" off on a 400 yard shot.
* At 15x zoom, the 3rd hash mark is 423 yards. That is a 3" vertical difference from max zoom which is already 2" off on a 400 yard shot.
* At 14x zoom, the 3rd hash mark is 441 yards. That is a 7" vertical difference from max zoom which is already 2" off on a 400 yard shot.
* At 13x zoom, the 3rd hash mark is 461 yards. That is a 12" vertical difference from max zoom which is already 2" off on a 400 yard shot.
* At 12x zoom, the 3rd hash mark is 483 yards. That is a 17" vertical difference from max zoom which is already 2" off on a 400 yard shot.

Just going from 16x to 14x and relying on the hashmarks can result in a clean miss on a whitetail.

 

[wadegarrett]

Hey guys,

Thanks a lot for those responses! I would have never even known to look up harmonics - that makes a lot of sense to me.

As for the hashes, I follow and totally agree. This scope upgrade was to dip my toes into shooting longer, I intend to get real familiar with both the hashes and dial. Previously, I was sighting in a little high working off MPBR, but I wanted more control over the shot - especially as I start shooting longer.

Thanks again for the advice! Looking forward to spending some time on the range.

 

[WCB]

It may not be "harmonics" but simply you are shooting a different shape projectile, that may or may not be harder or softer. The rifling may grab the bullet better or worse...different bearing surfaces etc. the barrel just basically throws the bullet different.