Today I tested 75gr Gold Dot at 10 feet, and at 100 yards. I chronographed 10 yards at each distance.

I used a 16.1" CHF barrel roughly 5,000 rounds old.

The 10 ft velocity average was 2527. The 100 yard velocity average was 2274.

This is all well and good, except plugging these numbers into JBM shows a ballistic coefficient of 0.32, and ATK states the BC is 0.4.

What could account for this?

good data to have in the 100 yds stuff, thanks.

maybe a head wind? i dont know shit about BC but i did watch a Holiday Inn commercial last night....

There was no wind to speak of. Environmental facts were not at play.

Good work on capturing 100 yd velocity.

A G1 BC of .400 is highly unlikely.

the 75 grain Hornady BTHP is closer to .360 or .365 for me and verified out to 1,000 yards.  

G7 of .183 is better correlation for all ranges on the Hornady which is alot more streamlined than the GD..  

Your velocity loss puts it at or below .300 G1 BC.  But I dont know your altitude above sea level or temperature which will effect results.  .300 is a good starting place however.

Did you use 1.75 inches above bore for your scope height?

No, my scope is 2.5", but that wouldn't matter. Environmental factors were not at play, either.

My BC calculator was 0.33 calling it. I mistyped 0.32 initially in post #1 (initial velocity 2527, residual 2277, initial velocity taken at 10 feet)
That said, I also ran some 77gr SMK, which is pretty well proven to be 0.362 at that velocity. I calculated exactly 0.33. Initial was 2485, residual 2241, the load was Norma's 77gr SMK .223)
I also ran some 55gr Gold Dot, which is listed as 0.222, and got 0.190 extrapolated.

This is a pretty consistent 0.03-ish low. on my calculation's part.

ATK also lists their 64gr Gold Dot as 0.27, and Federal lists the 62gr Gold Dot /Fusion as 0.31.

The bullets are IDENTICAL except that the 64gr version is slightly longer in the middle. The meplat, tail, and ogive are identical to the letter. This is an exaggeration of 0.04BC. It seems like maybe they are doing this with their newer loadings...?

Which would make sense with them calling their 0.36-7 75gr a 0.4...and would back up to the letter my consistent 0.03 less than published.

Also of note, I called and spoke with ATK's ballistic division, and was given 0.373 as the BC for the 75gr Gold Dot. I am inclined to believe that this data is accurate, as I am sure my margin of error could be 0.01ish, easily. The 75gr Gold Dot is longer than the T2 ever so slightly, I believe, and I would believe it to be of similar BC. It's a sleek bullet.

*Also of note, regressing ATK's data for 62gr Gold Dot nets a BC of 0.27.
Federal lists the 62gr Fusion on their page as 0.31BC.

http://www.le.vistaoutdoor.com/resources/catalogs.aspx
https://www.federalpremium.com/ammunition/rifle/family/fusion/fusion-msr/f223msr1
Of note, .362 becomes .372 above 3000fps. I think that the BC of the 75gr Gold Dot at 14.5"-20" barrel velocities may very well be literally identical to 77SMK at 0.36.


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Long story short, ATK's Ballistics division gave me 0.373 before any numbers were published on 75gr Gold Dot.
I have regressed its BC as well as 77gr SMK and they both gave identical 0.33 BC results.

It is my opinion that at the velocities I am seeing, 75gr GDSP is 0.36-0.373

They tested it in a longer barrel and had higher velocity?

Under 3000fps the BC won't shift much. They list it at 2775 from a 24".

I  would just  chronograph it at 10 feet while shooting at 600 yards and you will have a pretty accurate idea of its BC once you figure its elevation to get on target.  

Roger that. I wonder if there are any ranges like that near me, is the question.

I think it's been pretty well established that the bc for the 75gr stuff is .373.

That data came with no qualifiers from ATKs ballistics department.  It was not given with a velocity attached. That said, it's very believable. The 0.4 that ATK claims in their LE catalogue, is not.

I think some of the guys over on M4C did a regression on it, and it came out at .373. Originally, ATK was claiming closer to .400

ATK claims 0.373 when spoken to on the phone, 0.4 when you log onto their LE website.