groups suddenly opened up

[dmax1800]

I need some help with a problem. The problem is that I am finalizing a load for a rifle and the last 3 sessions at the range with the rifle has resulted in MUCH bigger groups. At 60 thou off the lands, before the last 3 sessions the groups were about 3/4 to 1 inch at 100 yards. In the last 3 sessions they are 1 to 3 inches. Same load as before. I shoot off a bench with a front shooting rest and rear bag. I also shoot a 300 win mag off the same shooting setup and it will shoot about 1/2 inch groups at 100 yards.

Here's the rifle: Rem 700 22-250 heavy sporter barrel (not bull barrel), H-S Precision stock with aluminum pillars, Nikon 4.5-14 Buckmaster BDC scope, trigger set at about 3 pounds (by a gunsmith).

Load: Nosler Ballistic Tip Varmint 55 grain, Varget powder 34.7 grains, Winchester large rifle primer, Norma brass.

Here is what I've checked/tried: different scope, both fouled and clean barrel, checked screws on bases and rings, checked action screws, bullet runout is 1/2 to 1 1/2 thou, case mouth runout on loaded rounds is about 3 thou, case mouth thickness varies by only about 1 thou, can put 2 dollar bills between barrel and stock, brass before and after annealing, avg muzzle velocity is about the same, standard deviation of velocity runs 10 to 15, barrel has about 200 rounds thru it

I found the best powder charge with ladder tests. Then I tried different seating depths and found 60 thou off the lands was the best. In the last 3 sessions I have been trying 60 and 70 thou off the lands.

I'm stumped. Anyone has any suggestions???
Many thanks in advance.

 

[bigedp51]

It has gotten colder, any chance the weather had a effect on your action screws, meaning did you check the torque settings for your new H-S Precision stock?

My second guess would be a carbon or copper buildup. Something changed how the barrel is vibrating and your node.

Long Range Shooting: Optimizing Your precision rifle
https://thearmsguide.com/6047/long-range-weapon-system-how-to-configure-your-bolt-action-rifle-to-shoot-long-range-part-2/up

 

[dmax1800]

BIGEDP51,
Temperature. Yes, the last 3 sessions has been colder than before. The last 3 sessions has been between 20 and 35 degrees. Before that the temps were 40 to mid 50s. I checked the torque of the action screws after next to the last session. IE 2 sessions ago, so I have shot it in cooler temps after re-torqueing the action screws. But that was a good thought.

I cleaned the rifle before the last session. I did get a lot of carbon and copper. Maybe I need to clean it thoroughly again. I'll try that.

Thanks for the input.

 

[AZShooter]

BigEd51 may have it right, the lower temperature could be the cause. BUT Varget is one of the very best temperature insensitive powders in existence!

Three things immediately came to mind that could cause groups size changes:

Action screws got loose. It often pays to retorque after a dozen rounds or so to learn if your rifle requires that second time. Can't tell you how many times my friend and I got caught with this one.

A nick or similar flaw in the crown:
A Q-tip dragged over crown can show a burr. Best way is to inspect with a borescope.

Fouled barrel:
Sometimes JB bore paste, Rem 40X or Iosso is necessary to help remove stubborn fouling. What products are you using?

 

[waltercrouse]

One of the best copper remover is Sweet's and Barnes copper removers. Do not leave this product in barrel for a long time and use barrel protector for action and magazine. If you have copper in the barrel the patch will be blue. Continue cleaning until you have no more blue in the patch. Clean the bore after use to remove the copper cleaner.

 

[gohring3006]

Carbon ring...

 

[dmax1800]

AZShooter: I checked the action screws 2 sessions ago (colder temps) and they were still torqued correctly. The aluminum pillars in the stock really help prevent the screws from working loose. Plus the torque on the HS stock (65/45 in lbs) is more than with a wood stock and don't work loose as easily.
I didn't know that a nick on the crown would cause a problem. I don't have a bore scope, but I'll check the crown. Learn something new every day.
I use Bore Tech carbon remover and copper remover (2 products). I did get more carbon and more copper than usual. After running 3 patches, brushing and then 3 more patches almost all of the carbon was removed (patch was almost totally clean). There was a little blue from the copper in the grooves, but it wasn't bad. I'm going to thoroughly clean the barrel again to get any residual.

Many thanks for the input.

 

[AZShooter]

Now if you could just get a look through a borescope you would KNOW what is happening in that barrel. If you lived around here we would be looking at it.

 

[Carbide]

Temp related but related to twist in barrel. 14 tw will barely stabilize the 55 boat tail and gets worse the colder you go. 12 twist is ok.
Carbide

 

[engineer40]

I don't always, but something I do like to do before starting load development is to get a box of factory ammo. Shoot a group of 5 and save the target and save the rest of the factory ammo.

Then in the future if something "random" starts happening with the groups from my reloads, the factory box of ammo is my consistent variable. If I re-shoot the factory ammo and it groups the same as it did the first time, then it's most likely my reloads. If the factory groups open up also, then it's most likely something with the rifle. I wouldn't trust anything less than a group of 5 shots for doing this.

Might be too late for you to do this now, but this has saved me a lot of headache in the past.

Good luck!

 

[cape cove]

I don't always, but something I do like to do before starting load development is to get a box of factory ammo. Shoot a group of 5 and save the target and save the rest of the factory ammo.

Then in the future if something "random" starts happening with the groups from my reloads, the factory box of ammo is my consistent variable. If I re-shoot the factory ammo and it groups the same as it did the first time, then it's most likely my reloads. If the factory groups open up also, then it's most likely something with the rifle. I wouldn't trust anything less than a group of 5 shots for doing this.

Might be too late for you to do this now, but this has saved me a lot of headache in the past.

Good luck!

Good practice, Great idea
TKS

 

[Pdvdh]

Failing scopes can cause groups to increase also. Swapping out scopes is a straightforward means of testing this possibility.

 

[Kansaswoodguy]

Did anything change in your components i.e. different powder lot, different bullet lot, or different primers. Your checking runout are you trimming your brass, I assume you are.

 

[dmax1800]

Same components. Only thing that changed was temp. Last 3 sessions were 20 to 35 degrees and previous were 40 to 60 degrees.
Trimmed to length and turned necks.