Seating depth consistency questions

B

Boyd Allen

Active member
Looking at a 6PPC, full house, top of the line sporter (10 1/2# short range Benchrest rifle, designed for registered competition) how much can actual seating depth variance (back of case to where the bullet contacts the rifling) would it take to degrade agging ability? Have you actually measured this potential variable? I am looking for actual experience.
 
Boyd,

Anything over.002 can degrade accuracy. The problems with bullet seating inconsistancy are (maybe not all):

1. Varying amounts of pressure to seat bullet can cause variances.
2. Bullet tip bottoming on seating stem.
3. Varying lube/pressure on bullet during pointing process in bullet making.

One thing I have found that makes bullet seating more accurate is matching a stem to the bullet being used. I do this by mating the the stem further down on the bullet. This seems to give more consistant seating, however, it does make the bullet harder to seperate from the seating stem by a little.

Hovis
 
Boyd-
One experience I had with the importance of seating depth came in my first year of competition. I was at Midland for the Buffalo. My rifle had been shooting great but on Sunday morning I couldn't find the recipe and came in 22nd place with vertical being the culprit. At lunch I realized that I had 300 rounds on the barrel since the last time I had checked the depth of the lands.

I tried seating the bullets .010 further out and the vertical disappeared and I came in 3rd in the afternoon. It was a great feeling to have thought my way out of a problem and illustrated to me how important are the little things.
 
Boyd,

I think the problem with this kind of question is any replies are anecdotal. It would take multiple tests with multiple barrels & bullets even to come up with a good generalization.

Both the other responders seem to say "it is critical." Here is one from the other side. A few years back, I got a new barrel for my 1,000 yard Heavy Gun. 1K HG's are close to point-blank unlimited rifles -- 85 pounds, shot off dual pedestals. BTW, even though it is a 1,000 yard rifle, I test at 100 yards. Anyway, to shorten this up, it shot the same size groups with a .010 jam, .010 off, and.020 off.

What I don't know is if it shot to the same POI with different seating depths, because I moved over on the paper for the each change. There was no difference visually -- but you wouldn't see a .1 change in vertical POI on the plain paper I use (aiming point is 24 inches below where the bullets hit).

It would take some testing, preferably with several rail guns, even to begin to get data we could count on. Maybe Jackie could give us a start when he tests his new 96-pounds of 8208?

FWIW, Charles
 
Thanks guys,
Hovis,
You are on the track that I am looking for. Thanks for the info.
Ray,
I have seen the same thing. In those cases moving the bullet out .003 cured the problem.
Charles,
Unfortunately, we live in an anecdotal world, but if you get enough of that together and you can start to draw some conclusions, however unscientific that may be. On that 1,000 yd. rifle, what is the barrel contour, and how is it supported?
 
Boyd, just to prove to yourself how seating location effects accuracy, take a target like the Skip Otto that has 6 mothballs in a row. Start at light jam, where the marks are about 1/4 as tall as they are wide. Then go in 0.003" for each of 3 3-shot groups, then out 0.003" for each of 3 3-shot groups.

What you will find is that each group will be noticeably different in size from the last. Generally bigger, smaller, bigger, smaller, etc.

I do this all the time with new powders or bullets.
 
Any of you guys ever measure a batch of loaded ammo, from ogive to head of case, to see how consistent your bullets are seated? Does your ogive length caliper attachment hit about where the rifling marks the bullet?
 
Boyd Allen said:
Any of you guys ever measure a batch of loaded ammo, from ogive to head of case, to see how consistent your bullets are seated? Does your ogive length caliper attachment hit about where the rifling marks the bullet?
Click to expand...

I use what used to be a Whitetail tool, then it was called a Stony Point, now a Hornady Lock-n-Load. I made a bushing that is exactly half way between the lands diameter and the groove diameter.

One other thing you will find out with this tool, if you use too much neck tension you will get variations in seating depth due to the shoulder flexing.
 
Boyd,
As was pointed out to me last year by friend and gunsmith Bob Kingsbury,It is not at all uncommon for a seating stem and top assembly to be"out". If you use a concentricity gauge to check runout,indexing the seater top around the clock you will ( with a top and stem that are "out")find runout get better or worse depending on clock position.An index mark at the best position is the cheap fix.Countless hours truing up your die set-up is the best fix and to quote Bob "that's why it's always been a gunsmiths game".Seating depth variations can be caused by this same unchecked rotation of the "out" seater stem/top assembly as well as by other variations that Hovis,and Jerry mentioned.
I am not a gunsmith ,I'm just a shooter, so my solution to the problem is a lot less refined. Find a competitive seating depth that is not so damned critical !
Different guys set up their guns differently.Some guys use loose neck tension, seat the bullet .010 past jam and let the chamber finish the job,some guys use heavy neck tension,hard jam and all the 133 they can get in a case, some guys use moderate neck tension,make a square mark and play with powder,etc,etc,etc. I prefer moderate neck tension,moderate powder charge and will start at jam and work my way back to jump and beyond in .005 increments. As Jerry was talking about you'll see the groups get bigger and smaller. If you look a little deeper you might see( all barrels are different) two or three settings that perform just as well as each other,right next to each other,that's what I'm lookin for( maybe .010,.015 and .020 off jam ?)take the.015 setting and put the powder to it,now take some powder away. You might find that.015 shoots 52,53 and 54 of 133 all real tight and all in the same hole, now stretch the seating depth longer and shorter in smaller increments at the different powder charges,does the gun still drill ?, at what point does it come apart? If you take a "middle ground" tuned rifle to the line it will be less sensitive to changes in environment and performance. If you need to make adjustments you'll rarely be lost. You've already put the gun through it's paces during initial set-up and evaluation.
If your barrel only has one spot that shoots( let's say .005 off jam and 55cl of 133)and at 54 or 56 it justs globs,or at jam or .010 off it just spits.Well,that's not much of a tune.And sooner or later it's gonna come apart on ya and you're gonna be lost.
I know you've been at this awhile and probably know this stuff,so my response is really for the benefit of the new guys that think there is only one way to tune,cause that's what "so and so" told them about 10 years ago.
Joel
 
From my experience, seating depth is extremely critical. I've also found that measuring jam must be done more often than most people think. I've had four good shooting Kreiger barrels in a row, and with at least two of the four the jam moved BACKWARD after a few hundred rounds! I actually had to seat the bullets deeper. In both cases the relative offset from jam I originally found was still the best depth for accuracy.

I suspect when I get around to measuring the other two barrels again I will find they have changed as well.

I shoot a .257 bushing, but I measure jam with a .258 bushing. I've found using less tension is better to get a consistent measurement.
 
Jam moving backwards

Steve,
That's pretty much impossible, isn't it ? Unless of course you made measurements, took off a barrel and screwed it on again tighter ? Or there might be some inconsistancies in your measuring procedure?Different neck tension? Clean chamber one time, dirty chamber the next?
Joel
 
Nader said:
Steve,
That's pretty much impossible, isn't it ? Unless of course you made measurements, took off a barrel and screwed it on again tighter ? Or there might be some inconsistancies in your measuring procedure?Different neck tension? Clean chamber one time, dirty chamber the next?
Joel
Click to expand...
If the throat is erroding, which it does with every shot, the seating die stem must move backwards (shorter) to maintain the same jam.
 
Jerry,
maybe I read it wrong ? but it shure looks like Steve is saying that his jam length is getting shorter( I am assuming that backwards means towards the shooter and forwards means towards the muzzle) as the barrel erodes and he is seating bullets deeper into the case( shorter o.a.l.) to compensate for it.That is impossible !
Hopefully he will clear this up at somepoint.
Joel
 
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It looks like this winter's topics are going to be the importance of measuring powder charges to .002 grains, and seating depth to +/- .0005. I guess that's better than a rehash of density altitude . . .

Some things to consider: The first is not proven, just theory. Powder charge, seating depth and neck tension all interact. Probably primer, too. They all have an effect on ignition and the pressure curve. Change one, and the optimum value of the others may change.

What does this mean practically? If you go out early in the morning & do your testing, you may well get different answers than varying match conditions dictate. Well, you say, I'll just hold seating depth and neck tension constant, and vary the powder charge for tuning. Be interesting to try a controlled study & see what happens.

The second thing is, are you good enough to trust one or two 3-shot groups to give definitive answers? I use to think I was. Less sure now. Yes, I can go out & shoot the three shot groups & get answers. But how good are they?

I remember early on determining that "just touching" gave me the smallest groups. It also gave me the most inconsistent groups, I think, based on a years worth of competition. But that's not proof, I probably learned other things during that year. Still, to this day, that's one seating position I never test; I believe it may give me the smallest groups on one day, but not the next.

If you are a new shooter, you can wear out a barrel determining the "optimum" settings. Now start over with a new barrel. Hmm.

These things do matter. Best answer is to find forgiving settings and concentrate on the wind flags, esp. if you are a new shooter. If you're experienced, do the testing over some time. Share if you're willing, it would be interesting.

Charles
 
jam is dependant on two things. Burn and wear in the throat and neck
tension when checking jam. Assuming you have not changed bullets,or
the lot. Given a new chamber, the intersection of the top of the lands
and the lead angle could be called sharp. That sharpness rounds off
in very few rounds. In a 6.5 x 284 , I have seen this to happen in as little
as 15 rounds, with a bore scope. The burn is not as noticeable, but in
reallity is the vaporizing of the steel surface. None of these items
add to the surface dimension causing the bullet to be pushed back.
Checking jam is on the other hand , incredibly dependant on bullet
friction in the neck of the case used. What often does occur is the new
throat being sharper , causes the bullet to bite a bit deeper and then pulls
the bullet slightly when the case is removed. Truely consistant numbers
in thousands are rare.
 
Charles E said:
It looks like this winter's topics are going to be the importance of measuring powder charges to .002 grains, and seating depth to +/- .0005. I guess that's better than a rehash of density altitude . . .


The second thing is, are you good enough to trust one or two 3-shot groups to give definitive answers? I use to think I was. Less sure now. Yes, I can go out & shoot the three shot groups & get answers. But how good are they?

I
If you are a new shooter, you can wear out a barrel determining the "optimum" settings. Now start over with a new barrel. Hmm.

These things do matter. Best answer is to find forgiving settings and concentrate on the wind flags, esp. if you are a new shooter. If you're experienced, do the testing over some time. Share if you're willing, it would be interesting.

Charles
Click to expand...
Charles, what we are doing is to try to confuse the new shooters. That will make 2010 much easier on some of us.

The 3-shot group thing, to clarify what Charles means is just a starting place. That, or those, best 3-shot groups must be verified by several repeats. I believe the 3-shot group, while not projecting an agg is the holy grail of starting points of tuning.

Remember, 3-shot groups indicate accuracy of the setup and tune, 5-shot groups indicate the ability of the shooter, aggregates indicate winners of the shoots.

Joel, you gotta' understand Steve. He's like the contrary Indian. He's the guy who tried to separate a throng of admirers from admirers of thongs when Tommy McKee broke the egg at Hickory last year!!
 
Jerry,
I have always had trouble with the southern dialect.This year was my first time at the Shamrock,I was loading in the open pavillion.Guys would walk up and talk to me and I could barely understand anyone,so I would just smile and nod until they walked away.During breakfast at the waffle house Sunday morning a rather pleasant waitress inquired of me " want some mo coffee baby", figuring this was my chance to fit in with the locals I responded " yes maam,baby want mo coffee". Judging by the way she glared at me while pouring 20oz of coffee into my 10 oz cup,this was not the correct response. I do learn quickly though,my response to "Baby Want Some Mo Cream" was simply " no maam"
Joel
 
Nader said:
Jerry,
I have always had trouble with the southern dialect.This year was my first time at the Shamrock,I was loading in the open pavillion.Guys would walk up and talk to me and I could barely understand anyone,so I would just smile and nod until they walked away.During breakfast at the waffle house Sunday morning a rather pleasant waitress inquired of me " want some mo coffee baby", figuring this was my chance to fit in with the locals I responded " yes maam,baby want mo coffee". Judging by the way she glared at me while pouring 20oz of coffee into my 10 oz cup,this was not the correct response. I do learn quickly though,my response to "Baby Want Some Mo Cream" was simply " no maam"
Joel
Click to expand...
Now Joel, I done gone an' tole' you to get a haircut and she won't call you baby no mo'.

She call you sugga' next time!!
 
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