Sierra .308 mk bc over-inflated?

[kweidner]

I was helping a nieghbor with his .308. We developed a very nice load using a 175 smk using 41.5 gr of 8108 xbr. 64 degrees 64percent humidity 29.940hg. we were shooting at 500 yds. 5 shot string 2546,2533,2544,2540,2530. Ballistic computer called for about 12.5 minutes. we dialed it and actually needed 14.25 to get to POA. moved to 14.25 minutes and ran the next group pretty much dead center. both groups had less than .5" of vertical but i needed 14.25 instead of 12.5 minutes to get there. I used the stepped bc in Ballistic FTE which gives a .496 2800 to 1800 and .485 below 1800. In order to match the flight I would need a bullet that was traveling abot 1410 fps. i feel my chrony is fairly accurate as I have used it to develop many long range loads and usually at most i would have .25 to .5 moa to adjust. Would you adjust the bc or run the 1410fps solution? This is the first time I have run into this so extreme. What bc do you run with the 175 smk. The scope is 1.7 above center line.

 

[green 788]

could be the scope's turrets, or just a bad 100 yard zero... that would be the best guess with that much variation... I know the 175's shoot as advertised by Sierra, so the BC is not inflated.

Velocity... I doubt that's off that much either.

That leaves the scope. Go back to 100 yards and see what you have there and you'll probably find the error... probably hitting low at 100, I'd say...

Dan