Temp sensitivity of WC872?

Discussion in 'Reloading' started by esshup, Aug 18, 2018.

  1. esshup

    esshup Well-Known Member

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    I'll be calling Kirby on Monday, but I had some weird fps readings while shooting today in 80°F temps. and know some guys here have shot their 7mm Allen Magnum more than I have so hopefully they will see this and chime in.

    180g Berger VLD pushed by 108g WC872 gave me 3350 fps in 40-50 degree ambient temperature 4 years ago Today I kept the truck running with the A/C on high, and the ammo and gun in the truck except to shoot, taking one round out of the truck at a time. 785' elevation then and today.

    Got gun from truck and shot 3 shots with that ammo from 4 years ago using brass that was lightly FL resized, shoulder bumped back .001". FPS today was 3564 clean cold barrel, 3553 and 3593 fps. Chronograph set 20' from muzzle. Gun put back in truck to cool down. Regular bolt lift, no marks on brass. Paper at 100 yds, got scope moved back to where it should be.

    After gun cooled down (I could still put my hand on the barrel when I put it in the truck, but knew I couldn't shoot another 3 shot string without it getting too hot).

    Grabbed a round that I loaded this morning. I anneal after every firing, I weight sorted the brass into 1 grain groups, but I only neck sized this time, neck dia of unloaded brass was same dia as unloaded brass run thru the FL sizer. Bolt still closed fine on the brass.

    Shooting at 100 yds, (had removed and reinstalled scope, wanted to make sure I hit paper). Shot one, What? Not even on the paper? bolt lift was normal, no signs of stress on brass Looked at chronograph. WTH? That can[t be right. 3660 fps?? I sent another one downrange. Nothing on paper. 3714 fps. No sticky bolt lift, no marks on brass. I have one more loaded round, I pulled the bullet when I got home. Yep, the powder charge was correct 108.0 grains. I'm assuming those 180g Berger VLD's that didn't hit the paper went "poof" between the chronograph and the target. after 200 rounds thru the tube, I had 175g SMK's going "poof" at 3475 fps. That was the accuracy node for them.

    Tried some 195g Berger EOL bullets. same annealed/neck sized only brass, 99.0 grains 3349 fps. 100.0 grains 3331 fps, 101.0 grains 3384 fps, bolt a bit draggy on opening, a hint of ejector mark on base. 100.0 and 101.0 powder charges had the bullet holes .060" apart.

    Cooled gun back down in truck.

    On to the 200g WIldcat bullets. These I had weight sorted into -.5 grain groups and then grouped by bearing surface (0.001" groups).
    99 grain - error. Dumb me shot at a different aiming dot and bullet was just out of screen range. moved chronograph 2" to the right.
    100 grain - 3251 fps
    101 grain - 3323 fps
    102 grain - 3314 fps with a draggy bolt and a hint of a wipe on the cartridge base

    100 and 101 grain bullet holes were touching, just horizontal spread. 99 grain 1/4" low right, 102 grain 1/4" low left.

    All bullets were loaded with the ogive 0.010" off of the lands. COAL don't mean squat to me, it's base to lands/grooves that matters to me and how far the bullet is "jumping".

    So, with that info. do you think it was a combination of ambient temps and just neck sizing that caused the excessive fps in the 180g Berger VLD load? Or something else? Or is WC872 that temp sensitive?

    A few years ago I zero'd the gun in 38°F temps with the 180g Berger VLD load, at 6500' elevation. 2 days later I shot a cow elk at 708 yds at -8°F and at 5,000' elevation and had to add 2 moa to the drop info I generated 2 days earlier. I had shot and missed and assumed that the bullet was hitting low because the elk herd ran towards me at the shot. I'm assuming the bullet impacted past the elk going under its belly. A year later, using date plugged into Exbal, (data from my Kestrel 3500) the same 180g Berger VLD load whacked a coyote at 867 yds with the first shot out of the gun in a year in 55°F ambient temps. I know a cold clean bore shoots 2 moa high for the first shot.

    I'm super busy on Sunday, I'll check back on Monday evening. Thanks guys!
     
  2. RockyMtnMT

    RockyMtnMT Official LRH Sponsor

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  3. Trnelson

    Trnelson Well-Known Member

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    I spent quite a bit of time with WC 872 and the slightly faster burning WC 867 in 264 win mag. The short answer is yes they are pretty temperature sensitive. That being said, they both behave in a linear, relatively predictable fashion as ambient temperatures change. Things get a bit fiddle to predict if you add in barrel temperature. As the more time the round spends in the chamber the more unpredictable it becomes.
    My results were not quite as pronounced as yours. Though I was only burning ~65% of what you are. I took the time to record velocity data in ten degree incriminates. I had ammo loaded for nearly every temperature I might shoot the rifle in while hunting with that temperature written on the case. It kept the rifle in it’s accurate velocity corridor with pretty predictable trajectories.