Seating depth variation

[VenatusDominus]

With my .223 I fired a ten shot group at 100 that was 7/16". Then I did it at 200 yards and the group was 7/8". If I fired twenty more at 100 and 200 respectively, would the groups get bigger?

Likely, but probably not by more than 30%. From a statistical standpoint, data on a bell curve usually starts to stabilize between 30 and 50 data points and after that you are simply filling in the curve between +3SD and -3SD. You have two ten shot groups at 7/16 MOA. The group would likely grow in size slightly as you increased the number of shots but not likely to go from 7/16 MOA to say 3/4 MOA. What shooting 30 at the same distance and aim point (in smaller strings allowing the barrel to cool between) provides you with a mean radius. If your group size is 1/2 MOA, the mean radius may be 1/4 inch or smaller, meaning 50% of you shots will hit a 1/2" target at 200 yards. You can then figure out 1 SD and know that 68% will hit within that radius and 95% within a 2 SD radius.

 

[Ckleeves]

I have spent quite a bit of time playing with seating depth and I'm in the camp that it doesn't make a huge difference (somewhat bullet dependent). I used to be a believer and would do all sorts of seating depth test and after "narrowing it down" in .02 increments would go as small as .003 increments. If you take the best and worst seating depths and load/shoot a large sample size of each the results have surprised me. There might be a difference but (most of the time) it's not going to be near as large as many would believe.

It's just like .2 grain powder changes making a difference, do the same test 10 times and you probably won't see the same results twice.

 

[bamban]

Digest this for a moment. Rounds were loaded to fit in magasine. 300WM, 190 SMKs, 2 test barrels each fired 14 five shot groups at 200 yards. That lot most likely accounted for lots of people on the receiving end instant dirt naps.

 

[Equalizer]

Btw for those that are mentioning the Bergers, the guys in episode #50 mention at least twice that the data was collected with Hornady projectiles and that with some bullet shapes seating depth was more critical to accuracy. They did use other manufacturers brass as well as their own. I wanted to throw that out there for the folks who won't watch the hour plus video.

 

[VenatusDominus]

With my .223 I fired a ten shot group at 100 that was 7/16". Then I did it at 200 yards and the group was 7/8". If I fired twenty more at 100 and 200 respectively, would the groups get bigger?

Just so we are on the same page, this topic is totally irrelevant if you are shooting inside 400-500 yards. This topic was meant for those shooting at ranges where the bullet time of flight is over 1 second (around 1000 yards). The difference on between .5 MOA and 1 MOA matters not on an eight inch vital zone at 500 yards. Establishing consistency of your load with 3-5 shots is plenty.

But consistency and statistical significant are not the same, and at 1000 yards the difference between .5 MOA (5.3 inch group) and 1 MOA (10.5 inch group) matters on an eight inch vital zone. At this range I want to know if the statistical probability of a shot landing within the 8 inch vital zone is 50% or 95%. You can absolutely establish consistency of a load with much less than 30 data points, but most statisticians agree that 30 data point is a minimum for establishing statistical significance, which is needed to produce valid probability numbers.

Understanding math, science and statistics is not necessary to shoot inside 500 yards, and many on this forum have no interest in or even detest them, I get that. But they become extremely important outside 1000 yards. I am passionate about both the math and the science of long range shooting and enjoy studying and having intelligent discussions about both and welcome all to join the conversation.

 

[RichCoyle]

If it's not a comp gun I'm betting I know the answer.

It is a factory stock plastic stock Savage .223 with a 26" heavy barrel. The scope is a Tasco 6-24X in Burris Signature rings.

 

[RichCoyle]

Likely, but probably not by more than 30%. From a statistical standpoint, data on a bell curve usually starts to stabilize between 30 and 50 data points and after that you are simply filling in the curve between +3SD and -3SD. You have two ten shot groups at 7/16 MOA. The group would likely grow in size slightly as you increased the number of shots but not likely to go from 7/16 MOA to say 3/4 MOA. What shooting 30 at the same distance and aim point (in smaller strings allowing the barrel to cool between) provides you with a mean radius. If your group size is 1/2 MOA, the mean radius may be 1/4 inch or smaller, meaning 50% of you shots will hit a 1/2" target at 200 yards. You can then figure out 1 SD and know that 68% will hit within that radius and 95% within a 2 SD radius.

I didn't let the barrel cool between shots. My hunting rifles are fired cold bore. I use a air mattress pump between all shots. The last three shot groups I fired at 300 yards were .7", 1.3", and .9" with an eight pound 7 - .338RUM.

 

[VenatusDominus]

I didn't let the barrel cool between shots. My hunting rifles are fired cold bore. I use a air mattress pump between all shots. The last three shot groups I fired at 300 yards were .7", 1.3", and .9" with an eight pound 7 - .338RUM.

That is some fine shooting for sure, especially with a rifle that kicks like a mule. Does your RUM wear a break or a can? I shot my 6.5 pound 300 Weatherby for 30 years before adding a break to it. If I shot 10 rounds in a range session, I would have a bruise the next day. Much more enjoyable to shot now.