Gun warms up- sighting drops

[BZR]

I have noticed that as my gun warms up the bullet point of impact drops. It is not very noticable at 100 yards but when I shoot at 200 yards it really starts to show.

I am not sure if this is a bedding problem or just the fact that I am shooting a factory rifle. The rifle was glass bedded and nothing else was changed.

Could the muzzle velocity be dropping as the barrel warms up or could the action be very-untrue?

Can anyone offer some more insight?

Cheers
BZR

 

[ThunderDownUnder]

It might not be so much temperature related if the first shot is thru a cleaned barrel. Less friction may mean a slightly higher velocity and a higher first shot than the following rounds as the barrel fouls.

Maybe if you could provide a little more detail we might have a better idea of whats happening and be able to diagnose more accurately.

 

[BZR]

What I have noticed that is when the gun is cold, the shot is on target. As the barrel warms up,warm not hot) the shot, in relation to the first one, drops. Then if I let the gun cool for a few minutes the shot goes back up.

The variation in the shot is about a half inch at 100 yards and a full inch at 200 between cold and warmed up shooting at

This happens without cleaning.

I am just not sure what could be causing this.

The rifle is a remington 700 VLS chambered in 243 Win. It has around 600 rounds through it now, but I have noticed this for a while.

 

[shadowdrak]

It could be the powder in your ammo. Do you let the rounds sit in the chamber before firing? This can cause the powder to warm up and since some powders are temperature specific, this could cause a problem. After letting the barrel cool down, you won't be heating the round up. Pretty common this time of the year.

 

[BZR]

I went out and shot today and played with this a little.

I shot till the gun got hot,more than just warm) then let it cool off and shot again.

When it was hot I was consistently hitting withing 2-3 inches of the bulls-eye at 500 yards. Once I let it cool off for around 20-25 minutes my shots were above the target... this was a change of at least 8 inches in calm wind.

The round is not in the chamber for more than 20-30 seconds before it is shot.

I was curious if this could be linked to my load...

 

[ThunderDownUnder]

One more question for you BZR, do you use a powder measure without checking each load with a scale? Inconsistent loads obviously will give a greater spread on the target.

The example you have given so far really does appear to be temp related! Is your scope mount etc all nice and tight?

 

[Travelor]

Your problem is not a powder amount variations or a fouling problem. I believe you are having a bedding problem. Check your barrel channel with a dollar bill by putting it under the barrel with the ends of the bill up,kind of a U shape with your hands on each side). Slide the bill back along the barrel in the barrel channel and see if it freely moves back to either the receiver or until it encounters the barrel bedding.

I suspect yuour barrel is "drooping" a bit when it warms. If the bedding does not extend about 1" or so down the barrel channel forward, you might want to have this bedded.

Also check the action screw tension. If the stock is wood and has not been pillar bedded, I woold suggest about 45 inch pounds of torque, if pillar bedded you can use this or go up to about 55 inch pounds - much more and some screws will break. Remember inch pounds are NOT foot pounds! One foot pound equals 12 inch pounds.

Another thought - is this a Savage? If it is, loosen the most rear action screw, the one that is behind the trigger, and this whould resolve your problems.

Good shooting,

George

 

[Twud]

You say "when the gun is cold", are we talking about 50 degres or 32 degrees or 70 degrees?
Every centerfire rifle I own has been bedded and freefloated. Might not help but it sure can't hurt.
Are you cleaning between groups?
Mark

 

[BZR]

Thanks for all the replies!!! This issue has frustrated me.

My barrel is completely free floated and the bedding does not support any of the barrel,only under the action and stops just in front of the recoil lug). I agree that the bedding is a problem. I would bet that the barrel is saging under when heated because of this issue!

I will bed the first inch or so of the barrel and see how this changes things.


Everything else on the rifle has been tightened with a FAT wrench to their recommended specs. I will have to look up what I did but the scope base and ring tops are around 15-20 inch/lbs the ring clamps are at 55 inch/lbs?? and the action screws were set to what the fat wrench recommended for a non-pillar bedded wood stock That number slips my mind.



I am using the term cold rifle as no shots through it on a cold winter day of 70 degree day in south TX,yes I was rubbing that in!!!). Warm is warm to the touch but nothing that would come close to burning your hand. When I got it hot, it was not comfortable to touch and I didn't want to leave my hand there for long. I didn't have a temp gun to specify specific temperatures.

Is there a temperature range you guys try to keep your barrel in?

 

[BZR]

I load with a Lyman DPS III. I do not varify with the balance but do check the calibration as I go.

The trend of the shots are not random such as that with a load variation. If the rifle is cold all the shots are grouped well, but if I shoot quickly and get it hot the group is lower than the original group.

 

[Akbushape]

Since this reaction is consistent it seems to me it is barrel
tempature related.
Cut a long narrow strip of steel off a thin plate and that strip will curl up like a piece of bacon because one side is hot and the other is cold. Heat expansion. On a gun barrel the
heat build-up after a few rounds would be uniform around the barrel and cause the barrel to expand uniformly,as long as the
molecular quality of the steel is uniform the full length of the barrel.) But a few factors could cause some minute shifts
in POI. Poor quality steel is one. A warped stock allowing the
barrel to touch it at some point is another.,this can happen
after free floating but you probably already checked that.)
Another thing I have often wondered about is,this may sound off
the wall a bit) but you have the top half of a barrel open to the air and a good portion of the bottom half schrouded with the forearm. It seems the top half would cool faster,not by much, but by a little) than the shrouded portion. As this happened the top half would contract ever so slightly more than the bottom half. Even the slightest change could make a noticable difference out at 3 or 4 hundred yards. Of course this
would have the oppisite effect from what you are describing.
I've never heard anyone anywhere ever mention anything about this but it is something I have often wondered about.
I've messed around in welding shops a little and have witnessed the situation where the quality of a given piece of steel will differ form one spot to the next especially in poor grade steel. Rifle barrels are made from high grade steel but
the potential may still be there to find one with a fault in it.

 

[Twud]

When in doubt call the manufacturer and ask to speak with a gunsmith. I've solved die problems this way and a trigger problem with Remington like this.
Can't hurt.
When you resolve the problem please post the cure.
Mark

 

[Twud]

I don't believe the barrel is sagging. Try shooting a 3 shot group waiting 20 minutes in between shots. You may have mentioned doing this already but I didn't quite get it.
I shoot Weatherbys that have thin barrels that heat up fast. For good groups I need to wait in between shots until the barrel is cold. Impractical I know but it's what works.

 

[mikecr]

I don't see where there is a 'problem' at all.
You are cursed with a hunting rifle that shoots cold barrel consistently? Seems to me that most hunters would hope for this..


I spend a lot of time shooting cold barrel groups in load development for hunting rifles. We're talking 1hr to 2hrs between each shot!
I endure this,for months) to eventually improve cold barrel accuracy,which is not related to grouping at all).

If grouping is your thing, then who cares where it shoots cold? Just get the barrel up to temps/fouled and tweak the scope to put groups where you want. Load develop for tightest groups under this condition. If your grouping expectations aren't meant, get a BR gun instead of a hunting rifle.