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Live Fire vs. Generated Drop Chart - Long(ish) Range Shooting help

813 Views 6 Replies 3 Participants Last post by  SABuzzard
This has been bugging me for two days after my most recent trip to the farm with my new .308

I am fairly new to shooting at these ranges (out to 700 yards so far) and it's been a fun learning experience.

I started off by using Vortex's LRBC App to generate drop charts for the factory ammo I was using.

My first 3 or 4 time out, the information was pretty darn close during live fire. It would tell me 20.43 MOA drop for 700, and i would dial it in (or hold over, i did both) and PLINK, on target all day long. This was with Federal 168g SMK BTHP. The stuff was good enough for me, except every other box the cases kept sticking when trying to open the bolt after firing.

Last weekend I decided to try some Hornady 168g A-Max. It shot wonderfully. just over 1/2" groups at 100 yards. I was pumped! So i generated my drop chart and went back to 400 and 700 yards. It called for 7 MOA drop at 400. so I dialed it in... MISS. way low. (Note: we had a mild tail wind all day, so I know it wasn't a strong head wind causing the problem)

Long story short I was shooting 16" low at 400 hundred. So i had to come up significantly to get on target. Once I was on target, i was stacking shots on top of eachother on the steel. I am 99.99% sure my zero was still on, only thing I did was re-center the turret cap back to the 0 position, i'm pretty confident I didn't accidentally turn the turret.

So, after that long winded explanation my question is:

Is it normal to experience variations THAT significant from live fire compared to ballistic app generated charts? Is it possible for a particular factory load to shoot very ACCURATELY out of a rifle but the velocity to drop off dramatically compared to a different brand? A chance that 2 boxes of this stuff was under-charged?

I know there will always been some difference, especially with variations in BC and muzzle velocity out of your particular rifle. I know eventually I should tweak and create my own drop chart for my particular rifle and variables. I don't have a chronograph and I haven't been back out to investigate more. I just find it weird that the charts were dead on for one type of ammo, and significantly off for the next.
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Are you a hunter or is this APP more for fun . One of my rifles is a 308 21 " bolt rifle and that type of bullet drop seems far close to 250 to 275 yards . I don't shoot a 168gr amax but that did seem like bad info in general . Watching vortex video on the APP shows that bullet with that same bad bullet drop info there . Might want to contact them about there data . Seems like that would be a zero for 250 to 275 yards but I tend to be a 150gr sst or nbt shooter with my 308

Don't put to much trust in the electronic toys .

Nothing like developing your own cheat sheet or log and at best a really good range finder for those longer shots as a hunter .
 

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Discussion Starter · #3 ·
Are you a hunter or is this APP more for fun . One of my rifles is a 308 21 " bolt rifle and that type of bullet drop seems far close to 250 to 275 yards . I don't shoot a 168gr amax but that did seem like bad info in general . Watching vortex video on the APP shows that bullet with that same bad bullet drop info there . Might want to contact them about there data . Seems like that would be a zero for 250 to 275 yards but I tend to be a 150gr sst or nbt shooter with my 308

Don't put to much trust in the electronic toys .

Nothing like developing your own cheat sheet or log and at best a really good range finder for those longer shots as a hunter .
this is mostly just for fun, although if I got the chance to rifle hunt somewhere (can't in my area) I would.

the more I think about it, it has to be something I was doing. Because at one point I dialed back down to 0 and just used my hold overs based off the reticle vortex generated. and it worked fine. And the hold overs equated to the same MOA adjustment. I just need to get back out there and double check everything.

thanks for the reply though!
 

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Tools like JBM Ballistics can be very helpful. JBM - Calculations - Trajectory

The two main pieces of information you'll need are velocity and the projectile's ballistic coefficient.

Do you have a chronograph? Or access to one? Velocity of the same ammo fired from different firearms will be different (barrel length, type of rifling, chamber set-up, etc.). It can make a difference when you get out past 500 yards. Knowing what your rifle is doing is important.

Ballistic coefficients are published by the manufacturer. Here's JBM's link page... JBM - Calculations - Links to Ballistics Coefficients

The two you mentioned:
175 grain AMAX = 0.475
168 Sierra Match King (Federal GGM) = 0.462

With velocity and BC you can punch the numbers into the JBM calculator or similar tool and get very accurate information. You can go crazy with all the different atmospheric condition variables, but using the defaults will get you close. Long range shooting is addictive. Good luck!!!
 

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Discussion Starter · #6 ·
Tools like JBM Ballistics can be very helpful. JBM - Calculations - Trajectory

The two main pieces of information you'll need are velocity and the projectile's ballistic coefficient.

Do you have a chronograph? Or access to one? Velocity of the same ammo fired from different firearms will be different (barrel length, type of rifling, chamber set-up, etc.). It can make a difference when you get out past 500 yards. Knowing what your rifle is doing is important.

Ballistic coefficients are published by the manufacturer. Here's JBM's link page... JBM - Calculations - Links to Ballistics Coefficients

The two you mentioned:
175 grain AMAX = 0.475
168 Sierra Match King (Federal GGM) = 0.462

With velocity and BC you can punch the numbers into the JBM calculator or similar tool and get very accurate information. You can go crazy with all the different atmospheric condition variables, but using the defaults will get you close. Long range shooting is addictive. Good luck!!!
Yeah i've heard of JBM, haven't used it. Vortex's Long Range Ballistic Calculator takes all that data into account to. the BC i had loaded in there, and the muzzle velocity (which was for a 24" barrel not a 20" so I know mine is a little slower) i do not know anyone with a chronograph
 

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Just re-read your original post and realized I missed some of your questions...

Yes - I've found that velocities can vary significantly between different brands of ammo that are firing the same weight projectiles.

I don't believe that two boxes of the factory ammo you mentioned would be undercharged or otherwise have lower velocities that would be noticeable. Federal GGM is considered to be a known go to and in my experience Hornady has been very consistent from lot-to-lot.

I put a couple thousand rounds of 308 Federal GGM through one of my original precision rifles shooting F-Class 300/500/600 yards. Proved to be consistent.

I shot a bunch of Hornady factory ammo through a second 6.5 Creedmoor at the same and longer distances. Never felt like the velocities were fluctuating. Confirmed this several times via chronograph.

That was years ago though, and I now hand-load for my precision rifles (6.5 Creedmoor, 6 Norma BR and the good ole 308).

Long range shooting is fun and definitely a challenge. I've walked away from the range on many an occasion scratching my head and wondering what just happened. Only to return the next time and be dead-on without doing anything differently from the last time... or at least I don't think I did. I guess that's what keeps me coming back.
 
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