Home Forums Reloading Big Game Rifles Seating depth trial

I've watched a couple of youtube videos by Erik Cortina and decided to try one of his suggestions, which is to seat bullets deeper in .003" increments. My normal procedure has always been to use much wider increments, like .020" or even .025". My old .243 AI is still a great shooter even with close to 3,500 rounds down the tube and I haven't adjusted the seating depth on its loads for a while, so figured it was a good candidate for testing.

Used a Stoney Point OAL gage to find the jam length, it was a good .1" longer then when last checked, then I loaded six sets of four rounds each starting at .020" off and increasing in .003" increments.

This morning was very calm with winds at 2 mph or less so it was a good time to try these. The picture below shows how the groups progressively tightened up as the bullet seating depth increased. The colored bars are 1/2" wide, aiming squares are 1.5", rifle is a Model 700 with a Pac-Nor Sporter contour barrel, Sierra 85 HPBT, scope is a good ol' Leupold VX-II 3-9x40. Cases are 10x fired but freshly annealed, neck sized and shoulders bumped .002".

It looks like this ended up right on the verge of an accuracy node so I'll load up some more starting at the final seating depth and continuing the seating progression to see when groups start opening up again.

Fwiw, I have a few issues with Erik's videos and also the statistical significance of one group at each depth but he does get results which can't be denied. This series of groups definitely shows a trend and warrants further exploration. If this works well I might rework up some loads for a couple of newer rifles which don't seem to be performing up to their potential.

Jim,
How close to the lands did you start with? I began using Berger's recommendation but seated bullets in .03 increments with success. But I have considered smaller increments. Interesting what you are doing.

As an added note, good to hear your 243 AI is still going strong with 3500 rnds down the bbl. My 243 AI is definitely easier on the bbl than my 243W's are. Doesn't foul as quickly either.


Oh, see where you started at now

Mike all the bullets you intend to seat for length consistency before seating. Depending on the bullet manufacture, I found up to ten thous. difference from base of bullet to ogive. That will not help your test. I normally move 5 thous. Develope all my loads this way. If you can, get bullets of the same lot. And, bullets of same manufacture but different box, can be very different. I assume coming off seperate production lines. Check them before you seat them.

I started at .020" for this trial. Used the OAL gage to find the lands by tapping on the end to seat the bullet just enough to hold it as the modified case was withdrawn, that length measured 3.345" counting the length of the comparator so I began at 3.325". No particular reason to start there except that was what Erik did in his video.

Thanks Jim. I'll be interested in hearing if seating the bullet deeper continues to improve groups.

It's definitely a worthwhile test to make. I have a 308 that is very happy with the 168 gr. Berger VLD well off the lands.

So far it's showing the truth of a rule of thumb that AussieGunwriter posted a few years ago. If your groups show two and one or in this case three and one, seat deeper. If they are evenly distributed but larger than they should be, seat shallower. You can see that flyer in #2, 3 and 4, and it's still there in #6 but it gets closer and closer to the main group.

One problem I have is implicitly trusting these samples of one each but they do show a definite progression which matches the idea of accuracy nodes being a sine wave, coming and going at different depths. With that in mind there's a good chance that if I was to start seating closer to the lands that group represented by #1 would start to get tighter.

interesting, I'm also in the process of following Eric's lead on this,

Liking where your headed, but like you said,

Can you repeat the same results.

Keep us posted Jim.

Don't know if it's just me, but it seems hunting bullets in recent times seem to prefer further from the lands than what previous wisdom generally said. But my target bullets still like to be close.

Gotta ask you Jim - what increments do you work with on powder charge weights?

I this weight range I go with .5 grain increments, that represents around a 1% increase.

Eric knows his stuff.
I have been doing this for several years now and bullet seating depth will turn a good shooter into a great shooter

Did this test over 10 years ago using a proven load of 117 pro hunters for a base line. Seating depth test 110 Accubonds. Seating depth in the last group ended up being 10 thou farther out than the proof load of 53 grains RL22 and WLR seated 3.165. 110 load was 54 grains RL22 WLR and the last group was seated 3.175. The 10 thou was the difference between the base to ogive difference of the 2 bullets.
I always use a sharpie to mark the bottom of the case and leave the color legend at home. The 2 in 1 out seat deeper that Aussiegunwriter mentioned has been around for 20 years at least and does work.
As mentioned my hunting rifles have all ended up with a fair size jump, but target rifles have responded anywhere from a light jam to a small 5-10 thou jump.

fascinating thread... following with interest...

Me as well. Great thread.

I've been playing the jam meathond and .003 increments. It works , some of my old but not shot a lot rifles came out to same measurement I currently use so ..
I did find interesting was the jam method + 20th. Was the same .001-2 off the measued lands on low count barrel , on 2 barrels anyway

Very interesting results Jim, thanks for sharing.
I'm close to finishing up OCW load development on a recently acquired T3x in 6.5 Creed. After I've settled on a powder charge this seating test will be the next thing I do to fine tune the load.
Glad to see from your results that Cortina's method works. I didn't have much luck with the big jumps suggested by Berger,

Start by kissing the lands. Come back in .04” increments three times for a total of four 6-round depths. You’ll find accuracy with one of the depths, you can fine-tune from there.




P

I've played around with this method/philosophy, and it works.

https://precisionrifleblog.com/2020/04/28/bullet-jump-research-and-load-development-tips/

I had the same results with 108 Scenars in my 260. Photobucket ate the pics though. Might do some digging.

Been doing the same with my 6.5CM - with whatever bullets I can find. Currently Speer HotCores but the theory seems to be working. Been having trouble finding a day where the wind is down but can still see the trends.

Good point. My seating dies, not the best available, tend to seat from the bullet nose, to the ogive, resulting in more consistent OAL than base to ogive measurements.

One curious finding when measuring surplus 7.65 Swiss ammo, which has always shot well for me, is that their manufacturing process ended with the same result.

Here’s a good look at the .04” method:

https://www.24hourcampfire.com/ubbthreads/ubbthreads.php/topics/11270854/all/Going_back_to_my_(ouch!)_roots

Can someone unphuck my link?

Seating Depth Trial

Here you go.

OK, did my seating test today on the Tikka T3x, 6.5 Creed using 147 gr, ELD-M bullets, 41.1 gr. of H4350, Lapua twice fired brass and CCI 450 primers.
One thing I have found with this rifle is that if I load to max magazine length I'm jammed .020" into the lands per the Hornady OAL gauge.
I used the Erik Cortina method, starting .020" off the lands (this puts me .040" off max magazine length) and bumping back in .003" increments.
Here's the results. I made two scope adjustments which are noted on the target. All I'm looking for here is group size, I've already settled on my charge weight at 41.1 gr. which is right at 2,650 fps.
I'm going with load #3 which is .026" off the lands. I'll shoot some at distance and hopefully the groups hold.

Interesting thread fellas. I’ve gone down this rabbit hole some but need to revisit.

In the case of Jim’s initial results, it would be neat to repeat the test in reverse order. Meaning starting from deepest seat to .020” off the lands. This would remove any possible bias your rifle might have for a fouled barrel.

Great link, Jordan.

When I first began reloading for rifles I played with seating depths extensively, and I thought I saw a sine function when I recorded results meaning there were actually multiple sweet spots. Now that I've read that article the concept of proportional changes in distance to the lands over time hits like a ton of bricks. During that time my uncle was mentoring me and we kicked around the idea of finding seating depth first, then the powder charge...but we never pursued it. Too bad, really because it means I've been missing the boat for almost 30 years.

The idea of reworking all the loads I've developed wrankles just a bit but one of the biggest reasons I play the game is that I like to experiment. It looks like I've got a fresh and fully loaded plate in front of me now.

I did a seating depth test with a Kimber Montana in 22-250 using Berger Flat Base Target 52gn bullets. (Unrelated to seating depth but noted on the graph - I was using Lapua brass, which was giving me clickers). This is with 2-shot groups, so the results are little rough, but both group size and ES show a cyclic relationship with seating depth.

Gotta wonder what's out there beyond 0.065", don'tcha think?

Jim,
Since I first posted that process, I have further refined by noting that the slimmer the barrel, the less the die movements need to be in order to tune further. In your group number 6, I don't know the barrel contour but would recommend another shortening of the seating die in a very small increment, even a couple of thou could bring that outlying bullet into the group.
Good luck with it and thanks for being open minded and trying the method.
John

Really enjoyed and have been enlightened. I’ve been pondering seating on a rifle I’m working so back to the loading bench.

We're talking about finding accuracy for hunting rifles here. There are typically two accuracy nodes corresponding to barrel vibrations - one at the top of the vibration cycle and one at the bottom. The OP says he works up in 0.5 gr powder charge increments and finds an accuracy node. No further effort is made to fine tune to the optimum charge weight at the node. We go right into 0.003 seating depth (SD) increments.

I spend more time up front find tuning the charge wt. before tuning the seating depth. 0.3 to 0.4 gr charge increments.

I've never seen a hunting bullet that is that sensitive to SD. A bullet like the TTSX could have a sweet spot anywhere from a 0.04 to 0.12" jump. Fiddle farting around with 0.003" increments is a waste of time, components and barrel life.

I'm not buying to two-shot accuracy vs. seating depth graph either. Statistically insignificant. I believe the graph poster said he felt there was a correlation (actual words were cyclic relationship - whatever that means) between the ES and accuracy, whch I don't see. That correlation is all over the map, er, I mean graph.

To each his own, I guess. Lots of different approaches for finding accuracy in a hunting rifle.

Every Barnes I’ve ever loaded has been its best seated to the front of the first driving band. 260,270,30-06,7-08,7RM, 338, 375.

That's typically where I end up. I don't even bother figuring out how far off the lands that is either.

Never said there was a correlation between ES and group size. And, yes, thank god, to each his own.

Your words: "both group size and ES show a cyclic relationship with seating depth."

So they are either in phase with each other or out of phase by my way of thinking. Maybe you could expound on this statement.

Not necessarily. They could have different oscillation frequencies in additional to any phase difference between the two.

ES cycles between high and low values as seating depth is changed.

Group size cycles between high and low values as seating depth is changed.

I understand what you're trying to say (and trying to see in your plot) now. It all depends on how much statistical faith you put in two shot ES and group sizes.

Yes, 2-shot groups is a rough data set. More shots would certainly be better, And, folks are free to disagree, but I see a pattern, especially with the ES. No doubt more shooting is needed to lock down what is happening.

I’ve never seen one that wasn’t. Did you click on my link? The only difference was seating depth.



P

I did read the link. By "that sensitve" I'm talking 0.003" makes the difference between nite and day in grouping.

Eyeball the first one, lock the seater, then measure for reference. Not too scientific.

Two shots won't prove a load to be consistently accurate, but two shots can definitely weed out the chaff.

I've used two-shot groups when starting out with new component combinations, looking for pressure/velocity. Might as well get the easy data while you're starting from scratch. Two shots in an inch and a half says "don't waste your time here.

My thoughts, too. Use 2-shot groups to find max pressure, and also to see what doesn't work, at the same time, i.e. a third shot never shrinks a group. Then, shoot 5-shot (or 3-shot) groups around the area(s) that first showed potential with the 2-shot groups. Same general process for finding seating depth, too.

Like this:




Resulting in this:

Copy. I agree completely.



P

Always a labor of love and components.