Different BTO (lot to lot) and bullet seating depth

[Ruger15151]

Newbie to Sniper's Hide here....

But I know you guys give great information.

I developed a 308 load using a specific lot of 168gr match bullets - OCW, bullet seating, neck tension, and primers. When I measured a new box the BTO was .026 longer than the lot I used for load development. My question is... is it more important to maintain the same bullet jump ( and thus the same ogive to case measurement) even though the deeper seated bullet will cause a pressure change (and velocity change).

Do I need to develop this load all over again?

I know everyone that has developed a pet load has had to deal with this once the purchased another lot of bullets.

Thoughts? (besides buying enough bullets for the life of the barrel)

 

[straightshooter1]

Newbie to Sniper's Hide here....

But I know you guys give great information.

I developed a 308 load using a specific lot of 168gr match bullets - OCW, bullet seating, neck tension, and primers. When I measured a new box the BTO was .026 longer than the lot I used for load development. My question is... is it more important to maintain the same bullet jump ( and thus the same ogive to case measurement) even though the deeper seated bullet will cause a pressure change (and velocity change).

Do I need to develop this load all over again?

I know everyone that has developed a pet load has had to deal with this once the purchased another lot of bullets.

Thoughts? (besides buying enough bullets for the life of the barrel)

Yes, a deeper seated bullet will increase pressure and change velocity, so I work at trying to keep the case volume the same (the volume after the bullet is seated). So, I'll change the seating depth to get there as that volume is more important to matching the load I've developed than the jump distance in maintaining the performance I get.

 

[winniedonkey]

I just readjust to keep the seating depth that I had settled on and go. I do rezero and run over a MS to see where everything lines up. Keeping data makes things like this "less shitty". There is a chance you may have to tweak a bit.

 

[918v]

It depends

 

[Jabot]

Seat it to the same cbto and shoot it over chrono this will tell u.
I've never worried about it

 

[straightshooter1]

Seat it to the same cbto and shoot it over chrono this will tell u.
I've never worried about it

When the BTO is .026 difference, seating to the same seating to the same BTO as before will produce a significant difference in volume inside the case and will change the POI due to the change in pressure. Whether that difference in POI is enough to be of any concern would depend on what kind of shooting one is doing. But there is enough difference to be considerable at extreme long distances.

 

[Jabot]

I got some juggernauts that r .024 longer .I'll test them at 800 this weekend and let u no

 

[straightshooter1]

I got some juggernauts that r .024 longer .I'll test them at 800 this weekend and let u no

The test I did a couple years back was at 100 yds with 168 SMK's that had a .033 difference in BTO.

 

[Ruger15151]

Im just shooting this 168 gr load to 600 yards. I have other loads out to 1000 yards. I'm not concerend with re-zeroing or adjusting my DOPE. That's easy. I was just curious if it would move the accuracy node. If I run this new load over a chrono and the velocity increased, I should be able to adjsut the powder down a bit to the original velocity and get the same accuracy again...correct?

 

[straightshooter1]

Im just shooting this 168 gr load to 600 yards. I have other loads out to 1000 yards. I'm not concerend with re-zeroing or adjusting my DOPE. That's easy. I was just curious if it would move the accuracy node.

The accuracy node is a function of the harmonics of the barrel, not the load. So, it's not going to move. But, there are more than one node and more or less of a charge can move into one of those other nodes. The way to "move" the accuracy node is by changing the barrel harmonics and this his what's going on with attaching a tuner and it will move also if you were to cut the barrel shorter.

If I run this new load over a chrono and the velocity increased, I should be able to adjsut the powder down a bit to the original velocity and get the same accuracy again...correct?

Yes, you can adjust the powder to get that velocity and back into the accuracy node. OR . . . you can adjust the seating depth to do the same. It's all about getting the same pressure for which velocity has a direct correlation.

 

[918v]

If you are say .050” off the lands with Lot A and Lot B is longer in the BTO department by .026”, that doesn’t necessarily mean you’ll be .024” off the lands with the new lot. Your comparator is not an accurate representation of the rifling in your barrel.

Second, even if this were the case, .024” off the lands is fine and the the two lots will prolly shoot the same. Jump is not as important as people make it out to be. Bullet base starting position, aka seating depth, is more important.

 

[Jabot]

This is not what the op is wondering. He is saying that if one lot of bullets is .026 longer in bto and u seat them to the same ctbo,so same seating depth. Th his means there is more bullet in the case causing more pressure, if this will change velocity

 

[918v]

So don’t seat them to the same CBTO. Seat them to the same approximate COAL.

 

[Jabot]

Seat then the same

 

[918v]

Yes. I usually do not fave to adjust about .005” up or down.

 

[Ruger15151]

This is not what the op is wondering. He is saying that if one lot of bullets is .026 longer in bto and u seat them to the same ctbo,so same seating depth. Th his means there is more bullet in the case causing more pressure, if this will change velocity

You are correct.

If I seat to the same COAL, then the bullet jump is .026 shorter which may effect accuracy. If I seat them to the same CTBO to keep the bullet jump consistent, then I am seating .026 more into the case which should change the pressure So.... which would have the biggest effect on accuracy. if you aren't shooting 1/2 inch groups or nothing over 300 yards then I'm sure it doesn't matter.

However, Everyone who shoots long range has had to deal with lot variances. I was just wonder how you deal with it in order to keep your long range loads accurate.

 

[spife7980]

I was just wonder how you deal with it in order to keep your long range loads accurate.

I buy big lots and I test them to know.

 

[Jabot]

I've used the same ctbo in my 6.5x47 for the life of the barrel 2700 rounds. I buy bullets in 500 piece boxes. So not same lot number. I tried my velocity at 600 yards the bc at 900 and checked at 1200. I've shot many matches with this the way it is.I've never changed the seating depth or added or lowered powder because bullets r .026 thousandths longer or shorter .my dope has never changed till I got to 2500 rounds it dropped 30 fps and groups started to open up,barell is almost used up.just my experience

 

[Ruger15151]

Thanks Jabot.

so what you are saying is that you have kept the same CBTO ( and same bullet jump ) even though the BTO may have varied box to box. So, that would mean you are actually changing the bullet seating depth ( when BTO varies ) but the velocity and DOPE for that barrel did not change.

Did I get that right because that is exactly the question I was posting.

Thanks man!

 

[Jabot]

Exactly

 

[Ruger15151]

Thanks for the suggestions guys! Stay safe.

 

[Steel head]

I seat new lots of bullets the same.
I’ve never had to make more than a slight seating change and usually nothing needs to be done.

 

[Ruger15151]

Awesome.. thanks Steel head!

 

[Jabot]

Update
Done a seating depth test ,ill trying to explain as good as a I can.
My testing was done on a 308 shooting 185 juggernauts.
Rem 308
185 juggernaut
Mv 2734
Ctbo 2.365
This is my ftr rifle I usually only run it single feed since it is throated long
As t 2.365 ctbo I'm jumping these bullets 15 thousandths.
So to do the seating test I shot this load to 800 and 900.
Then I took the same bullets loaded with the same powder charge, same cases ,same primers. The only difference is I seated the bullets to mag length a ctbo 2.226 that's a different of 0.139 seated farther in the case .
I shot the groups all prone from the same position within a minute from another .
1st pic 800 ctbo of 2.365
2nd pic 800 ctbo 2.226
3rd pic 900 ctbo 2.365
4th pic 900 ctbo 2.226
If u look at groups they all r slightly above water with the center of the groups virtually the same
The only difference is the groups shot with the ctbo of 2.226 r bigger. This us few to the fact that with the seating depth of 2.226 is shoots about a .75 inch group.
As compared to the 2.365 seating depth it shoots a group size. 2.
To me it doesn't seem to change much of anything .there was about a 7-8 mph wind

 

[Jabot]

Update
Done a seating depth test ,ill trying to explain as good as a I can.
My testing was done on a 308 shooting 185 juggernauts.
Rem 308
185 juggernaut
Mv 2734
Ctbo 2.365
This is my ftr rifle I usually only run it single feed since it is throated long
As t 2.365 ctbo I'm jumping these bullets 15 thousandths.
So to do the seating test I shot this load to 800 and 900.
Then I took the same bullets loaded with the same powder charge, same cases ,same primers. The only difference is I seated the bullets to mag length a ctbo 2.226 that's a different of 0.139 seated farther in the case .
I shot the groups all prone from the same position within a minute from another .
1st pic 800 ctbo of 2.365
2nd pic 800 ctbo 2.226
3rd pic 900 ctbo 2.365
4th pic 900 ctbo 2.226
If u look at groups they all r slightly above water with the center of the groups virtually the same
The only difference is the groups shot with the ctbo of 2.226 r bigger. This us few to the fact that with the seating depth of 2.226 is shoots about a .75 inch group.
As compared to the 2.365 seating depth it shoots a group size. 2.
To me it doesn't seem to change much of anything .there was about a 7-8 mph wind

 

[Jabot]

Dont no why it reposted