why are 2nd and 3rd shots of group lower

[dmax1800]

I've noticed a pattern with my rifle while developing a load. With different powder charges and different seating depths, the first shot is consistently higher than shots 2 and 3 in 3 shot groups. Shot 1 is from a barrel ambient temp and shots 2 and 3 are about a minute apart, so the barrel is slightly warm. Ambient temps range from upper 50s to lower 70s. Bullet velocities are real close between shots 1 and 3; typically less than 20 fps spread. Necks have been turned and the neck tension is real consistent at 2 1/2 thou. Its got a Hart barrel with a McMillan stock. Caliber is 300 win mag and I have about 150 pills down the tube.

As an example, one group was in a pyramid. Shot 1 was 1" higher than shots 2 and 3 which were 1/2" apart horizontally.

Any ideas why shots 2 and 3 are lower? Any ideas what I could do to get less vertical spread?

 

[dkhunt14]

Is the gun clean when you start? If it is the second and third shots are a little faster. They are lower because they are flatter. Matt

 

[dmax1800]

Gun is not cleaned every time at range. If it is cleaned 6 fouling shots are fired first.
Shots 2& 3 are not faster. It varies.

 

[justgoto]

Maybe a bedding problem.
I would check for contact with the stock along the barrel, receiver/stock screw tensions.

 

[FEENIX]

 

[Mikecr]

They are lower because they are flatter

They're not lower because they're flatter..
While bullet trajectory can be flatter, it's path is set by zero. With no change in zero, and velocity increasing, the path would extend at a set angle(POA) leading to higher POIs.

Something in your system is shifting with increasing temps. I suspect barrel, as it heats more than anything else in just 3 shots, and it sounds like this is pretty consistent.

 

[Lone Hunter]

They're not lower because they're flatter..
While bullet trajectory can be flatter, it's path is set by zero. With no change in zero, and velocity increasing, the path would extend at a set angle(POA) leading to higher POIs.

Something in your system is shifting with increasing temps. I suspect barrel, as it heats more than anything else in just 3 shots, and it sounds like this is pretty consistent.

+ 1 on this. I have had rifles shift impact because of no or improper bedding. Bedded them and this took care of the problem. I had an Anschutz .22 lr that would shoot 1" high left every time on a cold barrel. Once you fired 3 to 5 shots everything would be in the same hole. Good Luck on your weapon.

 

[FearNoWind]

First thing I'd check is bedding, action torque, and barrel clearance from stock.
Take three shots and run two or three thicknesses of 20# bond paper under the barrel from from the back (recoil lug) and if it doesn't pass with zero resistance it might be a good idea to work on the barrel channel of the stock.
Check the barrel for copper fouling ..... especially near the muzzle.
Incidentally, that circular green shooter's guide posted previously is for handgun training and, while some of the principals apply, it's not very useful for rifles. I use it (or one like it) in some of my classes. But it is cute ain't it?

 

[dmax1800]

I can run 2 one dollar bills between barrel and stock so barrel should be free floated.
If I torque the action screws any less than 40 in lbs they work loose. Factory spec is 35 to 40 in lbs.
How can I tell if the screws are causing stress???
I get very little copper fowling when cleaning, but I will try cleaning the barrel.
It's a McMillan stock and it is not bedded because they say it does not need to be bedded.
How can I tell if the action is properly bedded???

 

[Truc]

Since it is not bedded, then it is not properly bedded. Have it bedded w/ Devcon and your issues will most likely disappear.

 

[dmax1800]

Would bedding also explain vertical stringing I've gotten at times?
And I shoot 3 shots low, move scope up and the next 3 are high?
And one day they are right on and the next day they are either high or low?
Does sound more and more like bedding.

 

[FearNoWind]

I can run 2 one dollar bills between barrel and stock so barrel should be free floated.

If you're satisfied with that (I would not be) we can remove that from the list of considerations.

If I torque the action screws any less than 40 in lbs they work loose. Factory spec is 35 to 40 in lbs.

I torque to 55 inch pounds to a bedded base.

How can I tell if the screws are causing stress???

It doesn't take much stress to generate problems and I doubt you have the equipment to adequately measure the stresses (if there are any) with your action mounting.

It's a McMillan stock and it is not bedded because they say it does not need to be bedded.

As previously stated, if it's not bedded it's not right. I love McMillan stocks; but I bed them anyway.

Would bedding also explain vertical stringing I've gotten at times?

It could be a contributing factor. More likely a butt management issue though

i shoot 3 shots low, move scope up and the next 3 are high?
And one day they are right on and the next day they are either high or low?
Does sound more and more like bedding.

Yep, it sure does.

 

[cowboy]

My first thought on this would be a bedding issue.

The only other question I would have for you is how much confidence do you have with the scope that is on it. If you have a different scope to use you might consider switching out and see what happens.

The other thing you might try to verify if it is a bedding problem is to shoot one round and put it away for a day - use same target, same distance same setup and shoot one round, put it away etc. If your cold bore shots between days are acceptable then I'd really lean towards having it bedded.

Isn't this sport fun.

 

[dmax1800]

I tried putting dry erase marker on the action and put it back into the stock. I learned 2 things:
- the receiver and mag box slightly touches the stock in a few places. But the sides, bottom and front of the recoil lug do not touch anywhere but on the back.
- the block where the rear action screw goes in does not touch the stock, at all. There is an ear above and behind the block that rests on the stock, but only about 1/4 of it.

I'm going to sand the places where the receiver and max box touches the stock. I'm also going to put 2 thicknesses of elec tape on the rear block so that the screw holds on the block instead of the ear. I'll probably have the stock bedded at some time, just not before my elk hunt.

I also shot 3 shot groups with various torque settings on the action screws: 30, 35, 40, 45 and 50 inch pounds. Group sizes ranged from 3/4" to 2", so it is really sensitive to torque on the action screws.

After I sand the stock, I'll retry a group with the torque from the the best group today.

Do you think these actions will either improve the accuracy slightly or decrease the sensitivity to the torque amounts on the action screws?

Could the rear block not resting firmly on the stock explain either the inconsistent group sizes or why the first shot was always high?