.550" vs .600"?

[RichCoyle]

Do you experienced shooters think there would be any difference in cold bore shot with a muzzle diameter of .550" compared to a .600"?

I have an 8mm magnum with a barrel diameter of .600". The other da I fired a cold bore shot at 300 yards and then a follow up shot as though I was hunting. The "group" was 2 1/4". I'm thinking about a new 7mm magnum and using a .550" barrel. Could I expect that kind of performance with the lighter barrel in 7mm?

 

[lancetkenyon]

Going from a light sporter to a Featherweight in 7RM?

Pretty light contour for my taste.
I would prefer a #2b to #3 at least. Or a #4 CF.

Should be OK for a cold bore hunting rifle. Might start to move for the 2nd shot, probably more walking on shot #3.

 

[MagnumManiac]

Hmmmmm, I have 2 very skinny barreled rifles, one is a Kimber 8400 in 300WM that will print 2 rapid shots under 1 MoA with ease, but walk a third shot at least an inch away.
The other is my REM 700 SPS in 300RUM, it didn't walk 3 shots at all in rapid fire, all were under MoA in a clover leaf and with slow shooting it printed 10 shots into MoA @ 100 without issue while breaking in the barrel. I was very impressed with this performance from a factory barrel and factory Barnes 180g TTSX.

Normally, I would say that a skinnier barrel is more likely to walk shots as it heats up, but now I honestly think it depends on the barrel itself and not necessarily it's thickness.

What rifling profile are you thinking of doing?
I believe this also makes a difference, 4 grooves has always been the best performer for me.

Cheers.

 

[Critter Picker]

I tend to agree with MagnumManiac in #3 that walking will depend more on barrel than muzzle diameter. That being said,I still think the smaller diameter barrel is more likely to walk than a heavier one due to harmonics.

 

[RichCoyle]

What rifling profile are you thinking of doing?
I believe this also makes a difference, 4 grooves has always been the best performer for me.

Cheers.

I am thinking about using the Weatherby six lug ultralight profile from Pac-Nor. The Weatherby factory uses 24" barrels. Everyone knows you have to have at least a 26" barrel for a magnum. The goal is a seven-pound rifle with a twenty-nine-ounce scope.

 

[Phil Rizzo]

I know it can cause hysterics, but don't forget that most top tier barrel manufacturers offer fluting in house.
Best way I know to give a little and take a little.

 

[RichCoyle]

Thank you, Phil. I used Pac-Nor one time. Their fluting didn't take enough off to make much difference. I am going to gamble on a new 'smith in town. He has some CNC equipment, so at least if he programs things correctly my barrel will be fine. I hope.

 

[Phil Rizzo]

I've watched it done on CNC before.
Very interesting to say the least. This builder had something in the middle for support and to minimize vibes. He took several shallow cuts per flute, rotating 180* after each pass if I remember right. Modern Marvel!

 

[RichCoyle]

The first local guy has known me a long time. He knew I was going to have a barrel fluted. When he told me he was going to give it a try I told him,
"I have a new barrel you can try on. If you mess it up, it's on me. If you are successful I don't pay."

He jumped at the chance. It had chatter marks, but was fine. He solved the chatter problem.

 

[MagnumManiac]

I know it can cause hysterics, but don't forget that most top tier barrel manufacturers offer fluting in house.
Best way I know to give a little and take a little.

I didn't include my Win Model 70 EW fluted barrels in this discussion due to them being fluted and Magnums. They have 3 factory barrels in 264WM, 300WM & 338WM, but the 264WM rifle has barrels in 257 Weatherby & 270 Weatherby, unfluted at 26". The 338WM has another 264WM barrel that is unfluted, this was my first test at using the A191 reamer design and it worked marvellous in all respects.

I found these fluted barrels walked shots more so than unfluted barrels in the same cartridges. This may be the reason some barrels are worse than others for walking shots.

Cheers.

 

[lancetkenyon]

Good info on your experience Magnumaniac. I have always had small contours walk on me.

 

[milestown]

MagnumMania

Can you tell me more about your A191 reamer design and what you like about it. How does it shoot and how do the throats last in the magnum calibers ?

Thanks for sharing
Hal

 

[jrock]

I like to run smaller contours for weight savings and have not real issues. Of course the POI will shift and it has a lot to do with the overall rifle build and barrel quality. Barrel and cartridge combos vary as well. I typically get 4 to 5 shots with smaller cases and 3 or so shots with magnums. If the barrel is going to be threaded, I'd go for the 0.60 as the minimum. If you run shallow flutes on it, you can get the weight down a size but the flutes are really shallow and not worth the money for the weight savings, IMO.

 

[budlight]

A larger projectile will cause more barrel whip/vibration. 8mm to 7mm is 0.0393701 difference in thickness. So a barrel with using a 7 mm bore is roughly as stiff as a 8mm and .600 outside diameter