Trim to chamber length?


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ctrout
December 6, 2008, 11:06 PM
I posted this at a couple other sites but thought that it might be good to post it here as well. I read the following article from Varmint Al's reloading page and it made sense to me. I just bought 500 pieces of brass and have already trimmed about 200 of them to minimum length. It just so happens that based on my chamber depth as measured according to the procedure in the article, minimum length is about .015" shorter than my chamber will accomodate. If you were me, would you finish trimming all the brass to the same minimum length or would you trim to chamber length on the remaining brass and use the minimum length stuff for plinking and save the chamber length stuff for serious accuracy work?

From the following link... http://www.varmintal.com/arelo.htm

MEASURE THE CHAMBER LENGTH.... Here is another accuracy aid. I purchased the Sinclair case length measuring plugs for .17, .224, and 6mm calibers. These are very simple steel cylinders turned to the bullet diameter, leaving a rim slightly less than the case neck OD. To use, you merely drill through the primer pocket (so you can later push the plug out) on an expendable cartridge case and neck size it. Trim the case length about 0.100 inch short so the case mouth will not touch the rim of the plug gauge. Insert the Sinclair plug gauge as if it were a bullet, leaving it long and then chamber the case. The plug will be pushed deeper into the case neck and when you extract it, you can measure the chamber's actual length with a dial caliper. I usually find that the factory recommendations for case length leave a 0.050 inch to 0.070 inch gap between the end of the case and the actual length of the chamber. Click here to see a drawing. This is a factory safety concern and you can get high pressures by forcing a long case into a short chamber. The factory wants to be on the safe side. But like everything else, if you want the best accuracy, you can minimize this gap down to 0.005 inch and still be safe. You will get better accuracy and find that you can use much longer necks, in most cases, than the factory recommendations specify. The extra case length will better allow you to load the bullet farther out to touch the lands. I have found that, typically, factory .243 Win brass, is already 0.050 inch too short and there is no way that I can think of to lengthen it. I don't trim to the lengths specified in the loading manuals, but use my measurements as a guide. All of this sounds like a lot of trouble to go through compared to walking into a store and asking for a box of 30-30's, but it makes a big difference in the confidence level of your accuracy potential. If you have a lathe, the dimensions of the plug gauges are given for a few rifle calibers on my Mini Lathe Page. They are very easy to make. Check out Fred's page for more info on making tight-neck and long-neck brass.

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Walkalong
December 7, 2008, 09:28 AM
You can trim your brass to match your necks to chamber length, but it just isn't going to make much difference in accuracy, if any, and you are always on the edge of getting to long, getting the neck into the leade, and running up pressures to a dangerous level. Not worth playing with IMHO. There are better proven ways to increase accuracy, and that is not one of them.

moooose102
December 7, 2008, 10:02 AM
if you trim your brass TO chamber length, what is going to happen when the shell is fired? the pressure generated will strech the case (even though it is very little) and drive the pressures up. if i were you, i would experiment some. trim up in 10 round lots. start @ 0.002" shorter than chamer length, then 0.005", 0.010", etc. see if it makes any difference. chrono every round also, make certain to hand weigh every chage, and that the oal does not change. then you can see if there is any merit to this. anything else is speculation.

plinky
December 7, 2008, 10:23 AM
I read somewhere that longer necks were better for accuracy so I've quit trimming to minimum length. I don't shoot large volumes so I don't mind trimming every loading. In reality, the cases are .005" above minimum. I can't imagine how that helps accuracy but hey, I read it somewhere. lol

I would guess that there is more to be gained in neck turning. In fact, letting your necks run long might make any runout in neck wall thickness a more critical issue. Theoretically anyway. I just turned 50 Lapua necks which are said to be very good. Out of the box some had nearly .002" runout. After turning it's .0005" and that's with a fairly low end tool. They are some fine looking brass. :cool:

Your minimum trimmed cases should grow back pretty quickly so you can tighten that up if you want. Like Walkalong and Moooose said though, I'd be afraid of pinching the bullet by going too long. Maybe figure the max stretch you'll get and add .001 or .002" for safety. Trim at least that much from a chamber length neck.

It would be interesting to try brass at both lengths and stringently test for accuracy.

Great link BTW.

rcmodel
December 7, 2008, 12:10 PM
I personally think neck-trimming is going to do more harm then good in a standard factory rifle chamber.

They are generally over-size to start with, and neck trimming makes for a looser fit in the chamber, and a more off-center bullet start then if they were left alone.

rcmodel

Ol` Joe
December 7, 2008, 04:42 PM
I personally think neck-trimming is going to do more harm then good in a standard factory rifle chamber.

I believe you meant neck turning. :D

I agree. I`ve a 270 Win with a min SAAMI chamber that I turn for but only enough to insure clearance. The neck tension IMO is more important then a perfectly even neck in a hunting/factory rifle. There is more runout caused by the way the cartridge lays in the bottom of the chamber then produced by a slightly uneven neck wall.
If everything isn`t exact and at min spec you are better off neck sizeing and allowing the "funnel" shape of the shoulder to true the cartridge to the bore when it`s held snug in place by the bolt....JMO

rcmodel
December 7, 2008, 04:47 PM
I believe you meant neck turning.I believe I did too! :o

rcmodel

Walkalong
December 7, 2008, 05:14 PM
It would be interesting to try brass at both lengths and stringently test for accuracy.
If it helped, Benchresters would be doing it. There are ways to get more accuracy from a case, but as long as you have enough neck to hold the bullet well, extra neck length doesn't help.

Being dead straight in the chamber helps, but a longer neck does not help that. Neck turning for a standard chamber just makes it worse, although it may help consistent neck tension, depending on other things, which is good.

plinky
December 7, 2008, 06:32 PM
I think that it's going to depend on how good or bad your brass is to begin with. I remeasured some unturned Lapua necks and the worst variation I can find now is .0015", but I only did a few. They are thick at @.015" to .017". I believe the whole lot would clean up 100% true at .015" (still plenty thick). I went a bit farther at .014" just because that's what my last set was turned to.

I have checked some more generic cases in the past and found variations of .002 or more on a single case. This was the first set of cases I used (unturned) in my 700 Varmint and the groups were very so so. Then I switched to a set of turned FC cases and groups did improve. Of course, I was changing other things as well and the rifle is breaking in the whole time so I can say nothing for sure.

I'll experiment more with turned and unturned Lapua when I can but at the moment I tend to believe in moderate turning for a factory chamber.

Walkalong
December 7, 2008, 08:23 PM
I did not mean it made accuracy worse, it makes it less straight in the chamber when we remove brass from the neck unless we take other measures to get it lined up, such as partial neck sizing etc.

ctrout
December 7, 2008, 10:24 PM
Thanks for all the discussion. I looked at how much difference it would make (visibly) and also found that my brass is nowhere near long enough to come close to my chamber length anyway so I will not be pursuing this any further.

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