I have a few ballistics questions

[JamesnTN]

First why do we have a G1 and a G7 profile? Also between the two why is it better to use the G7 over the G1?

Next how important is it to know station pressure Latitude and Azimuth when plugging into the software.

I understand the need to know on MV loss per degree in temperature, but what about the importance of look angle and bullet length?

Then next question scope height how critical is this in getting the exact distance of centerline of the scope to centerline of the bore?

 

[Keith Glasscock]

It all depends on the requisite accuracy. If you are shooting at deer at 300 yards, you can pretty much forget all of it, on the other hand, if you are trying to nail a prairie dog at 1k, all of them can cause a miss.

Try varying each one and look at the results.

The following is a simplified version of why they matter. Bryan can step in and give an excellent description, or you can find one in his books.

Azimuth and latitude pertain to coriolis effect (bullets hit to the right in the northern hemisphere, and the rotation of the earth causes elevation variations if shooting east or west).

Varying station pressure is the same as shooting at a different elevation above sea level. For the most part, you are unlikely to be off more than a couple of hundred feet in your elevation above sea level (average change is 1 inch of mercury per 1000 feet of elevation). With that in mind, that number is used to compute density altitude which is a reference basis for the air density and relates directly to drag upon the bullet.

Look angle pertains to shooting up or down hill. Things aren't always as they seem. It is easy to shoot over targets. Think of it as slant-angle correction.

Bullet length (and twist and velocity - thus Sg) pertain to spin drift. Another component that makes bullets hit to the right at extended ranges (at least with our right-twist barrels).

Hope this helps,
Keith

 

[Bryan Litz Ballistics]

Keith did a good job addressing your questions, but I'll attempt to elaborate further.

First why do we have a G1 and a G7 profile? Also between the two why is it better to use the G7 over the G1?

G1 and G7 are different standard projectiles having their own drag models. The objective is to model the drag of your bullet with a standard which is the best match for drag over the range of velocities that the bullet will fly. For modern long range bullets, the G7 standard drag model is a much better match for drag than the old conventional G1 (which is a flat based, short nosed model that looks like a black powder cartridge rifle bullet). You'll see the inaccuracy of using a poor fitting standard (G1) more as the range increases. The problem will manifest as inaccurate trajectory predictions when using ballistic software.

Next how important is it to know station pressure Latitude and Azimuth when plugging into the software.

Again, these are things that become more important the longer the range is. If shooting within 600 yards, you can miss station pressure by a full 1" of mercury (ie, 28.92" vs. 29.92") but if you're shooting 1000 yards, you should nail this down to the 0.1".
Latitude and Azimuth are both used to calculate coriolis effect. This is minor even at long range, generally amounting to a click at 1000 yards.

I understand the need to know on MV loss per degree in temperature, but what about the importance of look angle and bullet length?

As Kieth stated, Look angle is there to account for gravity effects on your bullet for inclined shots. If you're close to level fire, you can leave this at zero. Only of you get beyond +/-5 degrees or more would you need to account for it on a long range shot.
Bullet length goes into the stability calculation, which is used to calculate spin drift. This is another detail that's usually only important at long range, amounting to about 10" at 1000 yards. It's typically difficult to see spin drift directly due to wind effects overwhelming, but it's there. You'll want to have a good level on your scope also to avoid dumping shots to the side due to rifle cant.

Then next question scope height how critical is this in getting the exact distance of centerline of the scope to centerline of the bore?

Do your best to measure this accurately, but as long as you get this within +/-0.1" you'll be in good shape. This is required for the software to know where your bullet starts out in relation to your line of sight.

Take care,
-Bryan

 

[JamesnTN]

Thank you both on your answers.

I was in a discussion on another forum of the importance of the questions I asked and I wanted to direct them to the answers of why they needed these inputs for a ballistics program to get them beyond 1000 yRds more specific from 1600-1760 yards.

 

[jcampbellsmith]

....Then next question scope height how critical is this in getting the exact distance of centerline of the scope to centerline of the bore?

I have in the past got this wildly wrong. I've now got an Excel table to track my scope heights.

Regards

JCS