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Thread: Case Trimming Question

  1. #1

    Default Case Trimming Question

    The maximum length for the 300 WSM case is 2.10 My once fired cases range from 2.082 to 2.92 I was planning on trimming them to 2.90 Should I trim to a shorter length? What are the probable differences in results fron longer to shorter cases?

  2. #2

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    correction... cases range form 2.082 to 2.092

  3. #3

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    My unfired Norma brass (from the same bag of 100) is much more consistant, ranging from 2.090 to 2.093. It looks like my fired and neck sized brass actually shrunk??? What gives?

  4. #4
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    I would not trim unfired unless they are greater than the 2.100" maximum or need it to get rid of ragged edges. Then if the fired cases are from 2.082" to 2.092" you could either leave them as they are or trim them to 2.080" .020" less than maximum length.

    Consideration for making them all the same length would be if you intend to crimp they will work best for setting the crimp die correctly. It will not hurt them to be a little shorter than maximum we just normally trim .010" less than maximum to give the case room to grow at the firing without interferring with the chamber throat. I've trimmed many cases to be shorter that the .010" of max and it is quite common for new brass to be shorter than the trim length. Also when neck expanding brass to another caliber it usually gets shorter than the design length. often the isn't considered in a cartridge design.

    Another reason for trimming is to get rid of an uneven case mouth and you can just set the trimmer to take off no more than is required to true up the mouth and that will be at the length that it is and we can live with it. Also I have found that uneven or ragged case mouths tend to lead to neck splits and I want to get rid of that. Most new brass, Norma and Lapue excluded, will be bent and ragged and uneven right out of the box and is not usable with out a trim. These new cases will get neck sized then trimmed to be true and unifrom. I have had such a problem with my new Winchester brass lately.

    Your WSM will have very little case stretch and rarely require trimming.
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  5. #5

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    Well this morning I decided to trim them to 2.085 and loaded up 42 cartridges in 3 shot groups of various powder weights, seating depths and 3 types of bullets.

    I didn't get out to shoot until late evening due to helping a friend move. Only got six shots fired, 2 groups of 3. first group was a max, and possibly a max plus load (66 gr H4350, 189 ETips). Second group was definitely max plus (66.5 gr). The chrony only gave one reading (probably because of twilight conditions), second shot of first group, @ 3236 fps. I'm skeptical of that...way higher than the last 2 shot group with 66 gr(about 3020 fps). Dont ask about the group size.

    Anyway, my intent was to let the brass grow eventually to 2.090... OK, so I get home and just measured the six fired cases. They range from 2.079 to 2.083 Cases are still shrinking. I am wondering if the fire forming, stretching in all directions is pulling the neck length down?

    I would not trim unfired unless they are greater than the 2.100" maximum or need it to get rid of ragged edges
    So you don't think different length case will affect accuracy?

    And right now, with these cases shrinking after each firing, I dare not do much more trimming.

    Side note - unfired case shoulder width is .532 - .533, first time fired/necksized case shoulder width is .537 - 538 as is the second time fired cases.

    The new Norma brass had fairly even mouths but many were irregular in shape and definitely needed necksizing.

    And Yeah, I have heard from others that WSM brass doesn't grow much, but I haven't heard about shrinking??? Hopefully this will stop soon

    Well tomorrow afternoon should be an interesting day at the range, hopefuly will get some chrony reading and better groups.

  6. #6
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    This shrinking of length is from the shoulder expanding into the chamber and pulling the brass down. Not a bad thing. The WSm design is very strong and very straight, very little taper. Likely they will not reach the 2.100" max length. Here is where neck sizing only will reduce working the brass. If your dies size that dimension down (shoulder diameter) the cases will grow a bit then shrink when fired again. We don't want to alter the chamber fit if it does in fact fit. SAAMI dimensions aside, we don't want to alter that case that has conformed to you chamber. This assumes the chamber dimensions are not lop sided or bulged on one side.

    And I do think varying neck length will affect accuracy, can't say how much but neck rtension is a factor and length of the neck to bullet bearing surface is a factor. I just suggest that you trim to same length and since some are 2.08 and some 2.09 trim all to 2.08" I would want all the same length evenif it was a few thousandths short.
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  7. #7

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    I only got another 6 rounds fired again today. I could have got some more but after the 4th shot it got too dark for chrony readings. Velocities are good, averageing about 3030, but group sizes are not good, 1.5, 1.7 and 1.8 so far.

    [quote]
    Quote Originally Posted by Murphy View Post
    This shrinking of length is from the shoulder expanding into the chamber and pulling the brass down. Not a bad thing. The WSm design is very strong and very straight, very little taper. Likely they will not reach the 2.100" max length. Here is where neck sizing only will reduce working the brass. If your dies size that dimension down (shoulder diameter) the cases will grow a bit then shrink when fired again. We don't want to alter the chamber fit if it does in fact fit. SAAMI dimensions aside, we don't want to alter that case that has conformed to you chamber. This assumes the chamber dimensions are not lop sided or bulged on one side.
    Yup, I agree, I like my cases fitting my chamber. I think if the chamber was lop sided I would get occasional hard locking cartridges. I did get one, but only one out of 12 second firings so far.


    I just suggest that you trim to same length and since some are 2.08 and some 2.09 trim all to 2.08" I would want all the same length evenif it was a few thousandths short.
    I hear ya and agree except, I don't want to be chasing these to shorter and shorter dimensions. I might trim to 2.082 and see what happens after the 3rd firing. Also, I wonder how the custom FL dies will affect the length once I start using them?

  8. #8

    Default The Latest

    When I neck sized the recently fired and shrunk cases they all grew about .001-.003 I trimmed the down to 2.083 and fired them again today and I think I found the sweet spot.

    I loaded 3 shot groups for 65, 65.5 and 66 gr of H4350 and the 180 gr ETips at 2 different seating depths. The first group @ 65 gr was 3.3", the second group @ 65.5 gr was 1.9" and the 3rd group @ 66 gr (max) was .545" That made me a happy camper This will get me through the antelope opener and then I'll fine tune some more.

    Also, noticed something else interesting. I pulled some bullets from some cartridges that were over max that had been trimmed to 2.085 and found many of them had shrunk to as short as 2.080. I'm guessing this was due to the seating process pushing the neck down.

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