Neck length to chamber length clearance consensus

[linebaugh]

Without going into detail on an individual chamber, caliber or cartridge is there a consensus on what the optimal clearance from case neck to chamber neck length clearance should be? I realize that many would suggest to trim to SAMMI standards but in this case there are no standards and that measurement does not really mean much if you don't know the chamber print used. I have the reamer and print in hand as well as fire formed brass.

Assuming my chamber print is correct I have measured 60 pieces of my 400 fire formed Lapua brass and have a .020 extreme spread with the average something closer to .008 for the brass I measured. The single short piece would be .019 clearance on chamber length and the longest couple would be over by .001.

I am going to make the inference that .010 would be ideal for cartridge to chamber length clearance and this would catch about 95% or my brass. If I took that trim length to .015 it looks like it would catch 99% of my brass. I am inclined to go with the .015 number but am open to other ideas which is why I am posting this.

For the record my fireform barrel has approximately .050 neck to chamber length clearance and that chamber shoots quite well and I have seen no more carbon build up than I have seen on any other chamber. I do wonder if I am leaving some velocity on the table and if such a long clearance would also induce increased throat wear? I know the present barrel is less than ideal and am not promoting it in any way but I do wonder what others experience might be.

 

[cw308]

Was hoping someone would try an answer your question with more technical advice then me . I only shoot benchrest 308 Cal. I would think as long as you know the length of the neck area trimming .010 - .015 would be fine , carbon build up I would imagine would depend on case expansion . .050 clearance is alot , wear by heat cracking if left to that instead .010 I would think would be a problem . I have a Lyman bore scope , it sure isn't the top of the line but it's better them eye balling . I wish I had it when I rebarreled to know what a brand new barrel compared to having almost 3000 .everything looks good some fire cracking at the start but you have to know what wear looks like . I would size and trim to the chamber measurements . Hope I helped in some way , maybe it will start things off for you .

Chris

 

[dmoran]

@linebaugh
As to how much neck length clearance is needed, personally feel that is dependent on a lot of scenario's (such as: how often the barrel is cleaned, how well the barrel gets cleaned, how often we plan to trim, how long of free-bore, etc., etc., etc.).
With that said and in my opinion, it's an individual answer. But .015" is not much clearance (open a calipers to .015" and see how small a gap that is).

You wrote that from a 60 piece sample of 400 fire-formed brass you have .020" variation in case lengths. I'm inclined to ask how did you get so much variation? Was the brass that uneven in lengths when new? Did you use different loads when fire-forming?
My reason asking and bringing this aspect up is: it is my experience that the cases "blow lengths" from fire-forming directly effect there capacity as well. And when the blow lengths have extreme variation, so will the case capacities. Sort to speak, say you measure 2 new cases and there .003" different in length, then fire-form them and they blow to .009" indifference, where did the .006" variation go? And is why I want my fire-formed cases to blow consistently in length, so that the case capacities end up consistent as well.

Just my 2-Cents

 

[243winxb]

measured 60 pieces of my 400 fire formed Lapua brass

When sized, case length will change. Measure it then. Trim .010" less then chamber length. My guess.

 

[cw308]

If we're talking fire forming with different lengths from base to shoulder datum , I would size to the longest case , test in chamber for resistance if none I would size 5 and test if cases get any longer or heavy bolt lift . I keep my headspace .001 - .002 no more or less for benchrest . Were you loading hot when fire forming your brass? Maybe expanding more then stretching .

 

[linebaugh]

Some of the variation is due to using a bunch of different charges, bullets, powders and even fireforming some with the cream of wheat method and then shooting a second time. I also know another reason why some of the varition exists. I can physically see the case mouths are out of square and I have encountered this many times in sizing down or up in case neck diameter. I did not trim anything after sizing and fire forming so the brass was formed, fired and sized again with shoulder pushed back about .0015 before measuring.
I understand most probably think I am nuts but the variation is not a big concern and I will clean it all up and uniform the necks when I shoot my last hundered rounds. I want to donit all at once so its all the same. I was just mostly posing a question of what seems reasonable.

I had not thought of what internalally the cases may be doing since I have used so many different scenarios to form. That is a reasonable idea to ponder and I may CC several rounds when done trimming.

As was stated above there are many variables such as how often one cleans and how often one trims. I was trying to avoid much of that by not getting overly descriptive and just asking what most would feel is an optimum clearance. I do realize now that I got to descriptive and opened my own discussion up to many other variables. None the less I would be interested in any input offered.

 

[dmoran]

@linebaugh
Some more input: myself will never load develop or component swap when fire-forming, because I want all the cases to be fired from one consistent pressured load, so that they all get ironed and stretched dimensionally the same. In fact, I fire-form x-amount of cases to be used as "match brass" and x-amount for "test brass", never using the "match brass" for testing at any point, so that it remains dimensionally consistent cycle to cycle.

 

[NorCalMikie]

Trim .010" less then chamber length.

Most chamber are cut maybe .020 LONGER than max trim length. What you need to do is measure your chamber and go from there. Sinclair sells plugs just for doing that and they work like a charm.

 

[mikecr]

My goal is 5thou clearance of chamber end. I simply let cases grow to no closer than this.
The tighter I keep it, the lower the carbon buildup at chamber end, and the quicker my necks seal.

 

[cw308]

Last 3 posts make perfect sense to me .

 

[Spike A]

I was told once to be careful getting them too close, as when actually firing they stretch than spring back some... and a lot of wear will be caused if they are too tight and ( bottom out ) so to speak... .. now that being said I have measured a few factory chambers and they seem to be considerably more room than the numbers gentlemen are talking about, but I normally trim to factory length and make sure to clean chamber good with no issues... I think I would be more concerned about case volume at this point being consistant more so than a thousandth of neck length...

Just my 1 cent
good luck happy shooting.. spike

 

[NorCalMikie]

If you're using shoulder "bump dies", your brass won't grow as much as using standard full length dies. No button to pull the necks longer.
Got BR brass with 25 to 30 resizes with bump dies and so far, no need to trim.