Range Report Sierra infinity .308 Dope is FUBAR, why?

[High Binder]

So I went out and shot a test load to bring back and create a rough elevation dope with but Sierra is simply not cooperating at all I suspect that I don't know how to tell Sierra how to account for the 20MOA base but I'm not sure. Can somebody tell me what I'm doing wrong? I'm shooting to 1200yds so here's my info: 700PSS...5.5-22 Nightforce on a 20MOA badger base with same rings and all the other usual stuff. I do everything in MOA. In Sierra, I amusing the Federal American eagle .308 FMJBT as the round I'm building off of.

5200 feet alt.
2820 avg. FPS
150gr .308 FMJBT
55 degrees
zeroed at 100yrds

at 185yds it says 1.2 but it should be 1.5


at 351yds it says 5.2 but it should be 5.25 (this one was on)


at 557yds it says 11.9 but it should be 12.0


at 1020yds it says 34.1 but it should be 36.0

So it's almost like the bullet speeds up around the 300-400 range and that's not happening so what am I doing wrong? I've messed around with the BCs but am clueless.

Thanks in advance!

 

[Lindy]

Re: Sierra infinity .308 Dope is FUBAR, why?

The slope of the base is irrelevant. All that matters is the height above the bore, and at what range it is zerored.

The BC for the bullet in that cartridge is 0.414. If that's not what you're using, it will affect your results.

Knowing the altitude is not sufficient for accurate predictions at extended distances. You must know the station atmospheric pressure at your location.

It's also not sufficient to know the muzzle velocity which the cartridge manufacturers supplies. You must know the muzzle velocity out of <span style="font-style: italic">your</span> rifle.

So, you are suffering from a lack of information.

See:

Sources of Ballistic Program Inaccuracies

 

[High Binder]

Re: Sierra infinity .308 Dope is FUBAR, why?

OK, the BC of .414 is a lot different than the .408 listed for that bullet in Sierra. The 2820 is the avg. FPS for this load out of my gun but (provided the chrono is good) the one thing I don't have is the pressure and like your article points out I did have variation of FPS like one would expect and on par with your predictions.

Lindy, that link is SWEET! Thanks for doing that write up. Does the 20MOA base cause any problems? Assuming that I did have the station pressure (lets just guess and say it was 24.89 for the sake of learning) is there any reason why I would need to change the BCs? Sierra gives you the ability to augment the BCs five times but what's the purpose of that?

Thanks in advance!

 

[Lindy]

Re: Sierra infinity .308 Dope is FUBAR, why?

The base makes no difference.

I haven't used Sierra's program in a long time. You might see how it compares to JBM Ballistics.

http://www.jbmballistics.com/cgi-bin/jbmtraj-5.0.cgi

24.89 would be the pressure in the ISA for 5000 feet. If you're at 5500 feet, it's probably a half-inch less.

At short ranges, that won't make much difference, but it sure will at 1000 yards. If you're going to want accurate predictions at those distances, you might wish to invest in a Kestrel, or another device which accurately measures the station pressure - I wear a Casio Pathfinder watch when shooting.

If you always shoot in the same place, and have an accurate source of sea-level-referenced barometric pressure nearby, you might wish to check the elevation of the range where you shoot.

Once you know that, use the chart below of the ISA, and figure out what the offset is from sea level in the ISA at your elevation. Then subtract that amount from your local source of sea-level-referenced barometric pressure.

Example: Let's say you're always shooting at 7000 feet. The pressure in the ISA for 7000 feet is 23.09.

So, the offset for 7000 feet from the ISA sea-level pressure of 29.92 is 6.83 inches. So, if your local source says the SLP on the day you're shooting is 30.2, your local station pressure is pretty close to 30.2 - 6.83 inches, or 23.37 inches.

That won't be as accurate as a device which measures the station pressure at your location, but it's better than guessing.

You may also wish to take a look at:

Barometric Pressure and Ballistic Software

 

[Swan]

Re: Sierra infinity .308 Dope is FUBAR, why?

Atmospheric issues aside, you may want to choose a better bullet if you plan on shooting to 1200yds. Odds are slim that projectile will make it and be remotely repeatable. Wind is going to kill you at that range with above mentioned bullet. Look towards the 155s from Berger and Lapua. Also, +1 on the JBM website for more accurate ballistics.

 

[High Binder]

Re: Sierra infinity .308 Dope is FUBAR, why?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body"> you might wish to invest in a Kestrel, or another device which accurately measures the station pressure
</div></div>

A Kestrel is how we figured that we were at 5200, it varied throughout the day but only by +/- 30' and apparently by just entering the alt. and temp into the Sierra program it will automatically figure station pressure but everything is suspect at this point. I'll try the JBM, thanks guys (Lindy)

 

[High Binder]

Re: Sierra infinity .308 Dope is FUBAR, why?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Swan</div><div class="ubbcode-body">Atmospheric issues aside, you may want to choose a better bullet if you plan on shooting to 1200yds. Odds are slim that projectile will make it and be remotely repeatable. Wind is going to kill you at that range with above mentioned bullet. Look towards the 155s from Berger and Lapua. Also, +1 on the JBM website for more accurate ballistics. </div></div>

I know that it's not optimum but I can consistently get on paper with the 150s and 168 match bullets @1000yds (low/no wind) but I'd really like to try some of the 155s, any suggestions on loads?

 

[Lindy]

Re: Sierra infinity .308 Dope is FUBAR, why?

If you have a Kestrel, set the reference altitude in the Barometric Pressure window to zero. When you do that, the Kestrel displays the local absolute station pressure, which is what you want to use in ballistic software.

See:

Barometric Pressure and Ballistic Software

I cited that link previously in this thread. If you're going to use ballistic software, it's a good idea to understand it.

There have been scores of threads on this site about loads for the Lapua Scenar 155, as well as the Sierra bullets. Use the search function.

A Google advanced search input screen is shown below. That search turned up more than 3900 links to posts with mil, moa, and hunting in them. Some of them are <span style="font-style: italic">not</span> related to hunting, but many are.

The Google advanced search, restricted to snipershide.com as shown in the input screen, is by far the best way to search this site. Feel free to click on "Date, Usage Rights,..." to further restrict the search.

http://www.google.com/advanced_search?hl=en

 

[High Binder]

Re: Sierra infinity .308 Dope is FUBAR, why?

Lindy, that's awesome! Never seen that before! Thank you!