S-D, velocity and groups

[Pheroze]

Please help me with this puzzle. I am confused by the apparent lack of value that consistentency seems to have on accuracy

308 Norma Mag, 180 gr A-Frames over 70.5gr of 4831 has the following variation:

But the group looks like this:

I increased the charge to 71.0 gr and the velocity did not change, but consistency became amazing:

But the group opened up badly:

So, I am confused by the fact the velocity didn't increase. I am also amazed that the more consistent load was not the better group!

There are no pressure signs. It is about 100fps slower than published maximums. I am considering increasing the load some more to see if the group closes while maintaining consistency. Woodleigh and Lee data have 73 gr max. But, Quickload with the A-frame max is about 71gr. Any insight would be appreciated.

 

[Pheroze]

Lol, my title had a fellow shrugging his shoulders. It has been transformed into the male sign!

 

[Opposite Pole]

Low velocity spread affects accuracy in long distance shooting as the velocity variation translates to differences in bullet drop, it doesn’t really affect group sizes at 100m or so. There are several factors that affect velocity consistency, neck tension being one, load repeatability being another (scales aren’t always accurate and repeatable be it electronic or mechanical). Large V spreads at loads significantly below max can also be due to inconsistent powder burn rate due to low powder density.

I find when I get a load that is persistent in velocity it’s worth exploring its where abouts, say several groups starting one grain below going to about a grain or so past it. Often times you’ll find a sweet spot. Then I fiddle with seating depth.

 

[fourfive8]

SD of velocity is not the only thing that affects accuracy. It is one of many. If you have a particularly accurate rifle and document that accuracy by shooting consistently tiny groups. Then introduce a variable such as a high SD, that one variable would then be isolated and you may see or be able to measure that it does affect accuracy even on a 100 yd target. If that variable is not isolated it becomes one of many- lost in a large crowd.

The other use I have for measuring SD when I'm developing a load is that it tells a lot about how consistent and stable a load is and I like those loads with no squirrelly velocity excursions.

Coincidentally, today I went to the range to do just that and was able to determine the SD for a load for my newly acquired 375 HH. The SD of the promising hunting load with a 300 gr. bullet today was 8.9 fps with a mean velocity of 2473 fps. The most common sample SD seen on calculators I think are provided for at the 98% confidence interval. A statistical parameter that simply says- there's a 98% chance that the next sample (shot) will be plus or minus 8.9 fps of 2473 fps. If I do that same test two or three times- I feel nearly 100% confident that load will consistently perform at that level. The powder I used was Varget, which is known to be very temperature insensitive.

 

[Hogpatrol]

What's your max shooting distance? If it's 100 to 200 yards, as long as the groups are good and consistently print in the same spot, forget about the numbers. If you're reaching out beyond 500, develop a stable load, i.e. where two groups are close in weight and POI and then try seating depth changes. For long range, I want single digit ES in a ten shot group. Aside that, flat spots in velocity with charge increases are not uncommon.

 

[bruce moulds]

s.d with 3 shots is absolutely meaningless.
you need a min of 10 shots.
sometimes s.d. will come down as you keep shooting., but not necessarily.
low s.d. does not mean an accurate load, but you can have an accurate load with low s.d. which is necessary for long range shooting to avoid verticle.
for 10 shot groups, the e.s. is usually about 3x s.d.
starting with a clean barrel is a no no for measuring s.d. as speed usually picks up as the barrel fouls, from 3 to 6 shots usually.
for shooting game out to 300 yds, s.d. is not a cvoncern as much as good grouping.
especially if you don't shoot off a rest.
sticks are not a rest in these terms.
bruce.

 

[Pheroze]

What's your max shooting distance? If it's 100 to 200 yards, as long as the groups are good

I don't ever see myself shooting past 300 yards, and most probably under 100. I do like tinkering with the loads though. Finding a good group early in development is both a blessing and a curse

 

[Ridgewalker]

Just an FYI, in one of my 300WMs I have owned the bullets didn’t seem to stabilize until they were beyond 100 yards. At 100 yards it consistently fired 1 3/4-2” groups. At the 200 yard target it was the same. Time after time. 1:10 twist. Same box of Nosler Partitions. Same day. Same rest. Strange harmonics and stabilization issue. JME

 

[Pheroze]

Just an FYI, in one of my 300WMs I have owned the bullets didn’t seem to stabilize until they were beyond 100 yards. At 100 yards it consistently fired 1 3/4-2” groups. At the 200 yard target it was the same. Time after time. 1:10 twist. Same box of Nosler Partitions. Same day. Same rest. Strange harmonics and stabilization issue. JME

That would be very frustrating!

 

[Ridgewalker]

That would be very frustrating!

It was until I figured out what was going on. I found the same happened to others on the internet and oddly enough also with the 300WM!