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How Many Thousandths

Bill Norris

Bill Norris

Back in the Day
Gold $$ Contributor
Was scanning thru videos on U-tube last night and happened to watch Erik Cortina (Pro-Shooter) Dont' Chase the Lands saying only .003 thousandths will show u a difference in group size & Sam Milliard (Panhandle Precision) Bullet Seating Depth saying it takes about .010 thousandths to show a difference in group size. Needless to say, if a hand loader had not tried for himself to find out just exactly how many thousandths it takes to show a difference in group size for his rifle and/or loads they would definitely come away from those two videos scratching his head trying to figure out who is correct. Not interested in starting an argument but would like to see a perspective from the group here.

EDIT : The groups below are an example of .005 difference in bullet seating depth in my Rem 40X 260AI.
 

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Since opinions are free....

I think they are both right in a way. When you run a seating depth test similar to the way you run a charge ladder, the size of the steps and the length of the sweep makes a difference. You might use steps that are too large and misinterpret the performance of a given single sweep.

My advice is to run a wide sweep with very fine steps. I would say 0.003" is a decent step size for a 6mm 105 hybrid for example. Keeping in mind that you are subject to small sample size issues, as well as human error issues at each step, what you are hoping to find are node widths that are more than a few steps wide in order to avoid missing the node in the noisy test. Done this way, you will likely go back and revisit the best looking nodes with more populated groups and then also explore the edges to know when to re-tune.

Something like this shows a fairly wide sweep and also shows the bullet can like to be jumped a fair bit.

Berger 105 Hybrid in 6 Dasher. MV v/s Seating Depth and Vertical MOA at 600 yards v/s Seating Depth.

1621633473208.png

1621633511453.png

Clear that any one sweep can be deceiving on its own, and that strong node at 0.035" is tempting but the main point is a coarse step would have missed that. I don't select the best little node, but I look for zones that are wear and temperature tolerant. There are often multiple good nodes, but make the test steps too big and you can't see them or know which ones are durable.
 
Bill Norris said:
Needless to say, if a hand loader had not tried for himself to find out just exactly how many thousandths it takes to show a difference in group size for his rifle and/or loads they would definitely come away from those two videos scratching his head trying to figure out who is correct. Not interested in starting an argument but would like to see a perspective from the group here.
Click to expand...
It depends on MANY other things - including how far out your loaded rounds are from being concentric, the burn rate of your powder, whether you are using a bullet that is forgiving of jump - or one that just plain likes it, etc. For example, Barnes lead-free bullets like to jump. I shoot a lot of Lapua Scenar L bullets and they don't like to jump in any of my barrels - but might in yours. Some guys get their velocity to where they want - then tune by adjusting seating depth. I start with a seating depth that I think is ideal for the cartridge (or magazine length) then fine tune depth after I find a powder that the gun likes best with that particular bullet. You will no doubt get more opinions on this subject than most - as you can attack this from so many angles and still end up with a great result. Some are just a lot faster than others for a given bullet and cartridge.
 
Had a question about the groups above. Yes, both groups were shot within 20 - 30 mins. No wind to speak of because my 100 yd. spot behind my shop is just a hole in the woods and I'm shooting from against the rear wall of my shop which is pretty much blocking from behind my bench.
 

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.003" makes a difference in an LR BR gun in 6 BRA for seating depths to around a .015" jump. In a 30 cal hunting rifle, .005" will usually make a difference to .040" or so. After that it will take .010" to see a difference.

The caliber, distance from the lands, and the accuracy of the rifle all affect how much change in seating depth is noticeable.
 
Some random thought from a random brain of seating depth:

Sierra published a paper on bullet seating depth several years ago. The gist of the article was that different bullets / rifles combos may actually benefit from a longer jump. After years of reloading for over 30 different rifles, I found this to be true in some instances.

For me, the seating depth must first accommodate the magazine. Secondly, I prefer at least one bullet diameter inside the neck to provide enough neck tension to keep the bullet in place during field use. I never seat closer than 0.010" to the lands, never saw the need in order to meet my goals.

For me, I never found seating depth with the limits previously mentioned, made a significant distance in precision. I imagine it does for bench rest guys and maybe long range precision guys but It doesn't for me.

Most factory ammo I've seen have the bullets seated considerably below the max SAMMI specs yet I've seen some amazing shooting at the range with factory ammo, especially Federal Premium.

PS: Hey, Bill - I see you watch Panhandle Precision's video - did you watch the one on cleaning - that one will blow your mind for sure. If what he asserts is true, the solvent makers are going to hate it. If you liked that one then you'll love the ones from gunblue490 and tiborauaurus rex on cleaning. I'm in the process of testing the "copper equilibrium" premise espoused in this videos.
 
Been loading Berger's, Classic and Elite, Hunters and I move them back, IN,.. .010 at, a time from, start with, the 95 gr. Classic's and 108 gr. Elite's in, the New, 6 XC.
Holy Dog Crap, WHEN you HIT, the "right seating, Number" do the groups, "tighten up" !
Tried starting at, .020 OFF ( heard / read that, they LIKED to, "Jump" ), then, at .050 / .055 "off",.. it's "Money" !
Went from, 1/2 - 5/8 ths inch groups, down into, the .2's and .3's for my 6XC with, BOTH Bullets !
My New .270 WSM load at, 3,175 - 3,180 FPS, with, the 140 classic's, is producing, Sub 1/2 MOA groups on my "good Days" of paying attention ( focusing ) and when, my old .338 Win. Mag "Flinch",.. stays away !
I started them at, SAMMI COAL per Berger's recommendation and just "played" with, powder charges.
Best groups before, were running around 3/4 MOA with, the AccuBonds.
I do believe that, the VLD's are a bit, "Fussier" on seat depth, but have, limited experience with, them.
Since the Throat on a Rifle, is moving further AWAY, every few shots, I like Tangent Ogive and Hybrid designed, Bullets "Jumping" for, my Rifles, since I'm sure that, my .270 WSM's Barrel will be, GONE at,.. 900- 1,200 rounds.
 
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K22 said:
Some random thought from a random brain of seating depth:

Sierra published a paper on bullet seating depth several years ago. The gist of the article was that different bullets / rifles combos may actually benefit from a longer jump. After years of reloading for over 30 different rifles, I found this to be true in some instances.

For me, the seating depth must first accommodate the magazine. Secondly, I prefer at least one bullet diameter inside the neck to provide enough neck tension to keep the bullet in place during field use. I never seat closer than 0.010" to the lands, never saw the need in order to meet my goals.

For me, I never found seating depth with the limits previously mentioned, made a significant distance in precision. I imagine it does for bench rest guys and maybe long range precision guys but It doesn't for me.

Most factory ammo I've seen have the bullets seated considerably below the max SAMMI specs yet I've seen some amazing shooting at the range with factory ammo, especially Federal Premium.

PS: Hey, Bill - I see you watch Panhandle Precision's video - did you watch the one on cleaning - that one will blow your mind for sure. If what he asserts is true, the solvent makers are going to hate it. If you liked that one then you'll love the ones from gunblue490 and tiborauaurus rex on cleaning. I'm in the process of testing the "copper equilibrium" premise espoused in this videos.
Click to expand...
Yes Sir. I watch as many as I can find by these dudes. Just like to see and hear different opinions and results from the different guys, guns, practices, and loads.
 
Too add to this, I just found a load for a customer's 338 Lapua Improved. 300 grain Berger EOLs seated .100" off the lands. Groups at 88, 89, and 100 grains of N570 went .447, .492, .228. ES of 8,15, 7. The cool thing is with a jump that far this load will likely never go out of tune.

I found almost as good of accuracy from another customer's 28 Nosler with the bullets .090"off the lands, and that cartridge is a throat eroder.

I am working on a customer 26 Nosler as well, and I will likely start tuning it with the bullet very far off the lands as well.
 
I think both could be right to a point, I also know that some calibers will only shoot with jam, you can and will get one group off the lands that will shoot better than the others but then when you jam it, it will shoot better than the other 4.
 
And the F-Class shooters are looking for both consistency , and close to , precision accuracy . Using lengthened chambers to accommodate long nose bullets , and longer loading lengths . And that seems to create a , "Find that seating depth" imperative . For the bullet you shoot in competition , and practice with . Many F shooters use .005 off touch , for a starting point , and graduate by .005 until the group starts to deteriorate . And it will . Very quickly . It really isn't hard to select a starting seating depth from a round of four , five shot seating depth tests . . The target will tell you . And if you feel the need to get closer , cut down the stepping distance between depth changes .
 
Some nightstand reading. This, plus the links to the seating depth series and you will see that some bullets like more jump, even across several rifles. A very rare opportunity to see lots of data outside of an ammo company or the GOV.

https://precisionrifleblog.com/2020/03/21/bullet-jump-and-seating-depth-reloading-best-practices/
https://precisionrifleblog.com/2020/03/29/bullet-jump-load-development/
https://precisionrifleblog.com/2020/03/24/how-fast-does-a-barrel-wear/
https://precisionrifleblog.com/2020/04/05/bullet-jump-load-development-data/
 
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