Calculating True 9mm pistol Base-to-Ogive from measurement taken on Hornady Bullet Comparator

Calculating True 9mm pistol Base-to-Ogive from measurement taken on Hornady Bullet Comparator


WHY:

I thought I would start this thread for pistol shooters interested in determining true cartridge Base-to-Ogive (“BTO”), and subsequently the correct length of BTO to provide their targeted amount of “Jump” in their pistol, via their handloaded ammunition.

I am doing this because I realized recently that although ai had set my Dillon bullet seating die on my Dillon XL750 very carefully, I was nevertheless getting a variation of as much as .010” in OAL as measured from base of cartridge to tip of bullet. I don’t currently know if that variation is a result solely of typical varying meplat (“tip”) length on the bullet I am using, or if some of it is a true BTO variation.

Determining TRUE BTO is especially important on my current 9mm 115g load because I am using a load from the Vihtavouri load tables, that is moderately vigorous (1324 fps / 451 ft lb), and so I am particularly keen on ensuring that (1) the COAL does not ever slip below the minimum recommended for pressure safety by VV, and (2) that I maintain at least minimal “Jump” in my tightly chambered SIG P210A Target model pistol, again for pressure safety.

But, I also want consistent and optimal BTO to help ensure optimal ACCURACY.

Determining “True” BTO is a challenge on at least this specific 9mm pistol cartridge, because:

- I am using the highly regarded Hornady HAP 115g JHP .355” bullet, whose ogive above the shank is basically a truncated cone with a 15 degree slope, topped by a meplat (“tip”) that is a hollowpoint of fairly broad diameter. Either end of the reversible Dillon seating insert fails to bear on the 15 degree ogive. The insert instead bears on the broad hollowpoint tip, whose length varies (as does the tip length of even the very best bullets, pistol or rifle). Normally, bullet seating inserts are designed to bear on the ogive for this reason, since the BTO on quality bullets does NOT vary to any discernible extent. But the specific combination of Dillon insert and this HAP bullet does not achieve that goal of ogive versus tip contact.

and

- Although Hornady makes a Bullet Comparator Gage specifically designed to help a reloader determine BTO, it is aimed at rifle versus handgun loading, and so does NOT provide any gage insert designed to work with a .355” diameter bullet. The closest I can get is a .338” gage insert. The next size up is larger than the .355” of the HAP bullet.


OBJECTIVES:

I THINK I have determined how to use the Hornady gage to:

- Determine where my variation is actually occurring (i.e. tip only, or tip plus BTO)
- Determine true BTO
- Determine current “jump”
- Determine the acceptable BTO range (minimum to maximum) that keeps my peak pressures safe
- Be able to adjust jump accurately within that range to optimize the load

In the following text, I try to describe my planned process. I would appreciate it if any experienced handoader could critique it and correct me where i am wrong, or simply improve the process. Please do not suggest I go to separate single stage handloading, as I shoot too many pistol rounds per month to make that an attractive option. I want to use the Dillon XL750 progressive machine.


PROPOSED PLAN:

Here’s my first draft of my proposed process, starting with a diagram:




My proposed process is:

Insert the .338” gage insert into the Hornady Bullet Comparator

Secure the Comparator to the moving jaw of my digital caliper via the slot and bolt that Hornady provides

Close the caliper with the gage in it, and “zero” the caliper, so that it is now measuring the distance between the face of the .338” insert to the other jaw of the caliper as it zero default

I know that the slope of the “cone shaped” ogive on the HAP bullet is exactly 15 degrees (per Hornady Tech Support), and that the slope is constant

I know that I want to measure the BTO, from base of the case to the .355” diameter of the bullet right where the bullet’s shank ends and the bullet’s ogive begins. But I am instead measuring to the point where the 15 degree slope of the HAP bullet is already down to .338” diameter

The difference in DIAMETER between the desired true BTO point and the actual point measured is .355” minus .338” = .017”

Therefore the difference in RADIUS is .017” / 2 = .0085”

But the slope of the HAP ogive is a constant 15 degrees, so I can use trigonometry to determine the distance along the bullet AXIS that separates the 2 points

The formula is:
.0085” / (axial distance) = Tan of 15 degrees = .26795

Doing the math, axial distance = .0085” / .26795 = .0317”

So, to get my True BTO:

Place the completed cartridge into the caliper, with the tip of the bullet resting inside the .338” insert, and the bottom of the case resting against the other jaw of the caliper

This gives me the Base-to .338” point on the ogive

To get the current True BTO, subtract .0317” from this measurement

I can then determine current “Jump” by making a cartridge deliberately excessively long (for my SIG P210A Target, this means a COAL (base to TIP) of about 1.153” or more). Then doing an iterative “plunk & rotate test”, gradually reducing the length of the cartridge until it just passes the test. That is then the maximum cartridge length to prevent the ogive of the bullet form contacting the rifling when chambered

Then measure that cartridge’s base-to-.338” length in the caliper/gage setup described above, and determine True BTO for zero jump by subtracting .0317” (as described above)

Decide what increments of Jump I want to test

Adjust the seating die to make those incremental changes in Jump.

This incremental testing with differing Jump will be difficult to do with the Dillon die, because by bearing on the hollowtip tip versus the ogive, this die invites variation in true BTO because the hollowpoint can be relatively easily deformed just a bit by seating pressure, AND since each bullet tip (unlike its ogive) varies, I would have to keep using the exact same individual bullet and work my way downward to shorter BTO lengths, hoping that I never accidentally adjust the die just a bit too much!

Fortunately, a partial remedy is on the way. Another forum member has made a custom die insert for me for the Dillon die for a very reasonable cost, and that new insert correctly grabs the ogive, not the tip, of the HAP bullet (He tested it with an XTP bullet which he has, which is identical to the HAP except that it has cuts in its hollowpoint perimeter to promote terminal expansion. That die has been mailed to me from The U.S. and hopefully won’t be too delayed by the COVID-19 shipping slowdowns. It will be at least 10 to 14 days though I think.

This incremental testing with differing Jump will be MUCH easier yet for me to do once the Redding Competition Bullet Seating Die finally arrives (a long time, since it too is coming from US to Canada during COVID-19 transportation slowdowns). This is because that die has a micrometer adjustment on it, with marking increments of .001”, and requires 7.2 degrees of rotation to move each .001”. And, the settings are reproducible. I am looking forward to getting THAT entirely new die setup.

So, this is my plan.

Please feel free to point out any errors or omissions! I want to do this right!

Jim G

noylj said:

. . .

I assume you know breech to lede distance and lede angle?

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No, i do not know the breech to lede distance and lede angle. How can I determine them?

But do I really need them? My most important goal in determining the point at which the bullet contacts the lands is to ensure that I have a jump to that point, so that I know I am not adding to the peak pressure by having the bullet restrained by the rifling (or lede) before the bullet has moved, as that would increase the peak pressure. Once I know I have at least SOME jump, I can iteratively try increasing it to see if it makes a difference to the bullet muzzle fps or accuracy.

Jim G

243winxb said:

Is there any upward movement in this area? View attachment 981080 The early 550 could have as much as .010" slop. There was a after market kit sold to lock it.

If any die contacts the shell plate (550) the OAL may change at each station. It acts as a stop. This may happens if the deck height (think shell holder) is different at each station. .125"

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You raise good points. I had thought about both, but am not convinced I have addressed them well enough.

I was careful in setting up the dies to ensure that no die contacts the shellplate. And I can visibly not see any movement of the tool head. But, I know the fit of the tool head has to be at least SOMEWHAT loose, so that you can change die heads to do different calibers, and that obviously requires SOME clearance, although not a lot hopefully. I was recently made aware that at least one source sells "clamps" to lock the tool head.

Your warning about shell holder height suggests I should at least CHECK the height of my shell holders, to make sure they are reasonably identical. I have no idea what tolerance Dillon applies in their manufacture, but I have never before had any reason to check.

I am sure I had read somewhere too that one of the loading equipment companies make shell holders of different heights. I don't recall the reason for having those.

After decades of reloading, I have been out of it for 5 years or so, and am now just getting back in. I'm trying to remember what I have elarned in the past, but at my age, I'm sure there are things I've forgotten.

Jim G

Jim G

fxvr5 said:

And consider that only the results from a Ransom Rest would be an acceptable method, and would have to require a huge number of shots in a single group in order to ensure that the conditions (Dillon die v Redding die) would be different enough in size to account for normal differences found when shooting the exact same ammo as explained in the two SSUSA articles on how many shots per group that have been posted here before.

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It is true that I have no idea yet which of the many details I am examining will make a difference. If the combination of the firearm's inherent accuracy and my own skillsets was only good enough to shoot 2 or 3 inch groups, it would make no sense to try to find out because a 2 or 3 inch group size would hide minute differences.

But, in too many tests by respected reviewers, some using Randsom rests, and some just sandbags, the SIG P210A Target has proven it can shoot 5-shot groups at 25 yards that are small enough that even the average of 3 such groups is around 2/3". So, maybe I might see incremental improvements once I have the right load and have gotten used to shooting this pistol. Heck, why not at least TRY?

Jim G

higgite said:

Seems to me that you’re making a simple process way more complicated than it needs to be. Since the ogive angle is a constant 15 degrees as you say, then it matters not what the distance is between the .355” and the .338” points on the ogive. Once you establish the max BTO for that particular bullet in a particular barrel/chamber by plunk testing, you can measure that BTO with any size comparator insert that you choose and subtract the amount of jump that you want. That will give you your target BTO for your reloads when using the same insert on bullets with the same profile.

I personally use a comparator insert with a .344” ID for 9mm because that was the size hole left by an 11/32” drill bit when I made it. ymmv

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I did not mean to imply there is anything "magical" about the .338" size other than that it happens to be the largest size that is still smaller than the .355" bullet diameter. But I would not consider a hole of any size simply drilled at home by a drill as being precise enough in its diameter and surface quality to be a great substitute for a gage insert that I hope Hornady took greater pains to make correct! The inserts they sell are not costly. I simply bought the comparator with the 14-bit set because of availability issues during COVID-19! If I could have found the comparator body alone in stock anywhere, and the .338" insert as well, that would have saved me a bit of money (but not much), but I could not find that body + .338" insert in stock at the same vendor anywhere. Shipping cost for 2 shipments from 2 vendors versus 1 would have further eroded or killed any saving.

Jim G

Barbaroja said:

I’d also take a slice of walkalong’s hat. I really feel that your over complicating things and making a lot of work for yourself that will yield you no discernible difference. I’d also think 243winxb is on to something asking about the tool head. I feel like .010 difference in overall length could be achieved by a press not being bolted down tight enough or even as simple and not getting a full stoke on the handle

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I made sure when mounting this brand new XL750 press that I used Dillon's mounts, and that all the bolts at every point are tight. I am also very careful to be absolutely consistent with my handle stroke (to the extent that a human can be consistent!). But that toolhead clearance that enables sliding toolheads in or out fairly easily does make me wonder if Dillon has made it tight enough. Given their multi-decade experience, I would certainly hope so. I have also made sure that the shellplate is secured to the platform via exactly the process they describe with the instruction manual for setting up the press, which minimizes the clearance needed to allow the shellplate to rotate between stations.

Jim G

243winxb said:

. . .

I have to wonder if your 9mm bullets taper may allow it to seat into the rifling? This does require an exact BTO. But depends on the barrels chamber.

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I have not figured out any simple way to get the exact shape and dimensions of the leade on my pistol, and yes, I know that the ogive shape does affect at what point the bullet actually touches the rifling. I would THINK that the perfectly STRAIGHT ogive on the HAP, versus a more typical rounded ogive, SHOULD make it engage LATER than a rounded ogive. (Even though in a seating die, that hollowpoint head is wider than the head on a round head bullet and therefor can trigger the premature conatct with the seating die insert that I am currently experiencing).

But I guess the definitive answer is gotten only by determining the exact real point of contact using the plunk & rotate test.

Jim G

fxvr5 said:

The real test of an improvement in mechanical accuracy requires a Ransom Rest.

The best method for testing just the ammo is to use a barrel fixture - where the barrel is removed from the gun and placed in a special device.

To repeat, 5-shot groups mean nothing. Lots of shots in one group is the proven better method, and even that has limitations as noted in prior mentioned articles.

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Even if I could shoot "just the barrel", I wouldn't do so, because in real life I cannot shoot just a barrel.I want to see how good I can make the combination of the (complete) pistol and me.

I understand, and accept, the limitations of the 5-shot group methodology. I'm not prepared to shoot 500 shot groups, and the statistical improvements of shooting any less than 500 shots are not good enough to persuade me to expend either the ammo or the time. Besides, as i said before, shooting 5-shot groups is the current accepted comparative standard across the shooting community.

Jim G

Sirclintler1 said:

It seems your making calculations to .00000 and your new die only adjusts by .001 increments. I absolutely do not fully understand this, but it seems your chasing numbers that your/any reloading tooling can remedy. I also use the original 550, I also was getting a deviation in OAL by .001-.006. My fix was to measure the tool plate receiver on the mast of the press and the tool plate locating flanges, subtract, split, shim top and bottom. Or maybe I should just go to bed. I will stay tuned, I may learn new things.

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I am open to the concept of shimming or clamping the toolhead. I agree that if the toolhead has APPRECIABLE play, it could cause variations. But, I haven't yet taken any measurements to see if the play IS appreciable. I will follow up on this though! Thank-you for raising it.

edit: Forgot to address your point on number of decimal places. I know that the caliper cannot display decimals beyond 4, and that indeed its accuracy could be less accuarte than that implies, but I carry 4 decimals in the math calculations and then round to the actual precision that the measurement can actually be made to with the equipment and process available.

Jim G

Warpiece said:

The comparator inserts are chamfered. You clearly want actual dimensions, you'll need a way to compensate or eliminate that variable

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Really? They chamfered the edge?? Why on earth would they do that? A chamfer by definition will modify the results, unless it is perfectly matched to the specific ogive of the bullet being measured, and STILL maintains the correct gage reading at the outer edge of the gage. You would need a custom gage for each bulelt out there to make that work. Wouldn't you?

I can understand a chamfer on the OUTER diameter of the insert, to make it more comfortable to handle, nut never the measuring surface!

Are you SURE there is a chamfer??

Jim G

noylj said:

Really surprised if you can prove any improvement, but I'll be excited to read about it.
I've fired a p-210 and could get groups of 2-3" at 25 yards. Only fired a few groups, but the owner was not doing any better. I actually thought that was great for a 9x19. My cz-75, which I bought a few years later, can give 2-3" groups. Sometimes it will, non-repeatably, produce sub-1" groups. It is called statistics, randomness, and standard deviation.
I read a lot of accuracy stories in magazines, but have a hard time believing such unknown shooters can so easily shoot such small groups, but I am a skeptic. Data is so easy to fake.

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I share your skepticism when reading individual forum postings, but not magazines, because editors are there precisely to validate and confirm their contributors' claims. I can see an individual seeking glory via great apparent groups, but it is harder for me to imagine an editor supporting an unsubstantiated claim.

Here is the best target I shot so far during this load testing process, with my P210A:



Jim G

fxvr5 said:

You missed the point. There are only a couple of ways to truly test mechanical accuracy, which is what you are chasing with all your ogive seating depth efforts. 1) Ransom Rest. 2) Barrel fixture.

You shooting it off a bench will not test that. Shooting a rifle off a bench can work okay if the shooter is up to task. But it doesn't cut it with pistols.



This means nothing. If you're truly testing accuracy, you have to go beyond the norm to more strict methods.

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Sigh . . . I don't have the time, money, or equipment access, or even any indoor facility out of the winter weather, to do things anywhere close to ideally, so I am stuck with my limited environment, equipment, and skillsets. I'm just trying to improve my final results when shooting, by maximizing what I can to the degree I can. And, I am ENJOYING the process!

JimGnitecki said:

Really? They chamfered the edge?? Why on earth would they do that? A chamfer by definition will modify the results, unless it is perfectly matched to the specific ogive of the bullet being measured, and STILL maintains the correct gage reading at the outer edge of the gage. You would need a custom gage for each bulelt out there to make that work. Wouldn't you?

I can understand a chamfer on the OUTER diameter of the insert, to make it more comfortable to handle, nut never the measuring surface!

Are you SURE there is a chamfer??

Jim G

Click to expand...

Warpiece: I just thought of another possible explanation for a "chamfer" being present. IF the inserts are "chamfered", are they really merely "coned" internally, where the larger end of the cone is intended to face the comparator, NOT the cartridge? I can see that being done to ENSURE that it is only the actual EDGE of the insert that contacts the bullet ogive. The cone shaped hole in the insert would ensure that as it is widening in diameter as the bullet enters, whereas the bullet ogive is getting smaller.

Naturally such a setup would require the manufacturer to make it impossible to fit the insert to the comparator body "reversed" in direction, via flange or other means..

Jim G