I opened up a fresh box of Hornady 75gr HPBT and was greeted with a little message (very fortune-cookie-like):

You may notice slight modification of the Boat tail to item #2279. This has resulted in improved accuracy and uniformity  with negligible change in the ballistic coefficient. There was no change to the profile so seating depth has not been affected.

The boat tail is shorter, and overall the bullet is shorter. Comparing it to a box of Nosler 77gr HPBT, they were *very* close in length. Also, the meplat seems a bit larger as well (as compared to my old box of the 75gr). Has anyone else seen this?

edit: I can provide some pictures tomorrow if anyone cares.

Pics of the old and new and 77 all pulled and side by side appreciated.

[center][url=http://mysite.verizon.net/~vze1nzcr/ar15/three-large.jpg][img]http://mysite.verizon.net/~vze1nzcr/ar15/three-small.jpg[/img][/url]
[i]From left to right: "Old" Hornady 75gr HPBT, "New" Hornady 75gr HPBT, Nosler 77gr HPBT
Click the image for the *big* version[/i][/center]

You can't really tell much from the picture because of the lens distortion. Maybe a scanner is more in order here. I'll see what I can do.

The boattail is where I perceive the most difference between new and old--the new one seems much closer to Nosler's tail.

Now I suppose the question is, with these changes (albeit pretty minor), will this change the terminal effects of the loads using these bullets?

-CH

My gut feeling is that all heavy OTM type bullets ought to fragment. The casing is so long, that the stress induced by bullet yaw would be more than enough to cause fragmentation at reasonable velocities. I'm not saying there's NO difference, but I would think that it would be "very nasty thing" (VNT) to be shot with.

That would be sweet and something I hadn't even thought about until you mentioned it! Man, I hope this turns out to be the case because having to buy 75 gr for my 1:7 and 68 gr for the 1:9 starts to suck. It would be nice to stock one ammo type for both types of rifles.

-CH

I imagine the terminal ballistics will change but not much, yaw time should atleast be about the same I believe.  On one hand you have a shorter nose which would probably make it yaw a little later.  On the other hand, you have more meat/weight around the bottom (heavy) area which should make it yaw a little sooner.  The biggest change will probably be in the external ballistics.  The change in the shape of the boat tail should have some kind of impact on how velocity is retained.  But again, there might be offsetting changes in the boat tail design.  If I had to take a guess I'd say it won't retain velocity as well because the boat tail is shorter and more abrupt.  As for 1:9 barrels being able to stabilize it, I doubt the changes will help.  The widest part of the bullet is what makes the most contact with the barrel and in this new round, the widest part of the bullet has been lengthend to run almost the full length of the bullet.  Seems to me this would make it seem like a longer bullet and would stabilize even less so in the 1:9 than the previous 75gr.  Just my 2 cents, do what you want with it.

I did some more comparing of various random samples of each bullet, the "new" and "old" Hornady 75gr and the Nosler 77gr.

* The nose length and ogives are very similar on all three bullets.

* The meplat diamter on the "old" 75gr is about 0.02" smaller, and the on the "new" 75gr it is just ever so slightly smaller, than on the 77gr.

* The boattail length on the "old" 75gr is 0.04" longer than on the "new" 75gr and the 77gr which are very close.

* The base diameter on the "old" and "new" 75gr are both about 0.02" smaller than on the 77gr.

* The "old" 75gr is about 0.02" longer than the 77gr. The "new" 75gr is only about 0.005" [very iffy at this precision] longer than the 77gr.

I have a computer program that lets me input various measurements and outputs a bunch of info based on those measurements.  At 2750 fps:

* The "old" 75gr has a BC [G1 drag function] of 0.369, an "optimal" twist rate of 1:7.59", and 71.2% "optimal" stability from a 1:9" twist.

* The "new" 75gr has a BC of 0.329, an "optimal" twist rate of 1:7.62", and 71.6% "optimal" stability from a 1:9" twist.

* The 77gr has a BC of 0.334, an "optimal" twist rate of 1:7.89", and 76.9% "optimal" stability from a 1:9" twist.

+ The "optimal" numbers were computed using formulas written by Robert L. McCoy of the US Army's Ballistics Research Lab.

This information is for educational purposes only and is not meant to replace "real world" experience because: a) You won't really know how it'll work in your rifle until you try it, b) even though I tried my best I still suck at measuring things [which is why I shared comparative numbers and not exact measurements], and c) the "real world" is what counts. Please don't sue me.

What computer program is that and how much did it cost?  Thank you.

Is this with or without the cannelure?

None of the formulas take "cannelure" as an option, so I would have to say without. Whether or not a cannelure makes a difference to those figures in the real world, I don't know.

I downloaded the program for free a while ago off a website by a guy named JBM. The link I have for it doesn't work anymore so I don't know if it's still available or not.