Finally gonna try and figger sumpin out

[john keyes]

about ten years ago I bought two brands of .308 180gr bullets.  ( I guarentee you DO NOT want to see the prices on the stickers on them). One the Sierra Gameking (flatbase softpoint) and the other the Hornady SST (ballistic tip boat tail). 
I immediately noticed that the Hornady shot 5 or 6" higher at 100 yds than the Sierra.    Now, was the SST leaving the barrel later when the muzzle was higher?   

I resigned myself to the fact that many factors influence Point of Impact and lived with it.
Until now.  I am determined to figure this out once and for all....

today I weighed them:
Hornady SST avg wt  180.6 gr
Sierra Gameking  avg wt 180.5 gr

then I measured them:
mostly about .3075 (my calipers are not perfectly zero'd) for about the same length ie bearing surface? of both bullets. 

I handpicked the same Federal brass for all and loaded up 40.0 gr H4895.
so up to this point I can not find a glaring difference in the bullets that would cause the change in POI.  except......
...I noticed to seat the SST bullets at 2.800 it pushes the Sierras down to 2.700 (crappy pic ensues)


Recently I bought a chrono and I'm off to the range to get some velocities and see what the heck is going on here.....

[Steve P]

Probably a difference in ogive and bearing surface.

Steve

[jedman]

   I had the same thing happen with a 280 Rem in a Handi rifle,  I worked up some loads with a 139 gr. SST bullet and when I shot Remington factory loaded 140 gr. " tipped bullets" the POA was off by 14" @ 100 yrds. They were very similar bullets loaded to the same OAL and velocity,  How that barrel can vibrate that much is a mystery to me.       Jed

[billy_56081]

A higher bullet impact on a target at 100 yards generally means that the higher impacting bullet is moving at a slower velocity. The reason for this is the bullet is in the barrel longer as the muzzle is rising from recoil. The lower velocity could be caused by several things. Jacket material and bearing surface are 2 of them.

[Graybeard]

I once upon a time tried to do that (try and figger sumpin out). It gave me a headache and since then I've decided I really don't wanna figure things out. 

Serious this is an issue that will just drive you crazy and when it's all over you'll be no closer to the answer than you were before you saw the difference. Fact of live is that any two different bullets or any two same bullets with different powder charge or different powder even if velocity is the same just ain't gonna do what you think they should.

In fact when working up loads I often use 2-3 bullets of same weight and 2-3 powders with each and go up in half or full grain intervals depending on which seems appropriate. It has proven to be very common that even one grain of powder difference with all else the same can result in a difference in POI of a few inches and in any direction.

Just accept it and save your sanity.

[charles p]

I once loaded some 150 flat point bullets designed for 30/30 in a 30-06, using the exact recipe I used for Sierra 150 BT.  The flat point bullets shot higher and delivered an outstanding group.  Never knew why.  Figgured it was bearing surface related.

[john keyes]

okay so much for my fact finding expedition.  Graybeard was right.  And Robert Stack is not around with his TV show anymore.  Heck with it.



the results from four shots each with each bullet:

Sierra 180 gr GameKing flat base soft point:   2401, 2407, 2420, 2409

Hornady 180 gr SST boat tail polymer tip:  2472, 2463, 2451, 2477

not much difference

time to close the books on this one



meanwhile, I'm done with 180's in the 308 anyway.  At one time I thought it was a great idea.  I bought some old Hornady 150's cheap at a gun show and I'm going with them from now on. 




there was a high note to the afternoon, turned out this 200 yd group with my ol 336W 

[jhalcott]

  I figgered out sumpin! That's why they put those adjustments on those scopes!! I had a friend that told me to "Just find an accurate FACTORY load, chronograph it, and load to THAT velocity!" This never seemed to work for him either!

[Dand]

Those Sierra FB loads sure had a consistent velocity. How was accuracy?

I figgered sumpin out - Send me the rest of those 180's. I'll be happy to use them in my '06 and 300 win.   

Nice shooting at 200!

[Lloyd Smale]

ive seen the same. Funny thing is I can grab one 06 and the two differnt loads will print 6 inches apart. Put the same loads in another 06 and they will print on top of each other. Personaly i think the main variable is barrel whip and vibration.

[SHOOTALL]

It could be as simple as jacket metal type or thickness . When shooting IHMSA speer bullets always shot higher and left the most copper fouling in my XP-100 . That led me to believe they had more drag and went up the tube slower maybe because the metal in the jacket was harder/softer/thicker and pressure had to peak higher to move it thru the rifling . I used same powder charge , primer etc. only changing brand of bullets as price was the deciding factor. I guess the same could be said of the core metal also. Some said bullet shape . It might be a combination of many things . Sometimes very different bullets hit same point of impact. Maybe they have offsetting differences .
Think i will go rest now , let us know what you find .

[huntswithdogs]

Ive always been all about finding a load that shoots well, adjust my scope to it and leave it alone!
My 243 shoots 100gr BTs and flatbases( same powder charge, primers and cases) to different places on the target but both will shoot under an inch. Suits me by turning the knobs.


HWD