Brass trimming: how close is “exactly” the same length.

In my experience, case length needs to be consistent and kept shorter than your chamber. When I adjust my Giraud, I just make sure they're shorter than trim to and are all done the same.

I'm also interested in how you lose 70 FPS with shorter case length. Are you sure it wasn't temp or humidity?

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Originally Posted by HKGuns

In my experience, case length needs to be consistent and kept shorter than your chamber. When I adjust my Giraud, I just make sure they're shorter than trim to and are all done the same.

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I grow mine all to 1.760" for a little extra neck contact to bullet. Especially since the Giraud chamfers the brass kinda deep. The extra length also shortens the trim time because I'm usually just "kissing" the mouth with a little neck chamfer.

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Originally Posted by markm

I grow mine all to 1.760" for a little extra neck contact to bullet. Especially since the Giraud chamfers the brass kinda deep. The extra length also shortens the trim time because I'm usually just "kissing" the mouth with a little neck chamfer.

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Mine are set to 1.760". I hold a +/-.005" tolorance.

Some are just too short to trim (many were 1.740 or less). I loaded a handfull and they were averaging 70fps slower than the other loads shot on the same day. Ill load more tonight and try again tomorrow.

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Originally Posted by MegademiC

Mine are set to 1.760". I hold a +/-.005" tolorance.

Some are just too short to trim (many were 1.740 or less). I loaded a handfull and they were averaging 70fps slower than the other loads shot on the same day. Ill load more tonight and try again tomorrow.

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That's remarkable. I've had a lot of FC brass over the years where they're way short of normal spec from the factory. The take a few firings before the trimmer will cut them, but I've never seen a dramatic velocity difference. I have not, however, been looking for that.

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Originally Posted by markm

That's remarkable. I've had a lot of FC brass over the years where they're way short of normal spec from the factory. The take a few firings before the trimmer will cut them, but I've never seen a dramatic velocity difference. I have not, however, been looking for that.

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Im not able to get get out today, but I loaded 10 rounds <1.740"

I also loaded 10 rounds within +/-.001" hand-checked powder of each round.

Then I have my "normal" ammo. Ill update when I hit the range.

Case length the only variable?? That's incredible.

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Originally Posted by markm

Case length the only variable?? That's incredible.

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Yes. All fed primers, 24.7gr tac, 69gr rmr. I did not isolate/sort brass as I havent seen a noticable impact from that.

Wow. I may have to try this in a caliber where I get case length variations. I've been wanting to get parts to set up 6.5cm on my Giraud trimmer. And if I could prove case length to be an issue, I'd damned sure buy the cutter parts.

Is oal independent of headspace? If I don't pay attention when lubing/sizing, I can have 4 thou plus headspace variance. (If I pay attention, 1, sometimes 2 thou extreme variance) If trimmer indexes oal, you get variation in neck length and therefore different bearing on bullet based on headspace variance. Of course, I guess inside neck deburring depth does same..... Does it matter?

I could predict headspace by oal because my kwicktrimmer indexed so well off of the shoulder (for consistent neck length). Hope I'll be able to drive the Giraud as precisely....

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Originally Posted by triggerjerk

Is oal independent of headspace? If I don't pay attention when lubing/sizing, I can have 4 thou plus headspace variance. (If I pay attention, 1, sometimes 2 thou extreme variance) If trimmer indexes oal, you get variation in neck length and therefore different bearing on bullet based on headspace variance. Of course, I guess inside neck deburring depth does same..... Does it matter?

I could predict headspace by oal because my kwicktrimmer indexed so well off of the shoulder (for consistent neck length). Hope I'll be able to drive the Giraud as precisely....

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Giraud indexes off the shoulder so consistently bumping the shoulder back is key for consistent case length. The hand trimmer indexed off the case head so every case is the same OAL. I'm not sure how much of an impact any/all of this has on neck tension and/or velocities.

I can tell you that I can certainly feel some variation in neck tension on the K&M Arbor press when seating bullets. I pulled 2 "loose" feeling necks out of my last run to shoot separately. I ought to chrono them against normal necks.