Home Forums Hunting & Shooting Long Range Hunting 162 SST interlock real BC?

Being a little burned out on chasing whitetails I decided to do a little shooting. I wanted to hunt with a different rifle next week so I decided to play with my 7WSM a bit. I have always used bergers in this rifle but decided to go a different route with the 162 ssts to try and find something that would load to magazine length but still shoot. I loaded a few to my desired length with 65grs of vv560 sighted in at 100 yards and dialed in elevation to get on my 800 yard target. I was approx 1.5 MOA low at 800 using my guesstimated velocity.

I tweaked my velocity in exbal and dialed in the new drops and the elevation was perfect. I was tired of driving to the 800 yard target so I decided to shoot a few on my 600 yard target since I had fresh papaer up and could see the hits through my spotting scope. I shot several rounds and was very satisfied with the amount of vertical in my load...next to none. One problem, my shots all hit 2 inches low on my 600 yard target.

My elevation is perfect at 100 yards, perfect at 800 yards but 2 inches low at 600. I did not drag my chronograph out since I assumed that the BC was correct, but apparently not. What are yall using as BC for this bullet. I would just calculate it if I had a confirmed velocity but can only guess with the info I currently have.

I would guess that the BC out of my barrel is a little higher than advertised and that I have compensated for this by decreasing the velocity below the actual bullet speed.

i would also calibrate or verify the drop on your exbal..maybe yu already did and got same result?

It is likley that the BC is off, but I wouldn't think that much. I would suggest running the load over your chrony to verify velocity, and start from there. With everything you described, its the only unknown.

R.

If you use the G7 BC your actual trajectory will be a better match to your calculated trajectory.

Not sure why the fuss over 2" at 600 yards you are talking about a little over 1/4 MOA there.

This is why you always field varify your charts. Run the same input in another program and you will get different results. Ballistic charts get you close practice get you where you need to be.

Ballistic coefficient also varies with velocity. Look at the Sierra manual; they list three different BCs for their bullets depending on velocity. Air temperature, humidity, and altitude all play a part in this. Most BCs are calculated under a standard ballistic ICAO (International Civil Aviation Organization) atmosphere and you have to re-calculate for your conditions.

Here's a handy ballistic calculator for you:

http://www.handloads.com/calc/

Ballistic coefficient doesn't have to change with velocity. Drag coefficient changes with velocity. The only reason BC changes with velocity is because the comparison to the model bullet is not exact. The G7 profile is alot closer to the 162gr sst than is the G1.

Nobody used this bullet I guess. That is okay I will derive a G1 BC that will work for my purposes. I was planning to run shoot 1400 with the gun today but guess I will set up my chronograph instead. I could figure out the true BC (well maybe not a true but working BC) and velocity with the data that I have now if this scope adjusted in MOA rather than cm/hm. Usually with data from at least 3 yardages I can get on paper at longer ranges without a spotter. I am guessing that being 1/4 MOA low at 600 would put me at least 1/4 MOA high at 800 probably more and that the error would compound as the range increased.
Maybe I can just change the outputs in ex-bal back to MOA and get a better guess on BC and velocity then switch the outputs back to cm/hm. I dunno, left my phone at home, so I cant play with it and see.

Does ex-bal allow you to input a G7 BC? If so then derive a G7 BC with your current data. For a starting point I'll bet the G7 BC will be about half of the G1. When you shoot at 1400 yards you will be alot closer than if you calculated with the G1 BC.The G7 BC for this bullet shouldn't change by more than a few percent due to decreasing velocity. The shape of the G7 model bullet is very similar to the sst/a-max bullets. The G1 BC will be double digit change from muzzle velocity to retained velocity at 1400 yards

You can use this calculator to run G7 stuff:

http://www.jbmballistics.com/calculations/calculations.shtml

How are you guys calculating G1 and G7 B.C. values?

The "form factor" value is what escapes me...

It is not a problem with the G1 BC that is causing the problem it is an input problem.
Even if your using the apparent end all of form factors you will still have to tweak it a bit to match your drops. I just have not had much luck plugging in BCs whether it be G7 or G1 and have my drops be as close as I like them. I have actually had better luck using the G1 factor with multiple BCs than I have had with the G7 BC....but it requires a great deal of work to tweak the BCs to exactly match your real drops.
I did not get to shoot through my chrono today because when I got home there were deer on the range and I had a continous parade until dark. I just got through playing with exbal and was able to get my inputs to match my drops after I switched the outputs back to MOA where I am more comfortable. Now I just got to do a bit more shooting.

I would check velocity before I do anything, of course it should have been done that day.