.308 bullet drop - different rounds

[margun]

I went to dial in my new Sako 85 properly today. I have been 'bedding' the barrel in according to instructions from the shop, 1 round/clean and repeat several times, then 3/clean and repeat etc. That was started last weekend, just to get on the paper.

 

Today whilst zeroing proper I started with Hornady Superformance 165gn, same as last weekend. After tweaking windage first, I got fairly repeatable 1inch groups at 100m, 4" high, before running out.

 

I then moved to Sako Gamehead 123gn and windage remained within 1" but they were shooting VERY low - consistently 12" low compared to the Hornadys.

 

I then tried a few Winchester Super X 150gn, everything returned back to where the Hornady was, if not better. So I got the rifle set up to shoot pretty well using those.

 

I had a gamekeeper friend with me who is more experienced/much better shot and he found the same pattern

 

The rifle was allowed to cool somewhat, moderator tightness checked regularly etc.

 

Has anyone heard of this or have any advice? I'm genuinely surprised that a supposedly faster and flatter (Sako) round would shoot so low under the same conditions. My keeper friend also said that it was the biggest variation between rounds he'd ever seen. Remember windage didn't budge with any round, but when returning to the heavier bullet, things improved by 12" higher elevation.

 

Weird or what?

[margun]

Thanks fister, I just thought it was off not to see some variation in windage also.

 

I plan to try some other brands and choose the one round out of them all which is most accurate and just stick with that.

 

Still baffled at a 12" drop @ 100m though. It's a hell of a lot!

[rjimmer]

More muzzle flip with the heavier bullet, perhaps.

[Vince Green]

The more you play this game the more you see things that suprise you. Imagine a fly fishing rod, if you suddenly flip the rod up from the handle end, like you would to cast, the tip initially goes down, then flips up to follow the rod. Well your barrel is doing just that (much less visibly) due to the recoil as the bullet is travelling down it. The whole thing takes about two thousands of a second. You can demonstrate the same effect with a fencing foil.

 

What part of the dip and flip the barrel is on when the bullet leaves explains the difference in impact. It happens with all barrels but lightweight stalking barrels are affected the most. You won't even notice it unless you keep changing ammo. As fister says its harmonics in the barrel

 

If I use apparantly identical Privi or S&B loads (same velocity same bullet style and weight) in one of my rifles there is a big shift in POI but both group well, just in different places. Thats why its so annoying to have to keep chopping and changing because one or the other is not available.

Edited March 12, 2016 by Vince Green

[Underdog]

It is to do with speed actually.

If it was due solely to barrel harmonics chances are it would be to one side also.

 

The slower loads have more barrel time and so the barrel is in a higher state of elevation from recoil when the bullet is exiting.

 

The difference in your example is alot and maybe due to the silencer mass. The heavier bullet loads inducing sufficient recoil to elevate the barrel and silencer where as the lighter bullet load has left before the mass can be moved.

[kent]

16 x 1/60th of a degree possible but never seen it have seen 5" mostly you won't get that much

[Grandalf]

Putting the science to one side. Everyone knows you have to find the right ammo for your rifle.

I can thumbnail my 22/250 at 100 yards with Winchester Silver Tips.

With Hornady I can't cover them with my fingers stretched out.

Nothing wrong with the gun or Hornady ammo. They just don't gell.

[margun]

Interesting insights from everyone. I know that finding the right bullet for the rifle is essential, but 12" is a big drop @ 100m in any calibre! I have confidence in the rifle and general setup, just surprised.

 

Underdog's remarks match my own expectations that oscillations won't be limited to just the Y axis...hence the strangeness of windage staying in zero. Not a comprehensive test has been done, granted.

 

Didn't notice any extra muzzle flip or recoil with the heavier ones compared to the lower grain Sakos.

Edited March 13, 2016 by margun

[kent]

Interesting insights from everyone. I know that finding the right bullet for the rifle is essential, but 12" is a big drop @ 100m in any calibre! I have confidence in the rifle and general setup, just surprised.

 

Underdog's remarks match my own expectations that oscillations won't be limited to just the Y axis...hence the strangeness of windage staying in zero. Not a comprehensive test has been done, granted.

 

Didn't notice any extra muzzle flip or recoil with the heavier ones compared to the lower grain Sakos.

It's not drop it's the bullet leaving while the muzzle is in a different plane

[old man]

Drop doesn't matter as long as you know by how much?

[figgy]

Also some twist rates suit different weight of bullets more.

 

Find a round that groups well and stick with it.

[Savhmr]

I've found that the difference in drop between 150 up to 175g in 308 isn't worth worrying about at close range (up to 150yds) for field work. I mainly use 155g MKs for target but switching to 168 Amax at 100 yds and the difference is minimal. Further out, the heavier rounds generally have better BC anyway so lose velocity less quickly. Drop isn't an issue. Not knowing what the drop is at longer ranges, irrespective of calibre, is, however. My advice is stick to a load, get used to it and know POI for all the ranges you're likely to shoot at.

[rjimmer]

Underdog's remarks match my own expectations that oscillations won't be limited to just the Y axis...hence the strangeness of windage staying in zero. Not a comprehensive test has been done, granted.

 

 

If I am getting V-Bulls at 300 yrds with Samson 180grn ammunition, with changing anything, PPU 168grn ammunition will be about a foot low and about a foot right. (.308)