Question on changing POI as my gun warms up

[justeric]

Has anyone found that their AR-15 POI shifts down when their gun warms up? I shoot about 1 shot per minute usually in 5 shot groups. I first noticed this when shoot at the 600 yard range and as the day went on I had to hold over more. I would think the gun warming up would increase the velocity but it appears the opposite is happening. A guy at the range suggested maybe my gas port opens up a little (its not adjustable) as it warms up. Any ideas why my POI is shifting down as the gun warms up?


Thanks,
Eric

 

[lmorrison17]

"As the day went on."

Possibly a change in head/tail wind condition as the day progressed.

 

[sgt23preston]

I've been shooting a long time & I've never heard of the POI moving more than a few clicks unless you have a bad scope or a loose scope mount or change your ammo mid day..?

How many rounds of ammo do you shoot on an average day..?

 

[Speed3]

I'm sure Ed shell can provide better info, but every time I shoot 600 in fclass, my come ups are different. Barometric pressure matters, how dense the air is....temp of the barrel and ammo etc.

 

[jimbobborg]

I've heard this about Mini-14s and pencil-barreled AR-15s. But usually, it's after the barrel has heated up. Is something loose in the rifle? Sights, barrel, etc.

 

[DaemonAssassin]

If you can repeat the process at more than one range outing, then that might be something to be concerned about.

It could also be that your rifle barrel is fouled and has found its happy place at the bottom of the arc.

 

[Broncolou]

With some powders as the chamber heats up and the case sits in the chamber it heats up too. This usually causes more pressure but I wouldn't think it would shift down in POI. I listened to people with more experience than me to come up with what I choose to load with...... and test from there. I use CFE in my 55gr(<100 yrds)rounds and AR Comp on my 75 gr (>100yrds) ammo. That works well for me.

 

[davsco]

"As the day went on."

Possibly a change in head/tail wind condition as the day progressed.

'twas my initial thought, 600 yards is a pretty long ways for an ar-15 projectile, with plenty of time for conditions to affect it.

how were your groups throughout the day, did they stay as tight and just move up or down?

 

[jimbobborg]

What ammunition were you using? Reloads or factory?

 

[dist1646]

Were you shooting prone with a sling or off a rest? Iron sights or scope?
My initial thought is that you have a position issue, ie; you are settling in through the string.

 

[clandestine]

I would NOT touch the Gas Port yet.

Is this issue with reloads?
Barrel Bedded to the Upper?
Fluted Barrel?
Front Sight on the Barrel or Handguard?
Using a Sling, if so how is it attached up front?
Using a Bipod, if so how is it attached and are you "loading" it properly
What kind of Trigger?

 

[jimbobborg]

I would NOT touch the Gas Port yet.

Is this issue with reloads?
Barrel Bedded to the Upper?
Fluted Barrel?
Front Sight on the Barrel or Handguard?
Using a Sling, if so how is it attached up front?
Using a Bipod, if so how is it attached and are you "loading" it properly
What kind of Trigger?

I'm guessing he's using this gun http://www.mdshooters.com/showthread.php?t=182760 ?

 

[clandestine]

I'm guessing he's using this gun http://www.mdshooters.com/showthread.php?t=182760 ?

Well, most of my questions are valid if it's the same gun. Stag 3g has:

Floated rail with front sight (POI can shift as the aluminium handhuard heats up)
Barrel is Fluted
Barrel is likely not Bedded
Using a Sling or Bipod off a Floated Rail with a Front Sight can change POI

He also could have added too much torque to whatever muzzle brake is on the gun. I have seen excessive torque effect consistency with a rifle before. For precision rigs, I prefer to use 15 ft/lbs maximum and use rocksett.

As others mentioned, if he is using optics, it could be the mount, the optic, or the mirage causing issues.

Im sure Ed has better answers. I'm not a shooting instructor, I'm just listing the mechanical things that could be a point of concern.

 

[jimbobborg]

Well, most of my questions are valid if it's the same gun. Stag 3g has:

Floated rail with front sight (POI can shift as the aluminium handhuard heats up)
Barrel is Fluted
Barrel is likely not Bedded
Using a Sling or Bipod off a Floated Rail with a Front Sight can change POI

He also could have added too much torque to whatever muzzle brake is on the gun. I have seen excessive torque effect consistency with a rifle before. For precision rigs, I prefer to use 15 ft/lbs maximum and use rocksett.

As others mentioned, if he is using optics, it could be the mount, the optic, or the mirage causing issues.

Im sure Ed has better answers. I'm not a shooting instructor, I'm just listing the mechanical things that could be a point of concern.

Since he hasn't posted back yet, I'm going with mechanical problems or ammunition problems. We'll see how this works out.