PDA

View Full Version : Trajectory/Ballistic Info for 308


BlindRacer
06-05-2012, 2:08 PM
I'm sure it's been hashed over beyond any other topic, but I just can't wrap my head around some of the stuff. I've been trying to figure it out all day.

I have a 26" barrel on an Rem700.

I'm trying to find out the Ballistic Coefficient, as well as the Muzzle Velocity.

Everything I'm finding is giving varying figures, which when plugged into the calculators give a VERY wide variety of results.

I'm looking for info on Federal Gold Medal Match 168gr, as well as American Eagle 150 gr.

Also, the drag number...which I'm thoroughly confused about. Some say G1, some say G7.


All I'm trying to do, is to get a rough estimate to be able to get me on paper at distance. I'll be shooting paper through 300 yards, and steel out to about 600 yards. I'd like to get data to at least 750 yards.

The main issue I'm running into, seems that no data is made for the 26" barrel. It will be a faster muzzle velocity than the others, and I don't know how much that would affect anything.


Thanks for all your help!

It would be wonderful if there were already charts somewhere for 168gr and 150gr from a 26" barrel!

Bhobbs
06-05-2012, 3:23 PM
The G7 BC of a 168 Matchking, which is what I believe the FGGM is loaded with, is 0.218.

Velocity is listed in the 2,600 fps range but I'm not sure what barrel length.

The higher muzzle velocity will give you a flatter trajectory. I would put in 2,600 fps and 0.218 G7 BC and see how it matches your drop. If it is lower, increase the velocity until it matches. Then shoot different distances to see if it still matches.

Pryde
06-05-2012, 3:28 PM
To get the Velocity you need to chronograph your shots.
There is no guideline as to a correct velocity for a particular ammo and barrel length because each barrel may shoot differently.

The same load shoots 50 FPS faster in my AI than in my FN and both have the same barrel length.

Bhobbs is spot-on about the BC, you have to look up the bullet manufacturer to see the BC, FGMM 168 uses Sierra Matchkings. (The BC is actually 0.462)

cfusionpm
06-05-2012, 3:29 PM
I use 2600 fps and 0.460 bc in my little Strelok ballistics app, and it's pretty close out to 600yds.

cfusionpm
06-05-2012, 3:30 PM
For 168gr GMM out of 20" 700 SPS tactical

Wolfie_AR
06-05-2012, 3:41 PM
Try the attachment as a start but previous posters are right, you need to chrono to get absolutely accurate data, of course temp, humidity, etc all play their part in making it slightly different everytime. Then there are differences in lot numbers, etc. but we can get to that later.

BlindRacer
06-05-2012, 3:44 PM
Thank you all for the replies.

However, this just reiterates what I was talking about. One of you said the BC with G7, and another the BC for G1?

I will get my measurements from my specific gun, and find out exactly where it is at. I just want to get a pretty close estimate to get me within a few MOA, so I can actually be on paper at 400/500 yards.

From specific data I've found at 500 yards, there is a drop between 50-65"

I don't know what to believe to get myself at least close.



My guesses from everything that I can find, is the Federal GMM 168 out of a 26" barrel is about 2700fps muzzle velocity. That sound about right?

24" looks about 2650fps, and 20" looks about 2500fps.

I'm just looking for approximate info to plug in.


So 2700fps muzzle velocity, G1 - .462bc / G7 - .218. Will this get me close?

BlindRacer
06-05-2012, 3:49 PM
Try the attachment as a start but previous posters are right, you need to chrono to get absolutely accurate data, of course temp, humidity, etc all play their part in making it slightly different everytime. Then there are differences in lot numbers, etc. but we can get to that later.

I have a chart that looks almost exactly like that. I was just surprised because all the other charts hat the 1000yard drop in the low 400" mark.

This one was done with G7 drag, but with the BC of the G1, I believe.

I really don't know if I'm understanding all of this correctly though, so I might not even be making sense.

Bhobbs
06-05-2012, 6:29 PM
I suggest reading up on the different BCs. It will help you understand why some people use one or the other.

Aeonstar
06-05-2012, 11:38 PM
Blind the chart is wrong. The bc should have been G1 not G7. The drop for the round should be near 400" at 1000 yards.

You need three things to calculate drop. The weight of the projectile, the velocity out your barrel and the BC expessed as a .xxx number with it's model type.

If you have a smart phone you can get a ballistics app and play with it all day.