Seating Depth Testing

[Savage 12BVSS]

Depending on the cartridge your probably talking 2000 rounds + before throat erosion plays a major role in accuracy imo!

I have not had such long throat life as you, 22-250 changed quite a bit by just 470 rounds and 7mm-08 was right at 850. I have always blamed it on close distance to lands. I've never bothered with any testing past .040 and both above rifles were .010 and .012 off. These thoughts about going on out to find longer jump points never really occured to me. Will try this myself very shortly and see what the results turn out to be. Food for thought Jud.........

 

[Savage 12BVSS]

At the end of the day every rifle, every powder, every projectile determines their own CBTO length!

I agree with this statement, however I believe there are several such points and doing a longer ladder test appears to reveal this in opening post.

 

[Shadowwalker]

That was a good article posted by PRB, and even better to see it put to the test.
0.070", what do you think made that group go tight while groups on both sides of it were more spread?

Anybody ever hear of 'Conditions"????

 

[mwkelso]

Anybody ever hear of 'Conditions"????

Yeah, all the time actually.
I'm guessing that is meant to imply something specific and I must be missing it. Please, explain yourself.

 

[Jud96]

I'm sharing this method for those that want to use it in their rifles. I think it's a very good, simple, and conclusive way of finding the sweet spot for seating depth in your rifle. A lot of people don't really understand how serious seating depth can change things. I was one of those people up until a couple years ago. I also think some guys aren't sure on how to really test seating depth or what to look for. I simply wanted to share my method with some results to show what I do, what works for me, and what I look for. It's weird to me to be shooting more than .030 off the lands, but my results show how well some bullets like to be jumped far. I think it's a good idea to do this test when working up loads in any new rifle or load combination. I also wanted to say that I recommend waiting a few minutes between each group, and if you have a sporter weight magnum to wait at least one minute between each shot in the group and again waiting a few minutes between each different group.!

 

[Okanogan]

The PRB series was very interesting. One of the things I saw pointed out elsewhere is that a lot (not all) the referenced testing was using Berger Hybrid type bullets.

I'm starting in with initial load development for a new 6.5 SAUM and was kind of surprised at the amount of difference that bullet selection/ shape makes for seating depth. I was going to try and load 143 ELDXs out to 0.100" off the lands but in my rifle that far off the lands would have put the bearing surface very close to or below the case mouth. In comparison, to Berger 140 gr Elite Hunters, the ELDXs had to be seated almost 0.200" deeper to keep them out of the lands. I went ahead and tried all the way out to 0.070 off the lands with the ELDX but ended up with the better groups around 0.040. Overall, I was not impressed with the ELDX in the new rifle which is kind of what I had expected after seeing how far they had to be seated into the case. I'm more hopeful future testing with the Bergers will fair better.

I think a good place to start load development might be with the Wheeler test for different bullets in your rifle before doing any reloading or shooting. If the bullet ends up sitting too deep or exceeds your mag length, it might just be better to pick a different bullet to work with from the start.

 

[Shadowwalker]

Yeah, all the time actually.
I'm guessing that is meant to imply something specific and I must be missing it. Please, explain yourself.

The best groups fired will be shot when every possible variable are absent. Zero conditions are really never zero if you know what to look for.

 

[mwkelso]

The best groups fired will be shot when every possible variable are absent. Zero conditions are really never zero if you know what to look for.

Got it, thanks.
I think this would be the scenario that we all wish we had a warehouse to shoot in perfect, zero conditions.

 

[Jud96]

I'm not sure what you're getting at here @Shadowwalker. Conditions were the same, all groups shot by me and conditions were calm with the same lighting and everything being done the same.

The purpose of the results is to find the seating depths that are less finicky and are more consistent. As you can see .090-.100 shot decent, but .060 and .080 weren't good even though .070 shot so well. The point is, the .070 load looks more picky than the the .100 off load.

 

[Bingoc]

Jud 96. I like your approach, and your article was very complete. Keep us informed with your follow on testing.

 

[aushunter1]

Actually your barrel can erode much faster than you think. With a hot magnum or fast 6mm you could erode .005-.010 of the throat in 100-200 shots.

But at what stage does it affect accuracy?

I have not had such long throat life as you, 22-250 changed quite a bit by just 470 rounds and 7mm-08 was right at 850. I have always blamed it on close distance to lands. I've never bothered with any testing past .040 and both above rifles were .010 and .012 off. These thoughts about going on out to find longer jump points never really occured to me. Will try this myself very shortly and see what the results turn out to be. Food for thought Jud.........

My barrel most likely started to erode was sooner, but I didn't notice any real drop off in accuracy until around 300 rounds. Mind you it was still shooting MOA at that stage!!
I have never had to seat any further back than .020 of the lands,

Also does anyone have a link to this PRB article Id like to read it.


Another thing comes to mind with this, that is you could be severely impacted with this method if you were using a powder on the slow side.

I mean if a load is already shown as being compressed in the manual you may not even be able to seat at .100??

So maybe with this process you need to use a slightly faster powder with less bulk density from the onset??

This is especially applicaible if this is bases on long VLD projectiles!

 

[kiwi49]

From what I've seen and read here and in other posts/forums - in all of the information, given freely, is this.
Bullet jump, powder, brass, primers, technique, posture, range shot, weather at time of the year - can have varying degrees of effect.
Just one thing I don't see mentioned here, is the freebore of the 243.
I'm guessing it maybe a custom barrel?
Jud96 - there's a lot of CBTO change from touching lands to 0.090"-0.100" off the lands and preserving powder space in the case.
Can you enlighten me please, as the process sounds well reasoned and has some barrel accuracy life challenges made easier to live with. TIA.

PS - I load AR2208/Varget in 308W/7.62x51, and find (for 155gn VLD type bullets), that any jump greater than 0.050", starts seriously compressing the powder column. To the point that CBTO becomes quite variable. Using 45.5gns powder in Lapua once fired to chamber brass (originally virgin at first loading).

 

[Okanogan]

Here is the PRB article in question but there are additional ones preceding it.
https://precisionrifleblog.com/2020/04/28/bullet-jump-research-and-load-development-tips/

 

[Shadowwalker]

Got it, thanks.
I think this would be the scenario that we all wish we had a warehouse to shoot in perfect, zero conditions.

Actually testing in the inside is not without it's problems also. Even tubes which I have tested in for years also have their internal Gremlins.
Always test in the best conditions available. If you do not you will waste a lot of time, effort and ammo. Just use your head and reason out the results.
Good shooting.