MOA changes significantly over distance!

[Tempest]

Need some input from more knowledgeable people on this.

I'm starting to beat my head against a wall on this. I bought a Savage 12 Benchrest used that had a low round count. I started w/ a .308 load similar to the load I used in a 10BA. The 10BA shot sub 1/2 MOA at 100 when testing loads and held .50 MOA out to 500 on a regular basis. I shot it many times at 1,000 at it did pretty well with this load.

I started out with same load for the 12 BR model and it did not respond. The rifle clearly liked a different load. So I played with powder to find a node and it also clearly liked a jump and I had to load them shorter.

So after I found a powder weight and velocity that worked well, I played with seating depth. The thing shoots killer at 100 yards < .50 MOA hole in hole type groups. However, when you take it to 500 or 1,000, compared to my other gun it shoots like crap. I've played with everything I can think about up and down and it seems like the gun just will not shoot. To remove the re-loading variable, loads done in the same equipment and process shoot great in my other rifle.

So my question is, what am I missing in this equation? It seems to like a lot of jump (around .035" has been best so far). I'm starting to think the barrel sucks and may need to re-barrel.

Here are the specifications:

Lapua brass, fired formed
CCI 200 or Fed 210 primers
43.5g Varget seems to be most accurate so far in Lapua, 43.7 in Win
175g SMK
Neck concentricity around .001"
Velocity is around 2750

I've chrono'd and had pretty good ES numbers. I'm at a loss with this gun!

 

[Immike]

Are the shots in ring around the center? I watch for this when a barrel is going south. Try running your jump test at a longer distance and see what you get. 300yds is where I jump test.

 

[Tempest]

Are the shots in ring around the center? I watch for this when a barrel is going south. Try running your jump test at a longer distance and see what you get. 300yds is where I jump test.

I ran the jump test at 500 today and that's what I'm saying. 100 was great, 500 would open up to 1 MOA from <.5 MOA.

 

[jonbearman]

Perhaps a different bullet like the berger 185's. Or try a different powder like rel 15 or 4064.Not all powders work well in all barrels and the same with bullets.

 

[6BRinNZ]

Are the shots in ring around the center? I watch for this when a barrel is going south. Try running your jump test at a longer distance and see what you get. 300yds is where I jump test.

I ran the jump test at 500 today and that's what I'm saying. 100 was great, 500 would open up to 1 MOA from <.5 MOA.

How do you choose your load at 100? IMO the process used to choose a load at 100 has a a large bearing on how well it shoots at distance.

 

[Tempest]

Are the shots in ring around the center? I watch for this when a barrel is going south. Try running your jump test at a longer distance and see what you get. 300yds is where I jump test.

I ran the jump test at 500 today and that's what I'm saying. 100 was great, 500 would open up to 1 MOA from <.5 MOA.

How do you choose your load at 100? IMO the process used to choose a load at 100 has a a large bearing on how well it shoots at distance.

Started low with powder and worked up for best group. Then I played with seating depth to find best again.

Thanks for comments. Trying to figure this out.

 

[Immike]

Mr Tempest, try something for me. Can you put a bullet in the case of a fired round from that chamber?

 

[wolley]

Better bullet.

 

[pacificman]

More than a few times I've had a few bullets do well at a hundred yards but when taken out to further distance one seems to always out do the other. I would try different bullet/s.

 

[6BRinNZ]

Are the shots in ring around the center? I watch for this when a barrel is going south. Try running your jump test at a longer distance and see what you get. 300yds is where I jump test.

I ran the jump test at 500 today and that's what I'm saying. 100 was great, 500 would open up to 1 MOA from <.5 MOA.

How do you choose your load at 100? IMO the process used to choose a load at 100 has a a large bearing on how well it shoots at distance.

Started low with powder and worked up for best group. Then I played with seating depth to find best again.

Thanks for comments. Trying to figure this out.

I'm not convinced that smallest group at 100 is the best approach for finding good groups at distance. 100 yards is very tolerant of loading practices. Not to say that this approach won't work it may not be as consistent across loads and rifles.

There are plenty of people that state they have found good loads at 100 that shoot poorly further out, and there are others (all good shooters) that state 100 yard load development is the way to go. The difference (from what I can tell) is what criteria they are using to select a 100 yard load.

This thread needs some context...you are comparing two rifles, can you state (or show) the groups at 100 and further out for both.

 

[A_Gamehog]

6BRinNZ

is what I would say . My other thoughts are the rifles have different twists...you never said..

 

[seymour fish]

OP, as previously requested, please define "like crap". Vertical, shotgun, minute of milkjug, etc.

 

[Syncrowave]

In any event, external ballistics, wind conditions, shooter error, etc., all increase group size in a non-linear fashion as distance increases, so precision (group size) in MOA always goes up as distance increases.

That's my experience...1 MOA at 100 yds never seems to = 1 MOA at 300 yds. Just as trajectory curves parabolically, so does windage, etc., and the further out you go, the more groups sizes increase in MOA.

 

[Tempest]

Mr Tempest, try something for me. Can you put a bullet in the case of a fired round from that chamber?

Why yes on the bullet fitting into neck.

Responding to the other questions:

*I don't think I'm expecting too much from rifle when I stated my other riflle (10BA) seems to have no issue holding .50 MOA out to 500 yards but a "target" rifle with longer barrel does.

*I understand the physics regarding the MOA will be harder to mantain over distance. I just think this combination is not what it should be.

*It's not a vertical issue on groups, more random.

*Correct on different twist. This is a 1/12 and other is 1/10.

*I will take some pics later today of groups.

*It has been suggested I look at 155 Scenars and compare. Might give that a try.

 

[Shynloco]

You may already know this, but the bullet you are using "SHOULD" work in that particular rifle. If it was me, what I'd do is consider changing from Varget to either RE15 or IMR4895 which are my fall back options in my .308 using the Sierra's. Now I have have seen Sierras not work in some rifles and then the Bergers did. But I've also seen it go the other way too. Just some options to consider. The other option might be to use the same powder load (Varget) and try a steady .010 change in bullet seating depth and see what that gives you at 1K.

Alex

 

[CatShooter]

*Correct on different twist. This is a 1/12 and other is 1/10.

This may well be the crux of your problem... I stopped building 12" twist 308's in 1998. For 175 SMK's I would never go slower than a 10" twist.

 

[BoydAllen]

Years ago there was an article published in The American Rifleman by Col. CE Harris. The upshot was that for .308 and .30-06 using heavy MKs that the 10" twist barrel had an advantage at longer ranges, and the 12" at 100 yd. The same barrels were rechambered in both calibers. I believe that they were Harts. Also, some time ago, Ruger tightened the twist of their 7 Mags because testing had shown that it improved accuracy with heavy bullets at long range.

 

[RGRobinett]

The following wind-drift figures, from "simplified trajectory" @ JBM Ballistics, provide an answer to this old question. The calculation was for .45 BC and 3000 Fps. MV: .45BC, 3000 FPS, 10 MPH wind, Std. conditions.

These are full value drift (dead cross-wind), from 100 to 1000 Yd, calculated in MOA: .6; 1.3; 2.0; 2.8; 3.7 . . . . all the way up to 9.2. Divide these numbers by 10, which will reveal the drift for a 1.0 MPH variation in wind velocity: so, at 300 Yd., a mere 1.0 MPH change - assuming a constant vector - amounts to a shot .6" out of a ZERO group; and a 2MPH change produces a 1.2" group . . . This is particularly cruel when shooting score-targets, as the targets are generally sized proportional to distance, as opposed to exponential increase in drift! ;D

Another way to look view it: for the same velocity/vector, drift more than doubles between 100 and 200 Yd; from 200, to 400, doubles, again. Between 100 and four hundred, drift, per 1.0 PMH change in wind velocity, is factored by 4.666. So, who can dope 1.0 MPH ? ;D

Now, if we consider this same scenario, using, say, a .5 MOA rig, the potential for BIG, becomes, well, BIG! It's seems a miricle that we accomplish what we do.

The short answer: tune at close range, where group shape/size is less subject to the dynamics of stability, precession, constantly shifting vector, velocity, etc. Then learn how to "read" wind and mirage (really, another wind-flag). RG

 

[Erik Cortina]

Check barrel twist, don't assume. It's easy to do with a cleaning rod.

 

[Tempest]

Starting to sound like the 175's are the problem w/ this rifle and the 1/12 twist.