M70 .300 Wby Mag throws bullets

[Magnum76]

I'm asking this question for a friend.

He has a Winchester Model 70 in .300 Wby Mag. 28" inch barrel, stainless, not sure on twist, 6 groove. After market barrel (Hart?)

Handloads, I forget what powder he's using but 88gr rings a bell for a certain weight bullet.

At 100 yds any weight bullet doesn't change POI. (crazy... lucky bastard)

Will cloverleaf three rounds if you do your part, @100yds.

After about 5-10 rounds it starts stringing the groups open. To the point of being unacceptable for accuracy purposes. Say about 3" groups. Barrel is given time to cool in between shots.

If the bore is cleaned, accuracy returns.

Does anyone know why the groups open up in a string pattern after a few rounds?

Is this crazy long barrel that subject to fouling?

Muzzle crown is fine.

Completely open to ideas and/or suggestions

 

[winmag]

Barrel fouling.

Possibly copper fouls quickly, or he's dumping so much powder that it shoots "dirty"

My Model-70 300wby (factory win) 24" pipe liked IMR 4831, but it fouled quicker with IMR stick powder vs Winchester 780 Supreme ball powder.
Apples to Oranges tho. Mine was a short factory pipe. His is a longer custom pipe.
Most custom pipes are not prone to fouling as quick as "rough" factory pipes.

Possibly the throat may be shot out, & he could have it set back 1/2 inch or so.
I'd have a competent smith scope the bore.

 

[Broz]

What bullets ? I have owned rifles like this that did the exact same thing with the solid copper offerings.

Jeff

 

[Pdvdh]

What bullets? I have owned rifles like this that did the exact same thing with the solid copper offerings.

Jeff

Interesting. Was it a factory barrel or an aftermarket custom barrel?

 

[Broz]

Interesting. Was it a factory barrel or an aftermarket custom barrel?

Weatherby factory tubes

Jeff

 

[Buano]

Sounds like a rough barrel that will copper-foui rather quickly.

I had a .300 Weatherby that would cloverleaf ANY load for 6-8 rounds. Thereafter it was hard to keep bullets on the paper, groups jumped from .6" to 4" and then to 10" as the barrel fouled. Stripping copper from the barrel would return it to clover-leafing.

If it was a cheap barrel I would suggest it be lapped or electro-polished. As an expensive custom barrel I would contact the barrel manufacturer and would consider sending it back.

 

[65WSM]

Have you tried HBN coated bullets? That might prove if it is copper fouling. With coated bullets it will still foul but not as quickly. I would polish the barrel with a brush patch and Flitz.

What are you cleaning with? Brushes, chemicals, how often?

 

[Magnum76]

Thanks for all the replies.

Not sure on his cleaning procedures.

Throat should be looked at. But chamber is so tight some factory ammo won't lock up.