Firstly I apologise for the length of this thread.

Anyway, I have been reloading for a few years and I think I may have been lucky thus far.
I reload for .17 Rem, 222,223,22-250 & 270. All of so far I have managed to get great results with all. My procedure is:
FL resize all my brass, weigh each charge on a digi scale. I tend to use Remington 7 1/2 and 9 1/2 primers as they have given me best results throughout. I have Lee and Forster dies and tend to use the Forsters most of all.
I have never trimmed my brass as I have found that I have got inconsistant results because of the amount of tension I use to lock the cartridge in place (Lyman Accutrim). I find that I get a greater variance than if I do not trim he brass at all. I do tend to use bulk Lapua or Norma brass when I can get it.
I have started reloading for 243 and have a Rem 700 and a Weatherby MKV UL. In either factory ammo is reasonable , but my reloads are a disater. On one outing I got 5 shots in on hole at 100. I made a batch of 100 of these (60 grain Berger over 42.7 grains of Varget at 10 thou off the ands) and they were grouping no better than 2" at 100 yards. I had a few other loads that were good and the same thing happened again.
As both guns are reacting in similar ways (more so the Remmy) I must be making an error n reloading. Temperature and wind condition not changed enough to make any dfference.
I've just made another 20 up with the load using new brass that I have neck sized with the collet die from Lee. The Lee die seems to give me a little tighter tension than the FL Forster die.

Any advice gladly appreciated

I vote for trimming the brass. The few .001" that your Lyman trimmer leaves has no bearing whatsoever on the cartridge over all length. That is determined by the seating die adjustment/bullet ogive relationship.

Even with mild loads, brass is subject to forces that stretch it, like repeated sizing/firing cycles for example. Mitigating factors like partial resizing and lubing case necks won't prevent this from happening.

Trim that brass!

I would also invest in a Stoney Point collimator and measure the oal of your factory loads. You rifle seems to like that length bullet. Seat your bullets to achieve the same seating depth. 10/1000s isn't necessary the best seating depth. Good luck and let us hear from you.

trim the brass.....as to neck tension.....forget it.....if you can't measure it, you can't manage it.

Many years ago, I too didn't think I had to trim if I only neck sized. I did experience the pressure rise associated with the chamber crimping the brass to the bullet. I was very fortunate, the only problem I had was in order to open the bolt I had to kick it open. The brass and the primer hole were ruined. I had the presence of mind to write to Peterson's magazine and had a very patient reloading writer explain to me how very lucky I really was. Regardless, if you fl size or neck size, TRIM THE BRASS WHEN IT NEEDS IT!

Trim your brass! don`t let it excede the max listed in your manual. If you don`t get good results with the Lyman trimmer try a Lee trim kit. They are cheap and very consistant. I`d recommend useing a dremal tool or cordless drill with them though as your fingers will take a beating otherwise.

I used a headspace gauge by sinclaire in the 223 and the maximum trim length was nearly 1/8" shorter than the chamber length. Is it worth measuring the actual chamber depth or is the trim length not supposed to be the actual chamber length?
That was the reason that I never bothered to trim. It seemed that I got the wrong end of the stick here.
Thanks for the advise so far.

Johnny,
What caliber are you concerned about?
What was the bullet weight of the factory load that shot so well? Kudude

The only cartridge I have problems with is the 243.I can not even emulate the performance of Factory 55 (Winchester Supreme).
I am out in the next hour to run a fresh load through that I took a few hours over last week. They are Hornady 65s and Berger 60s over Varget.
The 243 Weatherby is being bedded and the barrel cut for a moderator. I hate to admit it but seeing as I had so much trouble with the Remmy, I have asked the smith to do some Load development on the Weatherby.

I have invested in an auto zip trimmer from Lee as it seems to be the most foolproof system.