OzFclass

AlanF wrote:Norm,

I'll jump in with my 2 cents worth here.

Its good insurance to minimise your velocity variation in case your tune goes out of its node. Normally a tuned setup will tolerate reasonable velocity variations. However velocity sweet spots are only so wide and they can change up or down for various reasons, so if your velocity variation is low, you're less likely to go outside your sweet spot.

I actually test velocity variation at home without a target. When testing on the range I usually have a chrono but only to check whether high or low fliers coincide with high or low velocity.

But I look forward to what Dave has to say. His ability to tune a rifle has been speaking for itself lately.

Alan

I also 100% agree with this statement. If you want to tune for 1000 yards then understanding what is affecting velocity spreads is the biggest key - Velocity variations have much less effect at 300-700 yard shooting and indeed should only be approached after you have solved the short range accuracy issues of your rifle. I would love to get Peter Smiths approval to post some data/output from his software here but will assume I can make some statements without breaching his IP. When we started plotting velocity against height here at 1000 yards with these new chronographs we were suddenly greeted with amazing correlations. It soon became apparent that with rifles shooting 7-10 inch high groups the great majority of the elevation was due to velocity spread. When unwinding velocity most rifles we tested would actually hold under quarter minute (which is amazing as it also includes BC variation of projectiles and other range effects in addition to angular accuracy of rifle plus chrony errors etc etc). So to get the groups down it was obvious we had to work on velocity spread. We went about a process over the last two years of working through all the known variables one by one (and still have plenty to go). Some are quite dramatic, others less so but the old saying it is "the sum of all the little things that count" certainly holds true for 1000 yards.

One thing I would really like to try and emphasize for everyone though is that testing one factor (e.g. does my group size get smaller if I weight sort my shells) is very hard to do by doing it and just comparing sd of velocity or group size before and after. This is because it is not simply additive and it is often lost in the noise of other variables (including the measurement errors of chrony or other vertical effects of the range when measuring group heights) A more systematic approach must be made e.g. test a range of shell weights vs resulting velocities. Test if there is a correlation (ie in amongst the noise is there a general increase or decrease in velocity with change in shell weight). This is almost impossible to do on the target with before and after group sizes.

Dave a quick question if I may, and its probably not really related here but bear with me...

I shot a load at 500m a few weeks back and shot a 60.7 with it. Running it over the chrony on saturday I had an ES of around 50fps with most shots running 2910fps but a few slipping down towards 2860. On target at 900yds later that arvo I had two shots drop low in the 5 ring- I didnt think the condition was there for that as a few other rifles shot nice and flat. Would you think the few slower shots testing would work out to those lost 5's? I wasnt very happy with the vertical compared to what it has done with loads that I tested and developed at 1000yds without the chrony. At least I have a benchmark to go back to with the 2209 loads, but I dont think these lapua palma cases and CCI450's are up to the task of sparking up the 2213sc consistently. Both strings I chronographed (with different lots of powder but same charge) seemed to have a large ES with quite a few shots all very close but 3 or 4 blowing it out. Plus im already running a 10 inch drop tube to fill the cases and not getting any significant or consistant velocity increase.

Yes Brad, 50 fps at 900 yards would be approximately a minute so could be feasible. Also the same load could possibly shoot a 60.7 at 500m. Also plain old probability and chance will every now and then throw you a great 1000 yard range with the same load simply by missing out on those one or 2 low shots in the run of 10

The joys of long range shooting

Nothing like a challenge

Sure gives some food for thought.
Looks like I may have to upgrade my chronograph and get me one of those Magnetospeed rigs.
I have aways got variations in my chronograph from one day to the next so understand what your saying.
My way of testing for vertical is to test beyond the distance that I want to shoot at.
By this I mean if I want a 500 yard load for my 6BR then I test at 700 yards. If I want a 1000 yard load for my .284 I will test at 1200 yards.
So I suppose I am looking at group vertical ES and SD rather than velocity as my chronograph is crap and I have to work with what I have.

Dave,

From your experience and testing, have you found that if a barrel is able to shoot a low SD muzzle velocity, that this holds true thru a range of powder charges.

i.e. Barrel "A" shoots a SD of 5fps at "X" grains of powder. Change of powder load to "X+1" grains of powder and it holds the same SD of 5fps.

Craig.

this will probably be my last post before heading to Raton early in the morning so I might throw the cat amongst the pigeons here.

I do believe powder will burn more efficiently at a given pressure or charge and hence there is some merit in looking at sd over a range of loads.

BUT I will say this - it seems to me that it is often far less dramatic than people often suggest. Once you have discovered the best powder, primer and seating depth for optimum accuracy and velocity spreads - going up or down a grain hardly makes a "measurable" difference (in our test rifles at least). If you do see a difference - ask yourself could this be sampling error? and run it again - reverse the order and do a larger sample or just repeat several times.

There are plenty of people that disagree with this however and I am quite open to being persuaded - I would just like to see some solid data on it though.

So to answer your question. If you are getting low, repeatable velocity variation at a certain charge and want to explore up and down a little (within reason) then I would expect you will see a similar spread at the other charges.

Righto ciao for now