PDA

View Full Version : Seating off the lands



adamsgt
08-03-2019, 05:11 PM
At lunch yesterday with 6 shooters we had a discussion about seating depth. The caliber was a .308 Sierra 155 gn Palma bullet. The consensus was that you needed to start at .040 off and work back, up to maybe .100. I'm trying to visualize what's going on in the barrel after the primer pops and the bullet starts going forward that would result in better accuracy/group as the bullet is seated progressively more off the lands. Anybody got any theories as to why this happens? Is the bullet free of the case before it hits the lands? In our short range benchrest world we don't seem to operate in this arena.

retired
08-03-2019, 06:10 PM
is this a specific load..that is a palma bullet.
i start at the lands work up a load and tune length,
and try small powder adjust if i end up shorter.
so far have always found a load much closer than forty off.
i have heard those stories, just never fit for me.

adamsgt
08-03-2019, 08:05 PM
Yes this is a specific case and some end up at 70 off to get rid of vertical. We do all this testing of different seating depths until we find one that seems to work and then go on. But does anyone have any theory as to WHY this occurs? Why does a particular load perform better at 70 off than 40 off? Don't know if anyone has done any research into this but its just piqued my curiosity.

retired
08-03-2019, 08:14 PM
two cents worth

distance to the lands is time, time is velocity.
maybe the bullet needs velocity to engrave/fit the bore squarely.
maybe to little allows an off center start.

ya get what you pay for.
i have the similar lapua bullet, but have done
very little work with it.
mainly 175/185's

Pete Wass
08-03-2019, 08:39 PM
that barrels can be tuned either by going off the lands or into the lands. For instance, I have a friend who use to tune his 30 BR off the lands but only .009" and it shot one bullet hole. Most of the folks I know tune slightly into the lands. Me, it's .006" in but I believe both would work in any barrel.

Pete

fx77
08-17-2019, 01:28 PM
OP poses n interesting question
I never understood the physics of this issue and choosing OAL by experimentation was kinda Art or alchemy.
I have always wondered why OAL is an issue
I have read it had to do with angular exit from the case mouth, but that would not explain how some get better groups OFF the lands
Velocity is velocity, if that is true then your best group would be a given powder charge for a fixed distance or in said lands because you can vary powder charges and change velocities, but that group changes when you use a different OAL

Thus if there is a given velocity and that is the answer, then it matters not where the bullet is with respect to the lands , provided velocity is constant for more than one OAL, but in practice that is not the case.

Could it be the OAL has an effect on the barrel resonance for a constant velocity?

Beggs on this forum says that the bullet should be just touching of just off the lands, and changing the barrel resonance with the tuner will control the group size, and if that is so, it suggests that OAL without the tuner, resonance will change by changing OAL.

I would be grateful for some very smart and experienced shooters who could better explain the physics of Jump.

alinwa
08-17-2019, 01:46 PM
I have no foggy clue what "angular exit from the case mouth" means but IMO the reason seating depth plays a role is fairly simple....... getting the bullet going down the bore almost always involves some sort of "start/stop/start" cycle. For instance with a soft-seat and some jump the primer blast may advance the bullet to the lands where it will STOP until pressure builds enough to engrave/obturate the bullet and it continues down the bore. Any changes in the timing of this sequence will change vibration timing.

Because tuning is based on control of vibration, playing with the vibration impulse in ANY way (seat depth/neck tension/tuner/primer/ogive etc) can be effective.

Pete Wass
08-19-2019, 06:18 PM
Exact OAL" is so that each and every bullet will engage the lands and groves at exactly the same place every time. I have proven many times that Exact OAL's is the basis of real accuracy, that along with exact weigh powder charges. It's provable any time. Each bullet must be presented to the throat of the chamber the exact same place and time every shot. I have generally found that .006" into the lands, measured from where land marks are faintly visible on a polished bullet .006" is likely what some consider just touching but I almost always find that .006" in is where the magic comes.

My current load for my 30 BR''s is 35.5g of H-4198 and .006" in. One single bullet hole groups. Both rifles love it. It's a mid node but shoots great and no worries about getting the powder in.

Pete

I also have a great load of Reloader 7 worked up with the same results: 34.6g. I have used Re 7 for years in both the 30 BR and 30-44 with great results. It doesn't load as easy as H-4198 but it has worked for me as well. I tested 35.6g of Reloader 7 the other day with good results but had about a quarter of a bullet sideways to the left out of the bullet hole so don't know if I pulled it there or it needs another tenth. I'll try it again when I get time, along with an added tenth. It is so important to have exact everything when testing to know exactly what the rifle is telling you.

adamsgt
08-20-2019, 04:26 PM
Exact OAL" is so that each and every bullet will engage the lands and groves at exactly the same place every time. I have proven many times that Exact OAL's is the basis of real accuracy, that along with exact weigh powder charges. It's provable any time. Each bullet must be presented to the throat of the chamber the exact same place and time every shot. I have generally found that .006" into the lands, measured from where land marks are faintly visible on a polished bullet .006" is likely what some consider just touching but I almost always find that .006" in is where the magic comes.

My current load for my 30 BR''s is 35.5g of H-4189 and .006" in. One single bullet hole groups. Both rifles love it. It's a mid node but shoots great and no worries about getting the powder in.

Pete

Did you mean H-4198?

Pete Wass
08-21-2019, 04:22 PM
Did you mean H-4198?

I'm a bit dyslexic at times, sorry.

Pete

adamsgt
08-21-2019, 11:46 PM
I have no foggy clue what "angular exit from the case mouth" means but IMO the reason seating depth plays a role is fairly simple....... getting the bullet going down the bore almost always involves some sort of "start/stop/start" cycle. For instance with a soft-seat and some jump the primer blast may advance the bullet to the lands where it will STOP until pressure builds enough to engrave/obturate the bullet and it continues down the bore. Any changes in the timing of this sequence will change vibration timing.

Because tuning is based on control of vibration, playing with the vibration impulse in ANY way (seat depth/neck tension/tuner/primer/ogive etc) can be effective.

Al, some thoughts running around in my mind about this. Does the position of the bullet in the case neck at the times it enters the lands a factor in accuracy? For the 6PPC I've read that people use a 1.490 to 1.510 case length. I measured the chamber length on two of my 6 PPC barrels and IIRC the chamber length was 1.525. So if my brass is trimmed to 1.490 there is .035 of chamber where the bullet might veer, maybe. Cause at that point you now have space around the bullet for gas to flow as it is no longer supported by the case. It's probably as important as the number of angels that can dance on the head of a pin. But, I'm still curious as to why a bullet that is already seated off the lands can shoot noticeably better when seated significantly more off the lands.

alinwa
08-22-2019, 03:35 AM
Al, some thoughts running around in my mind about this. Does the position of the bullet in the case neck at the times it enters the lands a factor in accuracy? For the 6PPC I've read that people use a 1.490 to 1.510 case length. I measured the chamber length on two of my 6 PPC barrels and IIRC the chamber length was 1.525. So if my brass is trimmed to 1.490 there is .035 of chamber where the bullet might veer, maybe. Cause at that point you now have space around the bullet for gas to flow as it is no longer supported by the case. It's probably as important as the number of angels that can dance on the head of a pin. But, I'm still curious as to why a bullet that is already seated off the lands can shoot noticeably better when seated significantly more off the lands.


Hokayyyy, this is gonna' pi$$ some folks off, and I doubt I'll take my thoughts very far as that's just asking for the jackals to swarm in.... but your timing of this particular question is freaky so I will share :) --- I JUST ORDERED TWO IDENTICAL 6PPC REAMERS, ONE AT 1.500 AND ONE AT 1.550!!!! Both with with my currently f'rav'ritest .268 nd

So that makes at least TWO of us interested in this subject.

I call that 35thou you've noted above "gapspace"..... and I have spent time exploring this "gapspace" via my collection of 6BR reamers, all different..... but this is the first time I've actually ordered two otherwise identical reamers, but one of them 50thou longer than the other.

I don't have a way to answer your actual question above, " Does the position of the bullet in the case neck at the times it enters the lands a factor in accuracy?" because I don't understand what you're actually asking but YES, I explore "position of the bullet in the caseneck" a lot.

Do I have any "answers" to post here?

Absolutely NOT

LOL

alinwa
08-22-2019, 03:41 AM
I will say this.... my findings agree with Pete's. Without even entering the argument of "where" or "how much" I will opine quite strongly that THE SAME EVERY TIME is crucial. And add that this is a moving target.

Most of my newer grinds are "cupped leade" or 3-2 angles ground into the leades, generally ogive-specific, and I'm currently completely cornfused as to which angle, or combination of angles is "better"

K Hope
08-23-2019, 12:53 AM
I would be grateful for some very smart and experienced shooters who could better explain the physics of Jump..

Lacking on both counts but do have a few theory's:

Pressure and bullet motion in relation to time and location:

Itís the primer output that drives a stress wave / powder compression through the powder to the bullet. A shower of hot particles and gases start the bullet into the engraving process largely by a passive medium push. With no ullage the powder acts similar to an imcompressible fluid transmitting the primer force to the bullet with no significant chamber pressure mid case forward. This initial bullet movement reduces the amount of brass / bullet surface contact and the amount of tension or grip.

After the primer push at some point between engravement, and obturation, gas pressure is in short supply with somewhat of a pressure plateau or static load; bullet movement lacking. Shortly, significant gases and pressure build in the case from the rear to the base of the bullet and then the same at the neck. Pressure builds from the rear expanding the neck as it moves forward until obturation.

The brass not only has to withstand high internal pressures but, it also undergoes deformations in order to seal the chamber; case material properties e.g. strength and ductility consistency maintain the required level of tension.

Initial bullet movement and Initial Burn Rate have to match every time. Any pressure changes through case volume uniformity (e.g. compressibility % and Temperature) or neck tension (e.g. sizing, maintaining and detaching) and when blow by starts (seating depth) make a big difference as to the peak pressure value. (time and position)

Burn Rates are about how long pressure stays high along with velocity but, Initial Burn Rates and Initial Bullet Motion (Impulsive Force) are more about where the bullet is located in relation to peak.

Not mentioning vibration..........

BR

Ken

Lou Murdica
08-23-2019, 11:44 AM
great reply KEN
just to add the difference between in the lands and out, adds about 4000 to 6000 PSI to your load and changes the spike in the ramp up
in the case.

retired
08-23-2019, 12:57 PM
quickload has a generic 7200 psi add for seating at the lands vs off.


great reply KEN
just to add the difference between in the lands and out, adds about 4000 to 6000 PSI to your load and changes the spike in the ramp up
in the case.

adamsgt
08-23-2019, 11:27 PM
Lacking on both counts but do have a few theory's:

Pressure and bullet motion in relation to time and location:

Itís the primer output that drives a stress wave / powder compression through the powder to the bullet. A shower of hot particles and gases start the bullet into the engraving process largely by a passive medium push. With no ullage the powder acts similar to an imcompressible fluid transmitting the primer force to the bullet with no significant chamber pressure mid case forward. This initial bullet movement reduces the amount of brass / bullet surface contact and the amount of tension or grip.

After the primer push at some point between engravement, and obturation, gas pressure is in short supply with somewhat of a pressure plateau or static load; bullet movement lacking. Shortly, significant gases and pressure build in the case from the rear to the base of the bullet and then the same at the neck. Pressure builds from the rear expanding the neck as it moves forward until obturation.

The brass not only has to withstand high internal pressures but, it also undergoes deformations in order to seal the chamber; case material properties e.g. strength and ductility consistency maintain the required level of tension.

Initial bullet movement and Initial Burn Rate have to match every time. Any pressure changes through case volume uniformity (e.g. compressibility % and Temperature) or neck tension (e.g. sizing, maintaining and detaching) and when blow by starts (seating depth) make a big difference as to the peak pressure value. (time and position)

Burn Rates are about how long pressure stays high along with velocity but, Initial Burn Rates and Initial Bullet Motion (Impulsive Force) are more about where the bullet is located in relation to peak.

Not mentioning vibration..........

BR

Ken

Wonder if the Pressure Trace II system would provide detailed enough traces to show the difference between seating depths. How would you describe what happens if the bullet is seated with a hard jam?

K Hope
08-24-2019, 08:41 AM
How would you describe what happens if the bullet is seated with a hard jam?

I do not believe there is any one, fit all answer to your question and is dependent on the specific cartridge. Eg Compressed loads versus ullage, bullet design and distance of travel to obturation.

I defer to others more qualified to answer your question than I am.

I shot a bit with a couple of brothers from VT. Two top notch shooters; one was a strong believer in hard jam and spent a lot of time with his dies and in setting his loaded rounds straight. The other brother was a strong believer that more powder was the answer when he lost his tune. I tried both these methods but never had much luck. That being said these guys had years of experience and were using select bullets and blended powders. It certainly worked for them but for me it was like living on the edge.

BR

Ken

retired
08-24-2019, 04:25 PM
ullage
why do you insist on using this tern in the metallic cartridge world ?

meaning: the space in a closed container of liquid that is not filled by the liquid:
from the liguid world, as in cask filling.

we have always used % full

K Hope
08-24-2019, 08:09 PM
ullage
why do you insist on using this tern in the metallic cartridge world ?

meaning: the space in a closed container of liquid that is not filled by the liquid:
from the liguid world, as in cask filling.

we have always used % full

Ullage is a common term used in IB best describing free volume; it facilitates visualizing powder orientation within case that can produce changes in pressure and velocity.
Check it out on Google; here is an example:

https://ndiastorage.blob.core.usgovcloudapi.net/ndia/2017/armament/Lusto19293.pdf

BR

Ken

retired
08-24-2019, 09:33 PM
please try again.
the term was so uncommon/unknown it had to be defined at the very start of the article.
that does not sound like "common" to me.


Ullage is a common term used in IB best describing free volume; it facilitates visualizing powder orientation within case that can produce changes in pressure and velocity.
Check it out on Google; here is an example:

https://ndiastorage.blob.core.usgovcloudapi.net/ndia/2017/armament/Lusto19293.pdf

BR

Ken

alinwa
08-25-2019, 01:56 AM
I do not believe there is any one, fit all answer to your question and is dependent on the specific cartridge. Eg Compressed loads versus ullage, bullet design and distance of travel to obturation.

I defer to others more qualified to answer your question than I am.

I shot a bit with a couple of brothers from VT. Two top notch shooters; one was a strong believer in hard jam and spent a lot of time with his dies and in setting his loaded rounds straight. The other brother was a strong believer that more powder was the answer when he lost his tune. I tried both these methods but never had much luck. That being said these guys had years of experience and were using select bullets and blended powders. It certainly worked for them but for me it was like living on the edge.

BR

Ken

LOL!!

Both "strong believer" and "years of experience between them" could qualify as the understatement of the year ;)

And may I add that the elder brother..... the one who liked to tip the bottle to regain his groove, was #2 in Group BR HOF for a loooong time..... and simultaneously in the Top 10 for Score.....

alinwa
08-25-2019, 01:59 AM
BTW I'ma get with retired on this one...... thems some perty fancy verbiages to say "the bullet starts and stops some, and we want it to do it the same each time"..... and I LIKE verbosity!

:)

LOL

K Hope
08-25-2019, 02:30 AM
BTW I'ma get with retired on this one...... thems some perty fancy verbiages to say "the bullet starts and stops some, and we want it to do it the same each time"..... and I LIKE verbosity!

:)

LOL

I am still recovering from jet lag and a 12 hour time change.......... I will try and do better next time.......

BR

Ken

alinwa
08-25-2019, 08:17 PM
I am still recovering from jet lag and a 12 hour time change.......... I will try and do better next time.......

BR

Ken




LOL


please, allow me to comment without taking it as preaching :) you have the right to your way, I have the right to mine. It'd be pretty boring if we were all the same!

That said, I'm a wordy guy and I still thought it was a liddle high falutin'........it's nice when everyone in the room can understand what's being said.


I was in a pre-con meeting a couple days ago, Concrete, Excavation and Electrical Contractor, and this electrical engineer leads with "OK, since you deem this project time sensitive let me broach the salient subject, are you vendor agnostic? Open sourcing will significantly alleviate funneling and . . . . ."

At this point the GC raised his hand and asked "If you mean "can we buy or supply our stuff as we see best" then YES! That's what we hired you for....next question?"

Al Nyhus
08-26-2019, 09:31 AM
Not mentioning vibration..........Ken

Excellent hypothesis, Ken. Thanks for taking the time to post.

Good shootin' :) -Al

Pete Wass
08-26-2019, 11:40 AM
please try again.
the term was so uncommon/unknown it had to be defined at the very start of the article.
that does not sound like "common" to me.

education level I'd guess. I've always loved language and words that exactly describe what one wishes to say so I have done my best to learn every word and it's meaning someone was kind enough to offer me.

Pete

alinwa
08-26-2019, 01:15 PM
BTW, how many of you believe the powder "acts as an incompressible fluid?"

It takes around 600 actual pounds to start the bullet, ie if you push it with a stick it takes a 600lb weight on the stick for it to engrave the bullet.

If you push on the POWDER with a stick, does 600lb get transmitted forward? or does the powder compress/crumble some?

IMO the powder simply decreases the gas space and the primer pressure goes thru it as though it were porous.

FBecigneul
08-26-2019, 01:47 PM
I lost my head; I almost responded to A.

retired
08-26-2019, 02:26 PM
i agree,
but look at the meaning:

meaning: the space in a closed container of liquid that is not filled by the :
from the liquid world, as in cask filling.

LIQUID

it is being MIS-USED....


education level I'd guess. I've always loved language and words that exactly describe what one wishes to say so I have done my best to learn every word and it's meaning someone was kind enough to offer me.

Pete

alinwa
08-26-2019, 04:52 PM
I lost my head; I almost responded to A.

now that would be embarrassing......

A.

alinwa
08-26-2019, 05:10 PM
now might be the time to remind everyone, 'Forum'.... a forum is not a place for folks of like mind to gather and reinforce their collective views and ideas. A forum THRIVES on disagreement...... disagreement, argument and open discussion even passionate discussion are what makes a forum a FORUM as opposed to a church, a school or a group of fuddy duddy's striving to preserve a way of life past.

Definition of forum

fo∑rum
/ˈfŰrəm/
plural forums also fora\ ˈfȯr-​ə \
noun
1.
a place, meeting, or medium where ideas and views on a particular issue can be exchanged.
1a : the marketplace or public place of an ancient Roman city forming the center of judicial and public business
b : a public meeting place for open discussion
The club provides a forum for people interested in local history.
c : a medium (such as a newspaper or online service) of open discussion or expression of ideas
2 : a judicial body or assembly : COURT
3a : a public meeting or lecture involving audience discussion
The town has scheduled a public forum to discuss the proposal.
b : a program (as on radio or television) involving discussion of a problem usually by several authorities


Maybe it is best if those folks who're bothered by differing views just put each other on ig'noor after all! :)


I'll not ignore anyone though as I find ALL VIEWS to be fascinating.

alinwa
08-26-2019, 05:24 PM
i agree,
but look at the meaning:

meaning: the space in a closed container of liquid that is not filled by the :
from the liquid world, as in cask filling.

LIQUID

it is being MIS-USED....

I appreciate the difference and agree...... and use 'appreciate' appropriately ;) ..... also appreciate "like a liquid" so took it another way, laffingly

BTW I also very nearly laffed aloud to hear "vendor agnostic" in a meeting of grubby, mud-booted, unshaven ruffians the other day.... just how I is, gotta' find the humor when I can (which is OFTEN!)

keeps me healthy, so's I can still live on fried foods

Al Nyhus
08-26-2019, 08:08 PM
ullage
Why(pic) do you insist on using this term (sic) in the metallic cartridge world ?

Meaning (pic): the space in a closed container of liquid that is not filled by the liquid:
from the liquid (sic) world, as in cask filling.

We(pic) have always used % full

In the fluid world, head space is often used in place of ullage.

Term swapping knows no bounds...... -Al (notinwa)

FBecigneul
08-26-2019, 08:18 PM
Ullage is angelís breath, AlinSoDak.

John Kielly
08-26-2019, 08:29 PM
Back in my years in the retail fruit industry, ullage referred to the pieces of fruit that disappeared from the cartons during transportation.

FBecigneul
08-26-2019, 10:05 PM
I used to see a lot of ullage on the sides of the roads in and around ďThe Tomato Capital Of CanadaĒ, Leamington, Ontario and then there is always a lot of ullage on the sides of the roads in lemon season in Florida. Today thereís the ullage around Jeddo, Mi and that would be sugar beets. Let me tell you that you donít want that ullage coming through your windshield. Nobody tailgates a sugar beet truck.

zfastmalibu
08-26-2019, 11:16 PM
I have seen on many occasions where a shooter forgot powder and the primer did not push the bullet out of the neck. I have done it myself. My view on seating depth is that it acts similar to adjusting a tuner. In my opinion it varies the "effective" barrel length and related harmonics. Why certain combinations of components would need either a running start (jump) or a jam, thats more to do with the components preference for a steep or gradual pressure curve IMO.

Pete Wass
08-26-2019, 11:43 PM
I lost my head; I almost responded to A.

:)

Pete Wass
08-26-2019, 11:44 PM
I used to see a lot of ullage on the sides of the roads in and around ďThe Tomato Capital Of CanadaĒ, Leamington, Ontario and then there is always a lot of ullage on the sides of the roads in lemon season in Florida. Today thereís the ullage around Jeddo, Mi and that would be sugar beets. Let me tell you that you donít want that ullage coming through your windshield. Nobody tailgates a sugar beet truck.

after my last divorce :(

Pete

alinwa
08-27-2019, 04:57 AM
after my last divorce :(

Pete

language is fun

and that language is funny! :)

ouch though :(