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So first real use of my first batch of ammo and some good results if not what I thought.

So some advice appreciated.

At Bisley this weekend at 600 yards I used my club bought cheapo .308 ammo CGC?
This was 168 grain and I shot it back to back with my home loaded 155.

Now the CGC or whatever it was I used and zeroed on was remarkably different from my home stuff.
I was shooting 155gr over 40.5gr IMR4895 and in my mind it should have shot flatter being lighter.

The drop difference between the two was the 155s were a whole target lower.
I wasn't expecting such a large difference.
Good news was the 155s seemed infinitely more accurate.

Is this normal - seems counter intuitive that lighter would be lower and so much of a change?

Many more variables to consider - bullet shape and size and how well it fits your barrel amongst others.

Punch the relevant data into a ballistic calculator and you should see the answer. This is a free online one that's quite good:

http://www.gunwerks.com/ballistics

If your GGG ammo was the variety built with a Sierra Match King 168 grain bullet (part number 2200) the measured G7 BC is 0.218. Add the velocities and generate the drop charts.

For comparison, a .308 168 gr MK at 2600 fps drops 95.7" at 600 yards, whereas the 155 gr MK at the same MV drops 97.0" - an additional drop of 1.3". The heavier and slightly more efficient bullet retains more velocity, has a slightly reduced flight time, and drops a little less (but only a little).

However, reduce the velocity of the 155 to 2300 fps and the drop increases to 129.2" - nearly 3 foot more than the 168 at 2600 fps, or nearly half a 6 foot target frame. Without the correct BC and velocity for your loads these figures are purely an illustration.

Spencer54 wrote:Is this normal - seems counter intuitive that lighter would be lower and so much of a change?

Yes; all things being equal a heavier bullet of similar design at the same MV will retain more velocity and drop less, but not to the degree you are seeing. Either your 155 grain bullet is woefully inefficient (unlikely) or its velocity is significantly less than the 168.

..

Yes, you really need to chrono it to understand whats going on.

Haven't used the 168gr GGG stuff, but the 147gr is pretty feisty;

viewtopic.php?f=11&t=29794

As Rockhopper says, you really need a Chrono to help understand what's going on.

Spencer54 wrote:So first real use of my first batch of ammo and some good results if not what I thought.

So some advice appreciated.

At Bisley this weekend at 600 yards I used my club bought cheapo .308 ammo CGC?
This was 168 grain and I shot it back to back with my home loaded 155.

Now the CGC or whatever it was I used and zeroed on was remarkably different from my home stuff.
I was shooting 155gr over 40.5gr IMR4895 and in my mind it should have shot flatter being lighter.

The drop difference between the two was the 155s were a whole target lower.
I wasn't expecting such a large difference.
Good news was the 155s seemed infinitely more accurate.

Is this normal - seems counter intuitive that lighter would be lower and so much of a change?

considering you are 3grns below the manufacturers start load its not surprising you are finding the drops excessive enter your details into this calculator and see how you get on

Thanks guys I can see what's going on.
I used the book that came with my RCBS and it stated most accurate load was 40.5.
So that's what I did!

I'm at Salisbury plain in a few weeks so someone might be able to chrono them for me and then I'll know for sure

I had the same problem with my home loads shooting way low in my old T4. I made five batches of five rounds with half a grain extra in each one up to the maximum as shown in the Lee book then I shot them over a chrono and noted the results. All of them were quite a bit down on the desired velocity for that rifle but i think it was the middle load that gave the most consistent muzzle velocity so I didn't feel the need to push for maximum velocity given the maximum distance i was shooting at.

When looking at data always use load book data in conjunction with the latest powder manufacturers data, powders can change over time and data that was sound a few years ago and in a manual may be out of date so always check the manual loads against the powder manufacturers latest data. these differences combined with differences in cases, firearms and other components can lead to wide variations. Knowing your velocity is crucial to any understanding of a loads performance.

The Lee book is worthless for load data as the just copy other peoples data, they do not pressure test any loads and always keep to the conservative side anyway, best thing to do with a Lee load book is burn it.

Spencer54 wrote: Now the CGC or whatever it was I used and zeroed on was remarkably different from my home stuff.
I was shooting 155gr over 40.5gr IMR4895 and in my mind it should have shot flatter being lighter.

The drop difference between the two was the 155s were a whole target lower.

It worries me that people are using hand loads that they have no starting elevations for at 600yds & no real idea where they will go? 'A whole target lower' means they were bouncing off the mantlet or into the target number below?

What is the accepted safe practice for this scenario? - load shoot over a chrono on the zero range & calc? before a distance or shoot at 300 then 500 then 600 etc?

Do you have a table of elevations for Gun Sn Vs basic ammo specs, 147, 155, 168? if not please think about constructing one ....... or using the zero range first? Cadets are sent to the zero range if their first shot is not on the target frame (& some if not in the inner black).