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Old June 09, 2018, 10:42   #1
raubvogel
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Why trim the length of bottleneck cartridges?

Yes, I am that clueless. I can understand resizing the cartridge (be it full or not), but when you trim the length you are just cutting off the business end, where the bullet gets shoved in. I thought a cartridge like , say, 7.62x51 size is determined by the shoulder, and the OAL is measured from the bullet's nose to the read end of the cartridge.

I also thought that when you, say, load a round in a Garand/Mauser/FN49/FAL/700, the cartridge stops when its shoulder hits the chamber walls.



So, what does case trimming accomplishes? Are there chambers that have a little shelf to limit the neck length or something? I can see that in a straight wall cartridge (or slightly tapered like the 9x19), but not in a bottleneck.

So, since my brain hurts, can anyone shed some light here?
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Old June 09, 2018, 10:50   #2
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To help control the neck tension when crimping. Also, if the mouth of the case is too far in the chamber it may not be able to expand and over pressure.

Page to the 308 Win on this link and you'll see how the diameter of the chamber starts going down at about 2.05" You don't want brass pinched into this area

http://www.saami.org/specifications_...wnload/206.pdf

It's not uncommon to see brass after a firing be 2.030"
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Last edited by meltblown; June 09, 2018 at 11:05.
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Old June 09, 2018, 10:54   #3
tdb59
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In very simple terms, not being condescending.


The cartridge neck is larger diameter than the bullet and the bore.

Excess neck length can cause the mouth of the case to be shoved into the throat of the bore, causing high pressures as the bullet cannot release properly.

If you look a chamber drawing, it becomes more obvious.

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Old June 09, 2018, 11:08   #4
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Quote:
Originally Posted by tdb59 View Post
In very simple terms, not being condescending.


The cartridge neck is larger diameter than the bullet and the bore.

Excess neck length can cause the mouth of the case to be shoved into the throat of the bore, causing high pressures as the bullet cannot release properly.

If you look a chamber drawing, it becomes more obvious.

^^^^
This.
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Old June 09, 2018, 11:31   #5
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Every cartridge has a "max case length" to prevent the case exceeding the chamber length as shown in the diagrams above. Shooting a brass case and sizing a brass case BOTH lengthen the case. Thus you trim to keep the case length propper.

If you are reloading without a good loading manual?...not good as they explain many things you need to know that may not be obvious.
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Old June 09, 2018, 11:43   #6
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Although cases stretch when fired they don't "grow" in length until they have been sized ..... so, always measure and trim your cases after they have been sized. I prefer my rifle cases to be as close to max SAAMI length as possible. What I do with a batch of cases (I sort mine by headstamp and times fired) is to find the shortest case(s) then trim all cases in the batch to that length, culling out any cases that are under SAAMI trim length. Normally cases will be about .003~.005" short of max length.

Most bottleneck rifle cases that are full length sized will "grow" about .003~.005" each time they are fired. If you neck size only, cases still grow but only .001~.002" each time fired. If you trim to "trim length" (typically .010" less than max SAAMI length) you should be good for 2~3 reloads before you need to trim again. If you don't taper or roll crimp, case length is not critical as long as they don't exceed the SAAMI max length, however if you do crimp, you will find your crimps will be way more uniform when all cases are the same length. Uniform is a key word when reloading because it directly relates to accuracy.
I realize this is about rifle cases but thought I'd add a little about handgun cases too. Most handgun cartridge trim lengths are .005" shorter than SAAMI max length and for pistol cartridges (semi-autos) this is especially important because pistol cases headspace off the case mouth. That means any pistol case shorter than SAAMI max will result in excessive headspace. As an example, let's say you had a 1911 with a SAAMI spec headspace of .010". This is measured between the case head and breech face using a SAAMI full length case (.898" for a 45 ACP). If the case measured .893", you would end up with .015" headspace, which is excessive. The problem is ... it's hard to find 45 ACP cases that measures the full SAAMI length of .898" so you just have to use what you have available. I save my full length 45 ACP cases for match grade loads ... and yes, excessive headspace does affect accuracy. Rimmed revolver cases are not critical because they headspace off the case rims. That said, if you want optimum accuracy and uniform crimps, trim all cases to the same length.
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Old June 09, 2018, 13:38   #7
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Originally Posted by judgedredd View Post
Although cases stretch when fired they don't "grow" in length until they have been sized ....
Cases ABOLUTELY grow in length when fired. The body of the case expands to fix itself to the chamber and the case head moves to the rear. This "stretch" does grow the length of the case.

How can you "stretch" something and NOT have it "grow" in length. Ask someone who reloads .303. Cases lengthen when fired, and lengthen MORE when sized. Pulling themselves in half on firing after a few firings. Precision reloading (headspace measurement, neck sizing, etc) can minimize all this, but it happens.

Last edited by ammolab; June 09, 2018 at 20:29.
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Old June 09, 2018, 14:32   #8
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Originally Posted by ammolab View Post
Cases ABOLUTELY grow in length when fired. The body of the case expands to fix itself to the chamber and the case head moves to the rear. This "stretch" does grows the length of the case.

How can you "stretch" something and NOT have it "grow" in length. Ask someone who reloads .303. Cases lengthen when fired, and lengthen MORE when sized. Pulling themselves in half on firing after a few firings. Precision reloading (headspace measurement, neck sizing, etc) can minimize all this, but it happens.
I agree but Iíll be damned if I havenít seen some that got shorter after firing before sizing
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Old June 09, 2018, 20:43   #9
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I agree but Iíll be damned if I havenít seen some that got shorter after firing before sizing
Absolutely; happens all he time. The case expands to fit the chamber, then contracts slightly (springs back). It isn't uncommon at all for brass to come out of the chamber slightly shorter than when it went in. Serious benchrest shooters frequently don't do anything to a piece of brass until it's been fired at least twice, especially in a wildcat chamber.
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Old June 10, 2018, 08:08   #10
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Brass flow is a natural occurrence.
Wildcats that need to expand and fill the chamber usually end up shorter in oal.
Wildcats and others modern designs with minimal body taper and greater shoulder angle do more to resist brass flow and 'grow' less than historically typical designed cartridges.
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