Trim length, does it make a difference in accuracy???

[CA_Shooter]

While it seems .002 won't matter...

Can I ask the question: at what point does a variance in trim length actually matter?

For example, does .005 matter for a 6mm...or a 7mm? How about .008?

I found that at a certain amount of trim, it caused reduced neck tension and seated bullets much easier. I'd assume this matters on paper.

 

[mikecr]

at what point does a variance in trim length actually matter?

I the only condition I can think of where it directly matters is running most cases trimmed way short, causing carbon ring formation in the chamber (at case mouths). Then chambering a long case into that ring, throwing a shot.
As far as neck tension from reasonable trim length variance, I don't see how it could be measured as significant. Tension is neck springback force gripping an area of seated bullet bearing (PSI). If your neck sizing length is .200" and your trim variance is .005", that's a potential max of 2.5% area change.
Well, I imagine that your springback force varies more than that already. So I think the normal trim variance in itself would be lost in noise and unprovable.

 

[wboggs]

Some trimmers index from the shoulder, meaning neck length should be very consistent. However, one may notice some overall case length variance when using this type of trimmer, even though neck length is very uniform. Using a trimmer that indexes from the base of the case can provide very consistent case overall length values, but one may find some variance in neck length when using a trimmer of this type, even though case overall length is very uniform. I've used both types and that is what I have noticed over the years. I cannot reliably state that either system provided better precision in my hands. If there was a difference, I could not shoot it. I don't find that too surprising as having +/-.001" or so variance in bullet bearing surface engagement in the case neck is likely a very small source of error, especially as compared to other larger potential sources of error in the reloading and shooting process.

There is no "perfect" system for trimming cases. We try to make them as consistent as possible using whatever system we have available to us. If minor case length variance caused via some specific trimmer system is worrisome, one can always pick out a few cases at either end of the case length range and test them side-by-side to determine whether they can really shoot the difference. Alternatively, one can just hit the easy button and save themselves some time and reloading components by following Bart's advice above, which is backed up by winning performance.

Ned
Which ones index on the shoulder?

 

[Ned Ludd]

The Giraud trimmer indexes off the shoulder. There are a few other popular trimmers that also index off the shoulder, but I'm not able to give you a comprehensive list at the moment, and I don't want to provide you with false information. For a given trimmer, if the case is held by hand and inserted into a hole for trimming (as opposed to the case base resting on a solid support of some kind), there's a good chance it indexes off the shoulder. That's something that would need to be checked on an individual basis. Perhaps others will chime in on whether their trimmer of choice indexes off the shoulder or base. I haven't used anything but the Giraud in some time. It's pricey, but worth every penny IMO.

 

[1kbrxDave]

Does length matter YES it is your first CROWN .
test it and find out .

 

[Owens85]

The Frankford arsenal trimmer indexes off the shoulder as well and does a really good job as long as your shoulder bump is consistent then trim length will follow.

 

[ebb]

I started trimming on a Lyman trimmer thats like a tiny lathe, I don't know what its called. I could not get consistent lengths unless i push hard while trimming. Then the locking collar started moving and the cases were way off. I tried a Wilson trimmer and had pretty much the same results. A friend turned me on to the Lee trimmer and I could get fairly uniform results. When i hear people talk about cutting all the cases to .001, I am skeptical. I think Barts brass being within .002 is probably because he has to spend way too much time to get them any better. And that makes sense if it has minimal or no impact on groups. Until you can make the shoulder size till they are all exactly the same I think its a moot point. I decide I was spending too much time on it and got a Lyman Brass Smith trimmer, and it indexes on the shoulder. So, I have decided to use it on something that i need a quantity of and use the Lee for everything else.

 

[Kracken McGraw]

I tried a Wilson trimmer and had pretty much the same results.

I think we better hear some more reviews for the Wilson before someone reading this writes it off. Mine has been dead on and I think it’s great. This is one process you can learn the outcome with feel and hearing both, so it’s not the best time for music.

 

[ebb]

That may be the exact problem and It may need a new cutter. I have no idea. i just know that the results from a lee are way easier for me to get right than go on some frustrating learning curve that may or may not pay off in the end. I spent my life in construction and my beat up hands may be the issue. i would give any thing to have my shoulders and hearing back.

 

[mikecr]

The Frankford arsenal trimmer indexes off the shoulder as well and does a really good job as long as your shoulder bump is consistent then trim length will follow.

As long as your shoulder bump is correct, then trimming from the case head (OAL) seems the same result.
What's the magic method difference?

 

[Ledd Slinger]

I think we better hear some more reviews for the Wilson before someone reading this writes it off. Mine has been dead on and I think it’s great. This is one process you can learn the outcome with feel and hearing both, so it’s not the best time for music.

Yeah there’s nothing wrong with the Wilson trimmer, just takes a longer to change out cases so takes longer overall to get the job done. But it does a great job of getting a nice square cut in relation to the case body. Square cuts also equate to a nice square chamfer on the neck.

I can tell a difference in quality of cuts from cheap trimmers to the higher end stuff when I use my K&M chamfer tool. If the neck isn’t square, the K&M will show it in the chamfer cut.

 

[Alex Wheeler]

Yep, another vote to trim short. Minimum of .020" and a lot more in many of my chamber designs. With 65kpsi inside that case, you dont think it gets longer under that pressure? I shortened up some reamers back when everyone was so concerned about this carbon ring thing. Went right back to my long ones. Hell .050" at the end of the chamber wouldnt bother me a bit. .010" would though.
As to consistency, a lot of runout comes from necks that are not cut square. A few thousandths variation wont hurt you. All I worry about is that they are square and not too close to the end of the chamber.

 

[J.d. Popkes]

Before I got a Henderson I’d say it didn’t matter, solely because I hated trimming, chamfering, deburring. Purely out of hatred and laziness. Now I trim every time and I can’t prove that it matters but it only takes a couple minutes to do all three things on a hundred pieces.

 

[6BRA]

Forster..modified never varies. Never slips. Can control every case to .0005...don't ask. Not telling and it doesn't matter apparently anyway. It's possible on a good manual trimmer.If my trimmer would only cut to within .002, I wouldn't bother either.

 

[mikecr]

With 65kpsi inside that case, you dont think it gets longer under that pressure?

If you mean mouths close chamber end clearance, due to pressure, no they don't.
Mouths can only pull backwards -on firing, as shoulder bump is removed.

Cases stretch back to boltface, and a lot of sizing lengthens the case, so this leads to a need to trim.
For those of us who do not FL size, we don't do much trimming.

 

[Larry Koch]

If I shorten my case necks to much there won't be any room for powder. Right now thats the slowest part of reloading. Is case fill with room for bullet.

 

[Alex Wheeler]

If you mean mouths close chamber end clearance, due to pressure, no they don't.
Mouths can only pull backwards -on firing, as shoulder bump is removed.

Cases stretch back to boltface, and a lot of sizing lengthens the case, so this leads to a need to trim.
For those of us who do not FL size, we don't do much trimming.

I dont agree. Squeeze brass at 65k psi against the chamber and I garuntee it thins out, the only place to go is towards the end of the chamber. I am not saying it yields and stays longer, Im not talking about what causes brass to need trimming. We know this because the rifles shoot better when you dont crowd the end of the chamber, If the brass didnt grow when it was under pressure we could run zero clearence at the end of the chamber. If you dont fl size you shouldnt have to trim much, pressures are low. Run it up to the point your compressing lug seats and brass yeilds. Then fl sizing is nessesary.

 

[Straightshooter1]

If you mean mouths close chamber end clearance, due to pressure, no they don't.
Mouths can only pull backwards -on firing, as shoulder bump is removed.

Cases stretch back to boltface, and a lot of sizing lengthens the case, so this leads to a need to trim.
For those of us who do not FL size, we don't do much trimming.

I find my fired cases just don't grow much, if at all, in length until I FL size them. So, I agree that the cases stretch back towards the bolt face, because . . . the pressure that seals the case is equal on all surfaces on the interior from the base of the bullet back. The direction of the flow is going to be towards anywhere there's is less pressure, which is that space between the case and the bolt face. The shoulder is pushed by the firing pin to make contact with the chamber and everything there is then sealed and under the pressure all around the neck (no lower pressure there for the brass to flow towards). Because the case expands towards the bolt, it's why we need to bump necks and if the case headspace setting is too much the case is pushed towards the bolt face excessively resulting in eventual premature case head separation.

As to why a donut forms due to the flow of material from the shoulder into the neck, that appears to me to be from a brief pressure differential during the time the neck is expanding and forming a seal. And the pressure at that time is not anywhere near the max of the ignition sequence, but enough to get a little bit of flow into the base of the neck.

Because of my limited knowledge of physics, I'm all ears if anyone has a better explanation for how the pressure acts on the case.

 

[mikecr]

As to why a donut forms due to the flow of material from the shoulder into the neck, that appears to me to be from a brief pressure differential during the time the neck is expanding and forming a seal. And the pressure at that time is not anywhere near the max of the ignition sequence, but enough to get a little bit of flow into the base of the neck.

No, donuts form from sizing. FL sizing.
This is because brass only moves thick toward thin, and FL dies begin their sizing near webs.
This, rolling thickness upward from body to shoulder to neck to your trimmer to your trash can.
And every time that cycle happens your cases change in character.

 

[Bill Norris]

No, donuts form from sizing. FL sizing.
This is because brass only moves thick toward thin, and FL dies begin their sizing near webs.
This, rolling thickness upward from body to shoulder to neck to your trimmer to your trash can.
And every time that cycle happens your cases change in character.

That's the cool thing about the 40 degree AI chambers. Brass hardly moves towards the shoulders. I have some 260AI brass with 8 firings on them and F/L size bumping shoulders .002 each time with almost zero growth.