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.