Trimming 9mm case?

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Trimming 9mm case?

Post by jlow on 1/7/2019, 12:16 pm

I asked this on another more general thread but want to follow-up here specifically on it.

So the reason I ask is in an article on Accurateshooter.com titled “Precision Reloading for Handguns – Smart Tips from the USAMU”, there is advice that says the “importance of Uniform COAL”.  They also advice the use of consistent brass and the use of taper crimp.

Taper crimp only crimps the very end of the case and so to get a consistent crimp i.e. hold on the bullet, the thickness of the case wall (obtained using consistent brass) and length of the case (uniform COAL) would both be critical factors.

So why should this affect pistol precision?  Coming from precision rifle, there is a theory called OBT (optimal barrel time).  Basically it says that when you fire a round, the barrel rings like a bell.  The shock wave travels from the chamber where it originates to the crown of the barrel where it is reflected back, it then travels back and froth until it dissipates.

Why is this important?  It is important because when the shock wave is at the crown, there are minute movements in the metal and the crown can open and close very slightly.  So imagine a bullet exiting the barrel when this happens – it will affect the direction of travel of the bullet causing what we call a scatter group i.e. a wide group.  If on the other hand the shock wave is at the chamber i.e. as far away from the crown as possible, the crown would be quiet, and the bullet will exit without being disturbed.  This results in a tight or what we call an accuracy node.

So in rifles, it is actually possible to compute the amount of powder to hit the accuracy node with a program called QuickLoad which is affected by many things including the length of the barrel, bullet, case volume, seating depth of the bullet, etc.  (yes 9mm pistol reloading can also be determined) But in reality, we usually adjust the exit time of the bullet by changing powder weight and seating depth of the bullet.

So coming back to pistol, it of course is no different than a rifle, just shorter barrel.  One way to affect precision is to also adjust when the bullet exits the crown.  One can do this by adjusting crimp on the bullet because crimp can increase the hold of the case on the bullet, allowing more pressure to build and resulting in a faster MV.  Changing MV of course will change the time when the bullet exits the crown.

So sorry for my long windedness but this is why I am interested in talking to others who may trim their case and use a taper crimp.

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Re: Trimming 9mm case?

Post by Wobbley on 1/7/2019, 12:46 pm

Sort the cases by length.  Trimming is a big time waster.  Testing will determine the “tolerance” band.  It’s bigger than you think.  For the record, I don’t worry about case length, I just go shoot.  

https://www.mscdirect.com/product/details/00773051  For a fast way of measuring.  You might need to change the contact point to this.  https://www.mscdirect.com/product/details/35916113
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Re: Trimming 9mm case?

Post by jlow on 1/7/2019, 1:49 pm

Thanks for your input.  Coming from precision rifle, I of course have lots of measuring instruments like calipers and micrometers, but at least using headstamp sorted brass, there is significant variation after full length sizing - close to 10 thousands.  If one is limited to sorting and only using ones of a specific length, you would be limited to a small population of brass, so why not trim?

With trimming using something like a Lyman case trimmer, I can make all the cases within 1 thousand of an inch and with that, you can use the majority of your brass.

It's interesting but it seems like there is a big push back on trimming, I am trying to understand why.  Sure it takes time, but trust me, if you have turned rifle brass necks, trimming 9mm cases is nothing in terms of work.

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Re: Trimming 9mm case?

Post by cdrt on 1/7/2019, 2:00 pm

Pistol cases don't "grow" like rifle cases, when they are fired.  I have never trimmed a pistol case, but I sure have trimmed lots of rifle cases.
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Re: Trimming 9mm case?

Post by jlow on 1/7/2019, 2:12 pm

I don't think I ever mentioned that pistol cases grown when they are fired.  

They however do have non-uniform lengths.  If you do not believe me, take a batch of brass with exactly the same headstamp and measure their lengths - I have measured hundreds, they do as a fact vary in length.  With the FC brass I measured, extreme spread was around 10 thousands.

Look at it this way, why would you expect pistol brass even fresh from the factory to have no variance in length - that degree of precision is expensive and not necessary for plinking which is what most 9mm is used for.  I am sure they all are likely to be within SAAMI spec which is within 10 thousands of 0.754".

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Re: Trimming 9mm case?

Post by LenV on 1/7/2019, 2:38 pm

One of the big things you missed is the taper crimp. You mention that it only crimps on the very end. Not true. A good taper crimp will be almost a 1/16" long. More then long enough to make up for any case length differences. Then when you seat the bullet consistently the space in the case for powder expansion is the same whether the case has been trimmed or not. And all of that is mote when you bench the pistol at 50 yds. A difference in velocity of 200 fps only makes a difference of 1" POI. This case trimming is really not something you need to worry about with pistol ammo. But you can worry if you want too.

Len

Good taper crimp on a 45
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Re: Trimming 9mm case?

Post by SmokinNJokin on 1/7/2019, 2:52 pm

There was a huge discussion on here years ago about it, a lot of experts (real, not being sarcastic) pretty much put the subject to bed with the following conclusions:

1) Yes. Trimming may realize a small degree of additional accuracy with jacketed reloads. Far less important with lead. An easier solution is to use new or only once-fired brass of the same headstamp. Starline for example is very consistent when new, within a few thousandths. Yes I have measured. 10 thousandths spread is pretty bad for new brass.

2) This is a very extreme measure to take for pistol accuracy, and I think you will find every person on here would agree it is a waste of time even for those of us who are very, very fussy reloaders. If your goal is to get better at precision pistol, this is a pointless rabbit trail. The pistol and shooter ability are all far larger variables, so trimming brass is akin to shaving your head before running a race to decrease wind resistance.. If you are an olympian and everything else has been pushed to the limit, then sure. For most of us... waste of time.

3) If you wanted to explore this, just for the hell of it, to its logical conclusion, and get some real #'s on how brass length affects accuracy, I think the best experiment would be to take 9x21 brass (for 9mm) or .45 win mag (for .45 acp) and trim them down to exact headspace, and fire in a barrel tester. Or, you could deliberately short chamber a pistol (maybe something like .890 for a .45) and trim all brass down to that. But that would be an insane amount of work for an experiment. Sub 1" 50yd groups have already been shot with pistols using factory ammo.

Good luck with your experimenting, and don't get mad if you feel like you get flamed on this forum. Usually everyone means very well and the messages read much harsher than intended. We all have thick skins and are opinionated, no big deal.

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Re: Trimming 9mm case?

Post by jlow on 1/7/2019, 3:03 pm

LenV - OK, good.  Let's talk some details. 

Here is what is in the Accurateshooter.com website article – tips from the USAMU:

"Optimize the Taper Crimp
One often-overlooked aspect of handloading highly-accurate pistol ammunition is the amount of crimp and its effect on accuracy. Different amounts of taper crimp are used with various handloads to obtain best accuracy. The amount is based on bullet weight, powder burn rate and charge, plus other factors. It is not unusual for our Shop to vary a load’s crimp in degrees of 0.001″ and re-test for finest accuracy."


With the above article in mind, I am using a Redding Micro-adjustable taper crimp die.  Measuring one of my normal level crimps with a caliper, the crimp is about 90 thousands long.  So a 10 thousand variance would be more than 10% variance. 

SmokinNJokin – I do not feel being flamed at all.  Only asking questions to understand and I quoted the above article to show the folks here why I was following that line of investigation.  I also appreciate you guys taking time out to talk to me about this – it is exactly what I was looking for. 

I have no problem with thinking that this COULD be a waste of time, just trying to figure things out. 

So at least for me, I was thinking the degree of taper crimp was a variable which I could use to zero in on a precise load.  I guess the question is if that is not it, what does people do to zero in a load (assuming you already found the right brass, primer, powder, and bullet).  Do you vary the charge, or seating depth i.e. COAL?

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Re: Trimming 9mm case?

Post by kidneyboy on 1/7/2019, 3:40 pm

First find the COAL for your gun by doing a plunk test with your barrel. Plenty of info out there on how to do that. After you decide which powder you are using and what a max charge is for that powder and bullet combo do some ladder testing to see which charge gives you the best groups. Play with the crimp after that until you see it doesn't make that much difference in 9mm.
 Spend some time at the Brian Enos forum in the reloading 9mm section if you want to see long drawn out discussions about crimping 9mm. 
 It is way more important to work on shooting fundamentals. It's entirely possible you could halve your groups by refining your technique.

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Re: Trimming 9mm case?

Post by jlow on 1/7/2019, 4:29 pm

Thanks! Good info.  I have done lots of plunk test but only to determine when a reloaded round does not fit my case gauge whether it still fits in my barrel.  The plunk in this case looks like it is used to find what we call "touching" seating depth.

So I assume once you find the max length that does not fit into the rifling, that is the length you use because it tends to be the most precise?

I am tickled to see a ladder test mentioned here.  With rifles at least we are looking for powder charge that does not change elevation on the target for a few slight differences in charge - is this the same thing?

In terms of Brian Enos forum, it was a guy there that suggested I come here to talk about trimming....  Razz

I would not po po the importance of technique but I am a reasonable shooter - bump to expert at an IDPA state match, not that I could not improve a lot more.

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Re: Trimming 9mm case?

Post by SmokinNJokin on 1/7/2019, 4:55 pm

In my research, I have landed on the conclusion that crimp is critical for lead ammo - exactly as Len described. My crimps with lead look identical to his photo.

BUT for jacketed ammo, exactly straight wall (no crimp at all) is best. Not to go to deep into the weeds, all the best match jacketed ammo in 45 and 9mm (federal gold medal 185 jacketed, IMI match 185 military, ASYM, Atlanta arms, etc) is perfectly straight wall with no crimp. Even with 9mm's body taper this is still pretty easy to measure by laying a straightedge along the brass and looking for light under it.

That being said, Roddy Toyota swears by crimping the hell out of jacketed too, and he has some fantastic test targets to back that up and is very reputable.

So I guess that puts us back to square 1. Its probably safe to say consistency of crimp (which ties in with brass length) is more important than the amount.

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Re: Trimming 9mm case?

Post by jlow on 1/7/2019, 5:26 pm

LOL!  Very interesting read.  Maybe my career in crime, I meant crimp is not totally dead yet!

There might actually be an interesting parallel in this with precision rifle.  With that discipline, we NEVER crimp our match bullets, a bit of a blasphemy…., however, we control bullet release by the catch all phase "neck tension" which involves actual tension by a smaller neck but also things like friction, degree of work hardening, etc.  All of which we try to control...

However, some joker once demonstrated that they could get reasonable consistency in release by ignoring neck tension but putting a hard crimp on the bullet.  We surmise that it worked because the crimp had so much more effect on release than regular neck tension that it normalized all the poor neck tension consistency.  Maybe pistol crimp is doing the same thing....

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Re: Trimming 9mm case?

Post by kidneyboy on 1/7/2019, 8:32 pm

jlow wrote:Thanks! Good info.  I have done lots of plunk test but only to determine when a reloaded round does not fit my case gauge whether it still fits in my barrel.  The plunk in this case looks like it is used to find what we call "touching" seating depth.

So I assume once you find the max length that does not fit into the rifling, that is the length you use because it tends to be the most precise?

I am tickled to see a ladder test mentioned here.  With rifles at least we are looking for powder charge that does not change elevation on the target for a few slight differences in charge - is this the same thing?

In terms of Brian Enos forum, it was a guy there that suggested I come here to talk about trimming....  Razz

I would not po po the importance of technique but I am a reasonable shooter - bump to expert at an IDPA state match, not that I could not improve a lot more.
 
 Plunk test - once a dummy round clears the leades, in other words you can drop it in the barrel and it will spin and easily fall out, make it a couple thousandths shorter and call that your COAL. Example - an RMR 124gr JHP in a WIN case clears the plunk test at 1.110 in my sons Taurus 9mm. I use 1.08 as the OAL
 Ladder testing using the same example - I'm using BE86 powder. Alliant data says a max load 5.8gr for a 124 jhp. I reduced this by 20% and called that my starting load. I then loaded 5 each at 4.6, 4.8, 5.0, 5.2, 5.4, 5.6gr (that is the ladder) took those to the range, shot them from a rest and compared groups. In that gun 4.8 and 5.0 had the best groups. I made up 20 of each of those and once again shot them for groups. The 5.0 had the best groups of the 2. I then made up 50 of those and shot some from a rest and some offhand. I then made 10 at 5.1 and 10 at 4.9 and shot those for groups to see if there was any improvement by using 1gr more or 1 gr less powder. There wasn't so the cartridge I make for that particular Taurus 9mm is a Win case, 124gr jhp from RMR, 5.0gr of BE86, an OAL of 1.08 and no crimp. My press will hold + or - .002, YMMV.
 So the ladder test is for optimizing the powder charge. The COAL is determined by the barrel of your gun and the bullet is determined by what your barrel likes. Crimp for 9mm can be argued ad nauseum.
 If you have a ransom rest this whole process can be refined but it won't make you shoot any better.

BTW, You will also find excellent reloading info at thehighroad.org

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Re: Trimming 9mm case?

Post by jlow on 1/8/2019, 8:02 am

Thanks a bunch!

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