Hvy SS Barrels, & Cold?

[Bigwheels]

I have a question about my hvy barreled SS 300WM. It's a #8 contour 26" 1 in 10"RH & I now have 2 different loads that shoot great in the 70* temps I worked the loads up in, but INCREASE MV when I shoot it in the cold.
1st load was 210VLD with Retumbo @ 2900fps. Worked up the load in 60*-70* temps, & chrony'd @ 2916fps for 50 rnds chrony'd. Shoot in 35* & the MV chrony's @ 3000fps. Trajectory coincides with it. Shoots higher than normal.
2nd. load is 208 A-Max with H-1000 @ 2950. I have only shot it in cold temps 2 times now, but shooting higher than normal in he cold. .5 moa @ 700yds higher going from 70* down to 45*.
What gives?

 

[Bigwheels]

Re: Hvy SS Barrels, & Cold?

Nobody?

 

[USMCj]

Re: Hvy SS Barrels, & Cold?

Sounds like your powder is very temp sensitive. What powder u usin?

 

[gszeto99]

Re: Hvy SS Barrels, & Cold?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: USMCj</div><div class="ubbcode-body">Sounds like your powder is very temp sensitive. What powder u usin? </div></div>

It's in his original post; he is using Retumbo and H-1000 powders.

 

[USMCj]

Re: Hvy SS Barrels, & Cold?

Woops! Dont know how I missed that. At any rate, you have to expect some variation when there is a 35* temp difference.

 

[Hummer]

Re: Hvy SS Barrels, & Cold?

I have similar situation with a 7/08. I have to rezero when it turns warm and when it turns cold. I carry it out walking for coyotes around here and at 300 yards it will get "in the zone" warm or cold but at 600 yards is it is a hair low all of which reminds me I haven't rezeroed this winter.
You have nothing to worry about, just record your changes so you will know where you are.

 

[lanwickum]

Re: Hvy SS Barrels, & Cold?

When I was shooting in -10 to -20degF this winter it was a rather large change, as I expected. Still did good other than everything in the gun felt sticky and had to hold high. Need to find a good lube and get rid of the duracoat on my bolt.
Why yours shoots high. You got me.... Maybe you changed directions. Shooting east to west instead of west to east. Or is it the other way around. That is a joke.

 

[bohem]

Re: Hvy SS Barrels, & Cold?

#8 contour shilen? 1.26" OD

A 50* temp change should shrink the bore on that steel about .0005" so you might theorize it's a "tightbore" rifle and you're seeing increased pressure from that.

I have a hard time believing that it's a temp sensitivity issue that sped up your MV when it got cold.

Did you shoot these at the same place (same altitude actually)?

 

[Bigwheels]

Re: Hvy SS Barrels, & Cold?

I shoot at the same place. The first time I discovered this was with the 1st load with 210VLD, & Retumbo. I rechecked everything, including re chronying another 50 rnds. Shot higher than norm, MV increased by 100fps. I couldn't explaine it. That's why I switched to H-1000 in the first place. I had thought of the barrel shrinking too, but I thought, NAAAA, couldn't be that.

 

[FequalsMA]

Re: Hvy SS Barrels, & Cold?

The more heavy the barrel, the more the bore will change due to temp. I know its counter intuitive but it’s true. I do believe you are experiencing higher pressures due to a tighter bore.

 

[Ritten]

Re: Hvy SS Barrels, & Cold?

I saw something similar at the last sniper match I shot. Down here we don't face sub-freezing temperatures very often, but I missed the cold bore shot on the first morning of the match because I had not checked my cold bore zero below 50 degrees before. It shot 2 moa high......consistently with the cold bore that weekend. It was in the upper 20's. The following weekend, during 60 degree temps, I rechecked my cold bore shot and it was right where it was supposed to be, 2 moa higher than when it was at the match.

My barrel is a Rock #7 Max HV in .308 24" with an F.T.E. on the end.

 

[Bigwheels]

Re: Hvy SS Barrels, & Cold?

Thanks guys. I know it sounds crazy, but this is what I found when doing testing. When I first encountered it last winter, I spent about 5 weekends loading, chronying, & checking POI at range. I simply refused to believe what I was seeing. OBVIOUSLY MV DOESN'T INCREASE IN THE COLD!
I was proven wrong. And now that it's doing it again with a different load, I know it has to be the rifle. Barrel constriction sounds like the most likely reason.

 

[oneshot onekill]

Re: Hvy SS Barrels, & Cold?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Matthew Lundy</div><div class="ubbcode-body">The more heavy the barrel, the more the bore will change due to temp. I know its counter intuitive but it’s true. I do believe you are experiencing higher pressures due to a tighter bore. </div></div>
Interesting thought... But wouldn't the metal contract from all directions? In other words, the barrel would become thinner in all directions... thus causing a "LOOSER" bore. Not tighter. Afterall, it's the metal that's effected, not the air inside it. Or am I out of my ass???

On another note... Maybe someone should develop a "Barrel Warmer" for competition shooters who experience this problem. In the world of Motorcycle racing they use "Tire Warmers" that are thermal heated material that wraps around the tires and keeps them warm between races so they heat up and stick to the track faster. If you could slip a heated cover of some sort over your barrel during "down times"... Hmmm... Just a thought...

 

[FequalsMA]

Re: Hvy SS Barrels, & Cold?

I may have had it backwards. Either way, the more material the greater the effect.