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mag17

OAL or depth of projectile in case?

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Posted (edited)

I have been reloading for amount 8-10 years and I have also used OAL per manual for regular ammo (for my open ammo, I usual follow you guys on forum as starter and see what works in my pistols) but recently, one of the guys that I know and respect told me he measures the depth of the projectile in the case and uses that are guide - INTERESTING..... It made sense to me when he said it....

 

So question to group, OAL or projectile depth in case?

 

I did notice that same projectiles from same company, different batch numbers, the round nose profile can vary and gives me SLIGHTLY different OAL - all plated RN....

Edited by mag17

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Posted (edited)

Most of us load roughly as long as the weapons in choice will let us get away with. Consequnetly, we find the maximum OAL the gun will chamber via the plunk & spin test and then back off around .005” 

 

This length is different with every brand & weight of bullet, because the ogive (shoulder profile) of the bullet matters here. Not it’s tip.

 

From there you work your load up to obtain the desired power factor, then play with length and charge weight to dial in maximum accuracy with that firearm.

 

If I shot 9mm Open, I might be interested in this concept because they’re into +P+ pressure category, but the rest of us loading minor 9 are well within SAAMI specs for pressure and it really isn’t needed.

 

 

Edited by MemphisMechanic

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Posted (edited)

Make absolute sense what you are saying MM... Issue is that with my open ammo, I checked few rounds with projectiles from SAME COMPANY but different batches and the OAL was slightly different.... Took out the caliper and found profile of the RN was slightly different which would account for slight changes in OAL (using dillon seating die)... I am now just paranoid that I get some malformed batch of projectile and use them for my open 9mm major and results will be BAD!!!!!!

 

So I was just wondering how many open folks double check projectile depth?

Edited by mag17

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Curious to see what others say...

 

But I’d wager that most of us just pour them directly into the bulletfeeder from the box. :D 

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1 hour ago, mag17 said:

 how many open folks double check projectile depth?

 

Not directly.   No measurements involved.

 

But, I am concerned with how much powder there is and how high

it is in the case before I seat the bullet.

 

I've actually changed powders, to a slightly faster powder, so that

there is more room in the case for the bullet.

 

When I started out loading Major (about a decade ago), I had problems

with powder slinging and bullets not being held in place firmly in the

case, all due to the height of the HS6 powder I was using.

 

I changed to WAC so I could get more room in the case - again, not

a direct measurement of the bullet depth, but a desire to seat the

bullet a little deeper (same concept, but no direct measurements).    :) 

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For me it's pretty much OAL and will plunk, spin and work in my mags.  Though I do keep an eye on seating depth as it relates to the powder charge for the same reasons that Jack changed powders.  Currently my seating depth with a 124gr JHP is about flush on top of 10.0 grains of Shooters World Major Pistol.  That's just an eye test, I've done no measurements, and I've not experiences any bullet pushback from powder.

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For my 9mm major, 1.160 works great in all of the pistols and everything is case gauged that comes off the presses....

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When you say " slightly different " what are you talking about(+ - 0.003",+ - 0.005, + - 0.010' or + - 0.020")?

Edited by CZ85Combat

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On 1/9/2019 at 2:09 PM, clw42 said:

For me it's pretty much OAL and will plunk, spin and work in my mags. 

^^^

This. 

 

If loaded ammo works and meets PF why worry about seating depth? Assuming you’re not near or at max.

 

Per the CBC brass thread, depth does matter which makes  bullet selection important. 

 

 

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As others have said, you should start with every new bullet by determing your max OAL without rifling engagement upon chambering.

Regarding seating depth:

There are many factors that affect peak pressure at a given charge weight, which is what we try to control from a safety standpoint.  Among those factors are bullet weight, bullet diameter, surface area of the shank/bearing surface, friction coefficient between the bullet's surface metal and the barrel, hardness of the bullet surface in terms of how well it obturates (fills in the rifling grooves) and contains gas (and thus pressure), AND the initial size of the combustion chamber, which is a result of how deeply the bullet base is seated into the case (aka seating depth). 

 

We don't measure seating depth directly, however.  We calculate it with the measurable OAL, where seating depth is case length + bullet length - OAL.  Since the remaining volume leftover in the case to hold the powder is measured between the bullet base and the bottom of the case, it isn't necessary to measure every case length individually.  It's best to just have a single measurement per caliber so that you are comparing apples to apples from one cartridge to the next.  For 9mm, .750 is going to be within a few thousandths of every case, and it's easy to do math with.

If you have two bullets of the same/similar surface metal and of the same diameter and same weight, BUT different bullet lengths, you are going to get velocity closer by matching seating depth than by matching OAL.  When bullet lengths are different, the same OAL will produce different seating depths.  When you are looking at load data, and the load data is with a different bullet than the one you are actually using, just using the OAL in the load data by default may in fact produce significantly different seating depths and thus different results.

By the same token, you will have new reloaders start with FMJ or plated RN, loading out to 1.160, then they load their first JHP or FP, find out they need to load down to 1.080, and they will react to that every time -- "OMG!  That is so short!"  It can make people nervous.  But in reality, the seating depths may be much closer than the .080 difference in OAL.  They may only be .020, .030 difference, which is less dramatic.  In these cases, the difference in OAL is the distance from the bullet shoulder to the nose, which has no effect on internal ballistics.


With a pistol and powder I am familiar with, I can use seating depth to predict with reasonable accuracy what velocities I will get at what charge weights by comparing seating depths to my own load data I have already produced with other bullets of the same type and weight.

And to be clear, none of this is NECESSARY.  It's just another data point and another way to control your load development.  In all but extreme cases, if you are using load data for a bullet of the same weight AND type, using the published load data's starting load protects from dangerously high peak pressure, so long as you're respecting the max velocity of the published load data as the ceiling.

Hope this helps.

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I'm not loading major, but have been loading since 1972, and I've measured the OAL of quite a few cartridges in that span of years.

 

I can tell you without any doubt that the ogive of even best-quality bullets varies enough to change OAL by up to +/- 0.005.

 

I'm currently loading 147-gr 9mm and set up my seating die for nominal OAL of 1.160... I get a few at 1.165 / 1.167 but all pass my gauge test, plunk/spin test and sig-P320 Mags.

Most importantly, no malfs of any kind,

 

Capt_C

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9mm, 38 Super, 40 auto's feed best with OAL on the longer end of the spectrum. If there is published load data I always load to the max published OAL and check mag fit and plunk test and never look back. If no published data on a bullet load long with careful consideration it fits within the confines of your mags, has enough bullet seated into the case to be held concentrically and not easily separate during firing, and passes the plunk test in the intended guns chamber. Then test fire for function, adjust OAL if feeding issues occur. 

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