I know that with a Giraud trimmer, I'm going to get some variation in length, depending on how I insert, and how hard I press the case.  However, I'm getting anywhere from - 0.005" to - 0.009" variation on some 5.56mm cases (that is some cases are short by 5/1000 to 9/1000).

First, I don't think this is an issue, but I want to keep things as close as possible because I'm prepping this brass for test loads.  What techniques have my fellow reloaders found give them the most consistent trim lengths?  And what's the most effective way to adjust the Giraud case holder when making small adjustments?  The tool has markings to align things with, but the case holders don't, and there ain't much to grip the case holder with in the first place.  This may be part of my problem.

Also, what should I be looking for in trim length variation?  All the manuals say "Trim to 1.750 inches," and my training says that the final zero means "exactly that."  Practically, should I be happy with a ±0.003 or so?  If not, what standard should I set?

I'm sure that these slightly short cases aren't going to be a problem in "production" loading, but it bugs me.

As previously stated, it's your sizing step, not necessarily the trimming itself, that is leading to the variation.

If you use the Hornady headspace measurement tool you'll see what your variation in sizing is.

Based on my experience, your variation in trimmed OAL will match your headspace variation.

Honstly, if you are getting 5 to 9 thou variation, something is wrong.  

Eliminate the easy, simple stuff -

1.  Are your OAL measurements affected by damaged case heads?  
2.  Measure the variation in case "headspace" after sizing (before trimming).  Trimmed cases won't be any better than this figure.
3.  Clean you case holder.  You can get polishing media in there and not know it.

A technique I use with the Giraud is to spin the case while it's in contact with the cutter head to ensure uniform trimming.

Another trick - use a Sharpie to put a black dot on the case holder.  Referencing this black dot against the hash marks makes it much easier to see the small changes you are making when adjusting the case holder.  

Also, the case holder threads seem sloppy when loosened.  Teflon tape (plumber's tape) can remove the slop.

I get +/- .001" with both my Giraud and my Gracey trimmers. Technique makes all the difference.

I insert the case with enough pressure so it doesn't spin in the holder trying to hold it centered in the insert.

Once the cutters seem to be finished I reduce the pressure just enough to allow the case to slowly spin one full revolution (360 degrees) so any high side can get trimmed.

This eliminates any misalignment caused between the course threads of the case holder and cutter head.

Short cases are probably too short to get touched by the trimmer at all.

My cases are coming up SHORTER sometimes.  About 4 out of 12 test cases, and that's all I've done.  They were all sized with an RCBS standard die, well lubed inside and properly lubed outside.

All the cases both gauge correctly and measure consistently using my dial caliper.  There is no difference before and after with the gauge, but the cases aren't as close in length to each other as I'd like.

I think the variation issue is related to how I'm holding the cases.  I got new, rubber-coated gloves for better grip and less fatigue, but they are stiff andI feel like I'm fumbling a little bit and not holding cases consistently.  Which would mean that I probably have the case holder set for shorter cuts than I think.  When I get a chance, I'll readjust it and just hold cases with my fingers to see if that changes anything.  I'm going to aim for a trimmed length of 1.753 to 1.750 and see what happens.

If they were LONGER, then it was possible your technique may need adjustment.  If they are SHORTER, it is possible that case may have been short to begin with and hasn't gotten to the point where it needs trimming.

Measure sized cases with your comparitor and trim them in groups based on those measurements. Next time do better on the sizing operation.

I get +/- .001 trimmed don't matter how I push the case into the trimmer providing I leave it in until it stops trimming.

You need consistant sizing if you want consistant trimming.

I get consistant repeatable sizing with OneShot because it sprays on every case inside and out evenly just like how you spray paint an object. I keep my brass in lots based on times fired so they are of the same hardeness. You will still get one or two that will end up short or long but 99% will be in the +/- .001"

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Thanks for all of the suggestions, even if my original post wasn't crystal clear in stating that I was getting cases that were trimming shorter, rather than longer.  It turns out that the issue was caused by two things.  

First, some of my cases's shoulders were pushed back farther than others, including a couple of the "too short" trimmed cases.  When I gauged them, I didn't pay enough attention to the way the heads of these cases went a little too far into the gauge.  I'm using a Dillon .223 gauge, and the max/min ledges are nice and clear, but I was only paying attention to the cases not being unsized or not sized enough.  I'm sorting out those cases, with an eagle-eye on which ledge each case meets or misses.

The second issue was definitely those coated gloves.  I thought they might be an issue when I had the trimmer's switch catch on the glove and not go all the way to the Off position.  It looks like sometimes the glove would get in the way of pushing the case all the way into the case holder, so it was the glove stopping it instead of the case holder's stop.  Other times, the glove was out of the way, and the case went in fully.  Without the gloves, I had a much more consistent feel for when the case was really in the holder.  The glove covering my thumb and index fingers clearly changed things, but only on an intermittent basis.

This afternoon I backed the case holder out about a full turn and started adjusting in small steps.  I put a Sharpie mark on the case holder so I could keep track of how far I'd turned it.  And I held the cases carefully, near the head, and with only my fingers.  Once I got the case to contact with the cutter head, I used the caliper after each little adjustment until I got to about 1.752 or so.  Then I trimmed several more cases and calipered each once it was trimmed; they're all staying within 1.748 - 1.752, and I'm happy with that.  To finish up, I locked the die lock ring down tight (I use Hornady's rings), and made a pair of Sharpie marks on the case holder that line up with the 3:00 and 9:00 witness marks on the case holder mount.  That should make it easy to repeat the adjustment later.

I will caliper the next dozen or so cases, and if they stay within that tolerance, I'll only caliper every tenth or so case, just to keep an eye on it.  And of course I'll re-gauge everything to separate the "sized enough" and "sized more than they should be" cases.  Those "sized more than they should be" cases really only have the shoulder pushed back a little more than spec, but combined with the occasional effect of the gloves not getting in the way, they really messed with me.

Me too!  Now I have to find time to do the rest of the sorting and trimming - I'm working through the weekend and all of next week.  Crazy?  Yes.  Yes I am.

Check out this modified hand priming tool to hold cases.

I found that you need to keep a q-tip near by because sometimes you get a brass burr inside of the shell holder, and that can set you off on your trim length by about .003 – .005. You should be able to tell when this happens because it should be a little bit tight in the case holder when you go to pull the case out after trimming it.

Nice innovation to use the priming tool.

Foot switches are greatly under-appreciated.

I use that tool for trimming my 300 Blackout cases because those guys aren't terribly long and I need some help gripping them.  I used it with my first couple of test cases once I readjusted everything, but then I ran the rest just holding them with my fingers and that worked OK.

As a follow up, these cases were from a batch that I'd collected a LONG time ago (around 18 years!).  I sized them on a Rock Chucker using RCBS dies that I adjusted per RCBS instructions, and they've been put away pretty much ever since then.  

I've gauged the whole batch of about 300, and I came out with about 100 that went too far into the gauge.  I checked a bunch of these cases with the Hornady Headspace Comparator, as well as several that gauged OK.  All the "OK" cases were at the bottom end of the gauge, and a sample of those came out at just about exactly the minimum length at the datum circle.  The cases that went too far in have averaged about 0.004" short at the datum circle.  

I guess I wasn't as consistent as I could have been in sizing them way back then, and I certainly didn't have the gauges and comparator I have now to make sure I adjusted the sizing die correctly.  All the sizing I've done in the last several years has been with the die adjusted using the Dillon headspace gauge, so I don't have to worry about a pile of incorrectly sized brass, or worse, rounds that are made with incorrectly sized brass.

Moving forward, I'm going to be much more attentive to exactly how I adjust my sizing dies.  Since I have Dillon or Wilson gauges for most of the calibers I load, and I have the Hornady Headspace Comparator as well, it shouldn't be any big deal to know exactly what I'm doing when I size cases.

Thanks again for the feedback and advice.

I trimmed the cases that had gauged OK and spot checked as I went - everything was at 1.750 ± 0.003".  I've decided that I'm going to trim the ones that gauged short with the trimmer set the same, and just expect the brass to be worked a bit more when the rounds are fired.  I'll load them in a different run, so I can adjust seating appropriately, but they're all pretty similar in how much extra I pushed the shoulders back, so it shouldn't be an issue.

Again, thanks for the help!

I put a Hornady Die Lock Ring on every one of my case holders. Adjust once, lock and go.

Yep, I've done that.  I have a couple of case holders I haven't used in a while that I'll add Hornady rings to when I use 'em.  I got the idea from dryflash3, but didn't get around to implementing it until recently.  I like that the flats on the lock nuts let me secure the case holder in the trimmer, but let me take it out easily too.