Bullet seating depth measurments and what I have learned

[Linko]

I am learning from what I read on our posts. I have started late in life reloading so I am on a kind of crash course. Been at it now 2 years. long range is the main goal but I also am loading some Hornet and AR rounds.

I have read how some bullets are sensitive to seating depth some not so much. But all need to be set to some figure relative to the lands. So I am working on measuring the CBTO dimension vs the touching measurement from my stoney point measurements.

I have variance between the same bullets and the same setting on my whidden seating die (6BR).

I understand bullets from the same lot have a spread of measured base to ojive. hence the variance in seated measurements loaded. Think I have that right.

So is it correct to say seating depth is a range and after seating and measuring some additional adjustments of the seating depth would be necessary to get the same seating depth. or close to the same?

As for tools to measure since the ojive on every bullet type is different no one tool is going to have the perfect dimension to get measurements consistantly. correct? I am using the Hornady tool on a Caliper for my measurements.

The process is not exact round to round. I am close on my understanding?

 

[Tim Singleton]

You seem to have the jest of it.
All Bullets are not equal so the variation you see is more apparent in mass produced Bullets even if they say match or target on the box

The Stoney point tool used to find the lands is ok. All you need to do is establish a dimension to work from in finding the depth that works for that barrel.
I personally don't like the Stoney point tool as it doesn't replicate where the bullet will be when a round is chambered and the bolt closed.
IMO the proper way is to strip the firing pin mechanism from the bolt size a piece of brass seat a bullet long and close the bullet until it is pushed back this this the jam for that bullet reamer. Now you have a dimension you only work one way from deeper into the case

 

[mikecr]

I have read how some bullets are sensitive to seating depth some not so much. But all need to be set to some figure relative to the lands. So I am working on measuring the CBTO dimension vs the touching measurement from my stoney point measurements.

The only thing that matters about the bullet land relationship is that setting proven shooting best for you(it won't be the same as mine). That setting is logged as CBTO, which is actually relative CBTO(with your tool). The figure itself is pretty much meaningless, other than you need to reproduce it with each round you load.

I have variance between the same bullets and the same setting on my whidden seating die (6BR).

This could be a problem if your CBTO variance breaks out of tested best seating window. Usually the variance is do to inconsistent stem wedging, due to inconsistent seating forces. If your neck sizing leaves an interference greater than 1thou under cal, it's likely your seating forces are higher than normal, and so variance of this could be higher than normal.

I understand bullets from the same lot have a spread of measured base to ojive. hence the variance in seated measurements loaded.

BTO is meaningless to seating and CBTO.

 

[Ned Ludd]

Seating depth is largely dependent on the distance between where the seating die stem pushes down on the bullet ogive and where your caliper insert seats on the ogive when you measure seated bullets (diagram below). Bullet BTO dimensions are largely irrelevant to seating depth because the two contact points are outside where the sseating die tem pushes and where the caliper insert seats. To get the most uniform seating depth you need consistency in your bullets between the seating die stem and caliper insert contact points.

 

[Linko]

Seating depth is largely dependent on the distance between where the seating die stem pushes down on the bullet ogive and where your caliper insert seats on the ogive when you measure seated bullets (diagram below). Bullet BTO dimensions are largely irrelevant to seating depth because the two contact points are outside where the sseating die tem pushes and where the caliper insert seats. To get the most uniform seating depth you need consistency in your bullets between the seating die stem and caliper insert contact points.

Excellent description, and much better explanation of what i was trying to ask.

My question is how do I maintain consistancy between where the seating stem touches the bullet and the caliper insert seating point?

 

[Jet]

Bob Green's Custom Shop makes a tool that will fill your need. Link:

http://greensrifles.com/New_Products.html

I use one and it is an important part of my bullet sorting process.

Good Shooting

Rich

 

[Linko]

To have consistant measurements is a key to getting the critical seating stem and caliper contact point correct round to round.

besides the Bob Green tool there is the Redding Instant Indicator.

The redding looks to be a easily repeatable tool. is this a good choice?

 

[mikecr]

It might be repeatable if not for the slop of a press added to measure...
Repeatable for me is the good ole Sinclair 'nut' and caliper. I've also used a purpose built indicated stand with gizzy attached to the indicator(Similar to BSC, or Buhay).

I also use a BGC to qualify my bullet noses before proceeding with them. As mentioned, ogive radius variance is a contributor to problems in our measure.

 

[Shynloco]

I have read how some bullets are sensitive to seating depth some not so much.

Linko,
Its not the bullets...it's the rifle chamber. Each rifle is different and some like the jam while others like the jump. I have a variety of rifles that fall into each category...jam and jump. Do not make the mistake of discounting/ignoring a bullet based upon your initial statement as quoted above.

Alex

 

[Sheldon N]

So is it correct to say seating depth is a range and after seating and measuring some additional adjustments of the seating depth would be necessary to get the same seating depth. or close to the same?

I think a reasonable approach is to accept a "range" of seating depths when you are actually loading up rounds to shoot. I have a micrometer seating depth stem and I could conceivably measure every bullet and make fine tune adjustments so that each bullet measured exactly the same CBTO after the final seating. However, to me that falls into the category of being just a little too OCD. I measure the CBTO when I'm starting out to seat bullets and also maybe once every 10 or 20 rounds to double check that I'm still in the target zone. I will typically see variation of +/- 0.001 or 0.002 in seating depth as measured by CBTO.

I'm not sorting bullets by BTO and I'm not shooting benchrest so my approach may be different than others, but I do know from my seating depth tests that the variation of a thousandth or two is not going to make a difference on target.

 

[Slickrick0999]

Seating depth is largely dependent on the distance between where the seating die stem pushes down on the bullet ogive and where your caliper insert seats on the ogive when you measure seated bullets (diagram below). Bullet BTO dimensions are largely irrelevant to seating depth because the two contact points are outside where the sseating die tem pushes and where the caliper insert seats. To get the most uniform seating depth you need consistency in your bullets between the seating die stem and caliper insert contact points.

Thanks for this!
Could you possibly explain a double ogive bullet with a diagram like that? Sorry, I'm just not to informed on all this.

 

[Ned Ludd]

What do you mean by a "double ogive" bullet? Are you referring to Hybrids? If so, the difference is that near the bearing surface ogive junction, the Hybrid ogive changes from secant to tangent. A secant ogive allows for a longer, more streamlined nose and improved BC, at the expense of a sharper transition between the ogive and bearing surface, which may possibly narrow the optimal seating depth range. A tangent ogive provides a smoother transition from the ogive to the bearing surface, which typically means more forgiveness with regard to optimal seating depth. The Hybrids were designed to have the best of both worlds. As far as seating depth consistency, using a Hybrid won't really change anything from what is shown in the cartoon above. Due to the Hybrid ogive, the point at which the caliper insert seats on the bullet (tangent ogive region, blue arrows above) may be slightly different than where it would seat on a fully secant ogive bullet, but the distance between that point and the point where the seater die stem contacts the bullet ogive farther out remains the critical measurement.

As mentioned by several others, a tool such as Bob Green's comparator will allow you to sort bullets according to length variance within this region, whether it's a Hybrid, tangent, or secant ogive bullet.

 

[Linko]

I am looking for a tool more repeatable than the hornady insert to quickly measure my seating depth. I find the Hornady clumsy.

 

[Slickrick0999]

What do you mean by a "double ogive" bullet? Are you referring to Hybrids? If so, the difference is that near the bearing surface ogive junction, the Hybrid ogive changes from secant to tangent. A secant ogive allows for a longer, more streamlined nose and improved BC, at the expense of a sharper transition between the ogive and bearing surface, which may possibly narrow the optimal seating depth range. A tangent ogive provides a smoother transition from the ogive to the bearing surface, which typically means more forgiveness with regard to optimal seating depth. The Hybrids were designed to have the best of both worlds. As far as seating depth consistency, using a Hybrid won't really change anything from what is shown in the cartoon above. Due to the Hybrid ogive, the point at which the caliper insert seats on the bullet (tangent ogive region, blue arrows above) may be slightly different than where it would seat on a fully secant ogive bullet, but the distance between that point and the point where the seater die stem contacts the bullet ogive farther out remains the critical measurement.

As mentioned by several others, a tool such as Bob Green's comparator will allow you to sort bullets according to length variance within this region, whether it's a Hybrid, tangent, or secant ogive bullet.

Ok, the Barts bullets 68 grain dominator says it has a "double ogive 7/9" that is what I'm not understanding (or at least part of it)

 

[Ned Ludd]

I'm guessing this is probably analogous to Berger's "Hybrid" terminology, but a call to Bart's would give you the definitive answer.

Tangent ogive bullets are usually pretty forgiving with regard to seating depth, because the transition from ogive to bearing surface is very smooth. However, their ogive (nose) is not as "long and pointy" as a secant ogive bullet, which means they will experience higher drag, and therefore will have a lower BC than a comparable weight secant ogive bullet. Secant ogive bullets have a "longer and pointier" ogive, and therefore experience less drag and generally have higher BCs. Because the transition from ogive to bearing surface on a secant ogive bullet is much more abrupt than a tangent ogive bullet, this potentially comes at the cost of seating depth tolerance. In other words, the secant ogive bullets have the general reputation of being more finicky with regard to seating depth, largely due to the abruptness of the ogive/bearing surface transition (i.e. the approximate region where the bullet first encounters the lands). The images below depict extreme examples of tangent (such as a typical pistol bullet) and secant ogive bullets. It's pretty easy to see how different the region of the ogive at (or near) the bearing surface junction is and why that might affect seating depth tolerances in some cases. The Hybrids (and as a guess, the double ogive bullets), attempt to mitigate the abrupt secant ogive transition by altering the ogive shape in that region to one more similar to a tangent ogive bullet, while retaining the more aerodynamically efficient nose.

Again, the critical measurement in terms of seating depth remains between where the caliper insert and the seating die stem seat on the bullet. These types of alterations in bullet shape won't change that.

 

[Linko]

I am still (original post) looking for the best tool to measure seating depth of loaded rounds.

(Hornady comparator insert with caliper and then running the round in and out of die to get the depth exact)? this method is somewhat inexact as you twist the loaded round until a steady measurement if found. feel less than exact.

The Davidson comparator base has a recess for the cartridge base to seat , this looks a little more precise than the Hornady base if you are using the caliper comparator method to measeure seating depth. Anyone using this?

 

[snowpro440]

Linko have looked at the Bob Green caliper tool ? This might give you what your looking for . This matches the bullet ogive in the given caliber where as the model such as the hornady and Sinclair tool may not measure up to the correct diameters.
http://greensrifles.com/site/392f44e2d7f54b4fb1b48ea4d2d36157/default?url=http://greensrifles.com/New_Products.html#2784

 

[Linko]

Linko have looked at the Bob Green caliper tool ? This might give you what your looking for . This matches the bullet ogive in the given caliber where as the model such as the hornady and Sinclair tool may not measure up to the correct diameters.
http://greensrifles.com/site/392f44e2d7f54b4fb1b48ea4d2d36157/default?url=http://greensrifles.com/New_Products.html#2784

yes, I see that will perform the measurement and it lloks like the method for use will be more repeatable than the Hornady tool.

 

['Freak]

“The Davidson Seating Depth Checkers attach to the blade of your calipers with a small thumb screw and consists of a nose piece for the bullet and a base piece for the case head (sold separately). These two pieces together serve to align the cartridge for accurate measurements. The nose pieces can also be used without the base pieces in a similar manner to the other comparators. The nose pieces are machined with a taper for more accurate measurements.”

http://www.sinclairintl.com/reloading-equipment/measuring-tools/bullet-comparators/seating-depth-base-pieces-prod33946.aspx?cm_sp=Secondary+Banner-_-Reloading+Equipment-_-Shop_Reloading_Equipment

&&

http://www.sinclairintl.com/reloading-equipment/measuring-tools/bullet-comparators/seating-depth-checkers-prod33236.aspx?cm_sp=Secondary+Banner-_-Reloading+Equipment-_-Shop_Reloading_Equipment

 

[jsthntn247]

Sure wished Bob would make a comparator for .224 bullets.