Getting a half moon cut on case mouths, is this an indication of a mechanical problem with the chamber alignment or the bolt face not being perpendicular to the bore? Due to the weather in the noreast I've had plenty of time on my hands. Decided to true up some match 6PPC brass to get ready for the season. All the brass was fired five times. One box of fifty was long enough to get a full 360 cut on the case mouths using a Wilson trimmer. The next box were only .001 to .003 longer than my 1.490 trim length but decided to make them uniform. 35 to 40 of the cases came up with the half moon appearance where some part of the case mouths weren't cut at all, some more than others. How detrimental is this to accuracy? Am I thinking in or out of the box? Need your opinions.
Case Trim obsevation
- Thread starter stifffingers
- Start date
Can you post pictures of what you are trying to describe?
I think what he's describing is case mouths not being flush from the beginning. Therefore only half of case mouth is trimming.
I generally grab a handful of brass and measure for the shortest piece. Now i set my trimmer to this piece so as to trim and set all cases to same length. I feel you have a more consistent round especially if your crimping.
I know the half moon can only be the difference of a couple thou.
But consistency and accuracy is what I'm after.
Trim to length doesn't mean minimum case length, just a couple few thousands less neck. Coal is coal.
Straight walled cases that head space off of case mouth may be different tho. That question would be better for somebody with more experience than my self.
I generally grab a handful of brass and measure for the shortest piece. Now i set my trimmer to this piece so as to trim and set all cases to same length. I feel you have a more consistent round especially if your crimping.
I know the half moon can only be the difference of a couple thou.
But consistency and accuracy is what I'm after.
Trim to length doesn't mean minimum case length, just a couple few thousands less neck. Coal is coal.
Straight walled cases that head space off of case mouth may be different tho. That question would be better for somebody with more experience than my self.
I had mentioned that all the cases were fired five times. The first two firings were in a no turn chamber for fire forming. Then sized, trimmed to length for proper neck turning to .009 for a .263 neck. On second look at my records the second box of fifty were shot four times, thus the difference in length. I use a custom sizing die by Harrels.I wouldn't worry too much about it.
You didn't measure them when new, so you can't say for sure that they weren't made that way.
Not everything is Perfect all the time. (except me)
Last edited:
Deleted old_dood
three different trimmers I've used all did the same thing. that includes a Forster, a Wilson, and a WFT.
I'm not questioning the ability of the trimmers, but perhaps the differences may be a fault of the rifles chamber or bolt. If it's a realistic possibility then how much of an impact will it have on accuracy. That is the ultimate objective in Benchrest. I forgot to mention the cases are all Lapua 220 Russian formed to 6PPC. I am aware of variances in neck thicknesses but to have so many cases displaying the same attribute and especially after careful neck turning?
Last edited:
Deleted old_dood
I'm not qualified to answer that since I only shoot off a bipod to 200yd max. I've shot reloaded brass with up to .010" difference in length and didn't see an appreciable difference in accuracy that "I" would be concerned about. YMMV.
My Wilson did it also until I bought the sharkfin clamp.
For an answer to how trim length matters in Benchrest- go to You-Tube and search Jack Neary BR seminar at Super Shoot. IIRC he found holding the length tolerance to with-in 0.002" was sufficient.
Can you post pictures demonstrating this "half moon" shape? Thanks
May want to check case run out. Compare a normal cut case to the moon cut for concentricity
Maybe a quarter moon would be more appropriate? Reality is, the mouth of the case isn't parallel to the base so when trimming, only one side of the mouth is getting cut by the trimmer.
I fully understand why the cutter isn't trimming the whole case mouth, unless you want to trim off more than you like. My curiosity is why do the case mouths become not parallel after being prepped properly and being parallel beforehand.
Ive always assumed its because of the body of the case isnt perfectly equal all the way around in thickness so some parts squeeze more than the other. I bet you those half moons didnt actually remove much material at all either. Just chamfer and deburr and youll be fine
Brass is thinner on one side, it happens during the drawing process. It is perfectly normal and hard to overcome.
This causes the brass to grow unevenly, it’s normal and will arrest itself after a few firings.
RCBS tried to arrest case growth by the invention of the ‘X’ die. It works, but causes thick necks iver time.
Your trimmer is fine. I have the Redding mini lathe and the RCBS Trim Pro, both trim to exactly what I set them at if the burr is removed. I often get uneven cases and only one side is trimmed.
Cheers.
This causes the brass to grow unevenly, it’s normal and will arrest itself after a few firings.
RCBS tried to arrest case growth by the invention of the ‘X’ die. It works, but causes thick necks iver time.
Your trimmer is fine. I have the Redding mini lathe and the RCBS Trim Pro, both trim to exactly what I set them at if the burr is removed. I often get uneven cases and only one side is trimmed.
Cheers.
Upgrades & Donations
This Forum's expenses are primarily paid by member contributions. You can upgrade your Forum membership in seconds. Gold and Silver members get unlimited FREE classifieds for one year. Gold members can upload custom avatars.
Click Upgrade Membership Button ABOVE to get Gold or Silver Status.
You can also donate any amount, large or small, with the button below. Include your Forum Name in the PayPal Notes field.
To DONATE by CHECK, or make a recurring donation, CLICK HERE to learn how.
Click Upgrade Membership Button ABOVE to get Gold or Silver Status.
You can also donate any amount, large or small, with the button below. Include your Forum Name in the PayPal Notes field.
To DONATE by CHECK, or make a recurring donation, CLICK HERE to learn how.