AI AT Barrel change POI shift

[vno.helix]

Hello,

I am the proud owner of an AI AT rifle with a 24 inch, 1:12 factory LW barrel. In addition I have a 22 Inch #7 Bartlein 1:10 5R barrel chambered by GAP. Both barrels are shooters and one hole groups at 100 yards is common and fairly easy accomplished with either barrel even more so with the Bartlein shooting GM308M2 (175 gr SMK).

Now this is my question; While it doesn't seem to affect accuracy or consistency with either of the barrels, there is a 10 cm shift of impact from one to the other with the 24 LW shooting exactly 1 MRAD (10 clicks on the S&B) higher then the 22 Bartlein at 100 yards. (note: no lateral shift, just elevation)

Can someone explain this to me? At 100 yards there should not be such a significant velocity change to justify this large of a "drop" due to a 2 inch barrel length loss, correct?

(Unfortunately I do not have acces to a chronograph to support my question with more scientific data)

Thank you.

 

[rrflyer]

Its a different barrel. That makes it a different rifle so a change in impact is expected normal.

So long as it shoots well... Adjust the scope. Shoot it.

 

[ZombieMonkey]

It's not mainly an issue of velocity, it has more to do with harmonics of each barrel, etc. For example, my AT sees only a .1 MIL vertical shift between the stock 20" 308 barrel and the 24" 260 barrel, however I see a .5 MIL horizontal shift. All that matters is that your shift is repeatable, I wouldn't get too caught up on the shift difference between barrels.

 

[vno.helix]

Even in a near perfect theoretical world where all being the same and within say; 1/1000 tolerances, I was expecting there to be some shift, just not this much. Please don't get me wrong, I am not complaining. The rifle is beyond every expectation and ridiculously accurate with either barrel. I just could not wrap my mind about the magnitude of the shift. I have switched barrels back and forth several times, and even the POI change is perfectly constant, so as long as I correct for it the right way, the rifle is always zero'd for the particular barrel.

Thanks for replying.

 

[hk dave]

Barrel harmonics, barrel whip?

 

[orkan]

It would be wise for people to remember that barrels are not straight. Nor is the bore straight in them. Sure, they are "straight enough," but if you put them on an indicator, nearly all will be "bent" a bit one direction or the next.

Even in the DTA's we sell and have done work with, we sometimes see a mil worth of shift between barrels. Worst we've ever seen was 1.5 mils. Average is .3-.5 mils. That's in a rifle with 6" of barrel supported by the action. The AI has far less than that, so I'd consider a 1 mil shift pretty good!

Bottom line: It's normal. Set each barrel zero, log it, and roll with it. Now if each barrel's zero doesn't repeat... that is a whole other discussion.

 

[jake6547]

Have you ever seen a barrel spinning in a lathe while being chambered? I have seen quite a few and out of all them the muzzle end is always wobbling a little bit. No barrel is strait the only important part is setting it up in the lathe so the first three inches or so on te chamber side is exactly on center with the spindle of the lathe.

 

[jake6547]

Orkan come on man I was in the process of saying the same thing but you beat me to it.

It would be wise for people to remember that barrels are not straight. Nor is the bore straight in them. Sure, they are "straight enough," but if you put them on an indicator, nearly all will be "bent" a bit one direction or the next.

Even in the DTA's we sell and have done work with, we sometimes see a mil worth of shift between barrels. Worst we've ever seen was 1.5 mils. Average is .3-.5 mils. That's in a rifle with 6" of barrel supported by the action. The AI has far less than that, so I'd consider a 1 mil shift pretty good!

Bottom line: It's normal. Set each barrel zero, log it, and roll with it. Now if each barrel's zero doesn't repeat... that is a whole other discussion.

 

[vno.helix]

That is all I needed to know. As usual; you sharing your knowledge is much appreciated. I would not be able to have learned or continue to learn without it.

 

[AvsFan]

Here's a couple of great videos from Mark @ Short Action Customs which explains your issue...it essentially has to do with timing the curvature of the barrel to the action.





AvsFan

 

[vno.helix]

Just realize that 10 cm @ 100 meters is 0.06 degrees (tan-1 0.001=0.057) difference between barrels. I am most likely overthinking this but find it all way too interesting . So 1 click, 0.1 Mrad on my scope is 0.006 degree change. That is just overwhelming to me. I am going to have another beer!