Advanced Marksmanship SAC Zero for Light Rifle?

[Graham]

Have any of you put a Standard Atmospheric Zero on your light rifle (.308 or match rifle)?

If so, have you made new calculations on a zero sheet for new locations?

I would be interested on what tables you are using for the effects of barometer and temperature on your new sight settings.

 

[Graham]

Re: SAC Zero for Light Rifle?

That many of you, eh?

So, everyone zeros for conditions, then re-zeros before the match?

 

[Graham]

Re: SAC Zero for Light Rifle?

Wow. Excititng thread.

 

[Greg Langelius *]

Re: SAC Zero for Light Rifle?

Graham;

I'd be more responsive if I understood what it is you're referring to. If it's something that might be useful for my current kind of KD, SR/MR shooting, I'd like to understand it more clearly.

If I'm understanding you right, this value is a basic standard, into which various local values are integrated

Greg

 

[Lindy]

Re: SAC Zero for Light Rifle?

Greg: You zero your rifle in some conditions. Those conditions rarely correspond to some "standard" atmosphere.

The Army standard atmosphere is a BP of 29.53, temperature of 59 degrees F, and 78 percent relative humidity.

The ICAO standard atmosphere is the same temperature and humidity, with a BP of 29.92.

A SAC zero corrects your zero as if you had shot it in standard conditions.

There are some applications in which a SAC zero is useful, mostly for people who use a long-range zero. An example is that the original Barrett M82 came with a base with a 60 MOA slant - which made it impossible with most scopes to get a zero at less than 1000 yards.

However, that's mostly useful if you don't have a ballistic program which will adjust that zero for the current conditions, and must recompute by hand what your zero really is under the current conditions, <span style="font-weight: bold">and</span> then must compute by hand or using tables how much elevation change is necessary for a particular shot.

If you use a 100 yard or meter zero, a SAC zero is of little value, because that zero will not change much no matter what the atmospheric conditions are, because the distance is too short for air density to affect the zero significantly.

So, no, I don't use a SAC zero.

 

[Greg Langelius *]

Re: SAC Zero for Light Rifle?

Thanks much, Lindy. While it probably has little relevence to my own avtivities, I can now understand the value and significanc.

I learned something today..., I was a good day.

Greg

 

[Graham]

Re: SAC Zero for Light Rifle?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">There are some applications in which a SAC zero is useful, mostly for people who use a long-range zero... mostly useful if you don't have a ballistic program..... If you use a 100 yard or meter zero, a SAC zero is of little value, because that zero will not change much no matter what the atmospheric conditions are, because the distance is too short for air density to affect the zero significantly. So, no, I don't use a SAC zero.</div></div>OK. Good. 'Reason I'm asking is that I've been playing with my Horus reticle, zeroing at 500 yards (normally 3 Mils low), and weaning myself from my Palm Pilot/Exbal. I'm curious about whether I can get almost as accurate as Exbal using pencil and paper the morning of a match, then just writing the corrections on my arm in pen for use throughout the day.

Is this ambitious; or foolish?

 

[Lindy]

Re: SAC Zero for Light Rifle?

I'd say it was ambitious.

I wouldn't say it was foolish.

I don't think anything which enhances one's understanding of using a precision rifle is foolish.

Personally, though, if I wanted to wean myself from the ballistic computer in the field - which is a useful ambition, as you never know when that electronic gadget is going to break - I'd build myself a density altitude table for my load.

Which I did.

BTW, since I wrote the article on my website on making such a table, JBM will now do the work for you.

Mine:

Making Ballistic Cards Using Density Altitude

Theirs:

JBM Ballistic Cards Using Density Altitude

It might be useful to read my article for understanding, even if you use JBM for the calculation.

 

[Graham]

Re: SAC Zero for Light Rifle?

I am surprised that nobody told me to just zero at 100, then add 3 Mils.

I like that article. As usual, you got there before I did. Those appear to be the calculations. But I don't have FFS yet, which looks to be a MUCH better program for this kind of experimentation and data retention than Exbal. I guess I just need FFS, instead of a raw table of factors and a scientific calculator.

 

[Lindy]

Re: SAC Zero for Light Rifle?

Nothing wrong with the ballistic engine in Exbal. If you noted the comparison I did in this thread, it works:

http://www.snipershide.com/forum/ubbthre...690#Post1294690

But FFS has a more useful feature set for what I do.

A DA table makes a good backup, though.

 

[Graham]

Re: SAC Zero for Light Rifle?

On a recent trip I noticed and noted the changes from zeroing two factory loads at 750 feet and 65 degrees, then shooting them at 3000 feet and 88 degrees (although I don't use the altitude box in Exbal, I set it at zero and take station pressure from my Kestrel): My 168SMK factory ammo shot to my 170HPS Lapua factory zero; the 170 HPS was .3 Mils high all-around.

What I couldn't account for, and had no calulations to derive, was the change in rifle/ammo temperature.

I know what my loads do for muzzle velocity at 70 degrees and 80 degrees, and Exbal gets me close when the outside air temp changes, but I am noticing a huge difference when chrono'ing cold ammo and rifle (both at 60 degrees) on a 90 degree day, versus letting the rifle and ammo warm to 90 degrees in the sun.

So, whether I unbox from the air conditioning and shoot first, or last, on a 600 yard cold bore stage could mean the difference between a hit and a miss.

 

[Lindy]

Re: SAC Zero for Light Rifle?

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">What I couldn't account for, and had no calulations to derive, was the change in rifle/ammo temperature.</div></div>

Muzzle velocity changes with temperature, but not with altitude, as the oxidizer is contained in the powder, and the atmosphere is irrelevant. The only way to know how much that is to measure it by shooting over a chronograph at different temperatures.

Absent testing, I use a figure of 1 fps of muzzle velocity change per degree F. That figure is not far off for the lot of Black Hills 175 I tested. If you're handloading, Varget is about half that temperature sensitivity.

 

[Graham]

Re: SAC Zero for Light Rifle?

Thanks. I'll try testing that a bit this weekend.