Chamber length question

O

ozfclass

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I have two 284 Shehane barrels, chambered with the same reamer, and I use the same batch of Lapua brass in both. I found out quite soon that with one of them (call it barrel A), some cases needed shoulder bumping after the first firing on moderate loads (2830 fps), and after several firings, virtually all cases needed it after every firing. So for consistency I now FL size them every time. With barrel B however, shoulder bumping is hardly ever needed, even on hotter loads. I also noticed that much-fired cases from barrel B won't chamber in barrel A. So I did a measurement of the fired cases from both barrels to get a head to shoulder length comparison (using calipers and the body of a Wilson neck sizer die). The cases from barrel B are about 0.003 longer than from A, i.e the chambering in barrel B is deeper by 0.003.

Question : Is this difference in chambering depth likely to be the major reason for the different shoulder bumping needs with the cases, or is it likely to be something else wrong with barrel A?

The reason I'm asking is that I own the reamer and am considering deepening the barrel A chamber by the 0.003. One further question please - could this be done by hand i.e. without a lathe?

Any advice much appreciated.

Alan
 
In benchrest quality gunsmithing variation in chamber length (beyond about 0.002") is inattention by the gunsmith.

Chamber body diameter variation beyond about 0.001" is caused by an improper chambering process-like for example, using a "floating reamer holder that didn't float.

With your situation where f/l sized cases in one chamber does not fit the other could be caused by one of the conditions above PLUS a f/l die that is not sizing the cases enough.
 
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To answer part of your question: Yes, it can be done without a lathe. You still need a pretty good setup, so as to not put side pressure of the reamer; there are ways to do that.

BTW, you would be amazed at how little effort, even turning the reamer by hand, is needed to remove .001 or .002. Fairly hard to do. First time I tried this, I went about .002 too far, and then had to do the other barrel... ;-)
 
What you need to check is the diameter of the fired cases. A chamber that allows the brass to expand too much, (especially at the web), will cause the senario you are alluding too. The problem might be that the cases are not sizing as they should, giving the false impression that it is the 'bump' causing the problem.

Your measuring technique might be adding to the problem. You really need the proper tools, that being, a gizmo that seats only on the shoulder with no body involved to avoid getting false readings.

It sis sort of like the shooters who insist on measuring the shoulder bump by the "feel" method. Other factors can give you a false indication, resulting in an excessive amount of clearance between the case head and the bolt face.........jackie
 
Thanks all for the advice.

The main reason for the difference in chamber length is that it was 2 gunsmiths. I should have supplied a fired case for the second one.

It sounds like none of you are saying that the shorter chamber was too short for the brass? I know that some experts say you should make the reamer to suit the brass, but I guess that's more for case diameter near the head rather than length to the shoulder? It isn't rediculously short because new brass chambers without a problem.

Jackie,

I have a Redding instant indicator with 6.5-284 insert, but the 7 Shehane is too fat for it. I may see if Redding's custom die shop will come to the party and make one for the Shehane.

Alan
 
jackie schmidt said:
What you need to check is the diameter of the fired cases. A chamber that allows the brass to expand too much, (especially at the web), will cause the senario you are alluding too. The problem might be that the cases are not sizing as they should, giving the false impression that it is the 'bump' causing the problem....
Click to expand...
Jackie,

I've had another think about this, and believe it is the 'bump'. Because I've occasionally picked out the one that's hardest to chamber and used Redding competition shellholders with the FL die, sequentially starting with the 0.010 shellholder until the case chambers easily (usually the 0.004 does the trick). Doing this, the change from hard to easy chambering has always been quite sudden. If it was a body diameter issue, I'd expect the improvement to be more gradual, particularly as the Shehane has a relatively mild body taper.

So if it is a length to shoulder issue, do you have any ideas different from the other posters?

Thanks for your time.

Alan
 
This is a good argument for the use of a steel go gauge to set chamber headspace...

I think it would be easier to set the one barrel back 3 thou to match the other barrel than for an inexperienced person to deepen a chamber by 3 thou.
 
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