308 - 175-gr SMK at 1200-yards

[Alpine 338]

A friend and I moved targets around for the Winter shooting season at his place, managed to get full size IPSC steel targets hung at 1200, 1100, 1000, 900, and 800 yards yesterday. We ran out of time to get more hung, but still had a little time to put a few rounds down range.

This time I was shooting a Sako TRG-22 w/20" barrel. I wanted to see if it was still zero'ed since I pulled the scope/mount off of it earlier that day to see if it would work on a new rifle I just acquired.

Using my The Field Density Altitude Compensator (FDAC) and my Kestral, I calculated drop the best I could because my FDAC is an older version, and the lowest velocity card I have is for 2600-fps. My rifle will only do 2570-fps using FGMM 175-gr SMK suppressed, and I'm guessing that it's running closer to 2550 without the can. I was shooting it without the can.

At 800, 900, and 1000-yards I had consistant hits, tried to shoot a 12" plate at a 1000, but my windage was off from removing the scope earlier. After making a few adjustments, and still missing the plate by only a couple of inches. I decided to jump up to 1200 since I was running out of time and ammo.

My FDAC has a max. distance of 1000-yards, and on previous engagements on warmer days I was making 1200-yard shots with 13.1 mils dialed. Well my S&B ST turret has a max. of 13.2 mils. It took the 13.2 mils plus 2.0 mils of hold to hit that 1200-yard IPSC target. Was able to hit it repeatedly and consistantly.

Today, I calculated the firing solution using JBM, and it called for 14.2 mils, even less if I throw into the calculation a 15-deg slope we where shooting up for the 1200-yard target.

Anyhow, elevation was 7500-ft, and temp was 25-deg F.

So, I'm just wondering what other factors would cause me to need an addition one mil of elevation?

 

[mudvayne]

Re: 308 - 175-gr SMK at 1200-yards

Well your DA will obviously cause a shift. Also if you dont tell JBM your shooting at a specific angle, it only spits out straight line elevation. In the LOS section you have to put your angle in. And since a 175 is closer to the end of its parabolic curve at that range, uphill/downhill will greatly lower your dialing. In your case roughly a mil. Hope this helps a bit.

 

[Lindy]

Re: 308 - 175-gr SMK at 1200-yards

You mentioned the elevation, but not the actual barometric pressure, which can vary over a range.

There are many factors which can cause variations between calculated and actual performance at long distances. Some of them are listed here:

Sources of Inaccuracies in Ballistics Programs

 

[Kanati]

Re: 308 - 175-gr SMK at 1200-yards

Very broad question with too many variables that can effect that elevation difference, even with the info that was given. Density Altitude charts are the way to go too. The link that Lindy attached outlines the possible variables, good link with excellent information.

-tf

 

[Alpine 338]

Re: 308 - 175-gr SMK at 1200-yards

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: mudvayne</div><div class="ubbcode-body">... And since a 175 is closer to the end of its parabolic curve at that range, uphill/downhill will greatly lower your dialing. In your case roughly a mil. </div></div>

I can see the greater variation with a bullets flight path at the end of its parabolic curve, which makes sense. Although, shooting uphill should have given me an advantage (i.e. about one mil), but in my case I was at a disadvantage.

Not sure what the Barometric Pressure was since I only had my low-end Kestrel with me. I found my FDAC to be fairly accurate. Today I was shooting out to 1000-yards, and it was accurate to within a tenth or two tenths of a mil.

 

[oinco]

Re: 308 - 175-gr SMK at 1200-yards

Alpine338,

An interesting problem- I've been thinking about this since you posted-

if my inputs are the same as yours in JBM...it looks like a MV of 2470 -(of course disregarding factors environmental and others not depicted)-would put you at about 15.2mils/1200y no angle compensation- that delta seems quite large at roughly 80fps from your norm?

If the MV variance was within normal range- barometric pressure looks like a high of about 30.06hg in town on that day -hardly seems high enough to cause that much drop difference unless you were zeroed at some ungodly low pressure gradient not accounted for in JBM?

Were you shooting from the same ammunition batch earlier Today?

hope you are well!
O

 

[Alpine 338]

Re: 308 - 175-gr SMK at 1200-yards

O, yes same lot of ammo, but I was shooting my 5R with 24" barrel yesterday. Took a poke at the 1200-yard target, but the scope on the 5R has an eye-releif that is unforgiving, and I couldn't see where I was hitting (or missing), so I gave up vs. wasting ammo. Also, the sun was on the horizon and the temp was dropping fast.

Are you going to join Kevin and I at Chaffee the Friday before Christmas? Eric said he might be there as well.

Alpine

 

[jbell]

Re: 308 - 175-gr SMK at 1200-yards

Is your velocity correct for that air temp?

 

[Alpine 338]

Re: 308 - 175-gr SMK at 1200-yards

From what I remember, temp difference between the day I chronographed the rifle/load and the day I was shooting at 1200-yards was around 10-deg F. I'm also est. the velocity because I was shooting suppressed when I chronographed, and unsuppressed when shooting at 1200-yards.

 

[Alan_H]

Re: 308 - 175-gr SMK at 1200-yards

tag

 

[diego-ted]

Re: 308 - 175-gr SMK at 1200-yards

We shot at 1200 last weekend. I was dope on @ 850 but only had dope to 1k. It took me 7 shots to hit steel @ 1200, i had no idea another 200 yards requird so much more evevation! Tikka 308 vamint, LRS1,175SMK, 45gr Varget, lapua brass

 

[Alpine 338]

Re: 308 - 175-gr SMK at 1200-yards

I think I figured out where my problem is. I found I was seeing the same inaccuracies with my 338LM ballistics beyond 1000-yards. I had set barometric pressure in JBM to the standard 29.92 and never really measured what it was while shooting. Going back to JBM and increasing the barometric pressure value's, I was able to model my true ballistic performance to what I was seeing in the field, with both the 308 175-gr SMk and the 338LM 250-gr SMK.