Case trimming question?

I feel I am missing something obvious but I see case trimmers that rest on the shoulder to determine how much of the neck needs to be trimmed. It is touted as being more accurate than trimming from the OAL of the case.
Wouldn't the shoulder method require all shoulders to be the same requiring tighter tolerances when sizing or mixing sized and unsized cases? How can it be more accurate than just trimming off the OAL of the case?
Not trying to start a war here but just looking for info.


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I think it's relevant to neck sizing(fire formed)


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Alderman,

Typically trimming is done after sizing. As such your case shoulders should be fairly uniform (from the chamber if neck sizing as MJC indicated or from the sizing operation if using a FL die) and thus a consistent shoulder should be a given. Brass spring back and other issues may cause some shoulder variation, but if it is enough to significantly affect the trim length you normally will have other things to worry about.

Accuracy claims are based on the shorter distance being used for trimming (less wobble, no "run-out" as you can get with a lathe or weakly supported cartridge). You also don't deal with the nicks, burrs or ejection marks on the casehead that mess up the case to cutter alignment or add phantom length to the case.

Edit - I had a lathe style trimmer with which I achieved a consistent variation of 0.003-0.004" on the trim length and uneven mouths. I'm not saying the trimmer was at fault, just that it didn't work well in my hands for my guns. For guns that mess up the case heads (semi's, plunger ejectors, etc.), a trimmer that indexes off of the shoulder seem to work about as well as can be expected.

I could easily be wrong as I have never used a trimmer which indexes off of the shoulder, but when I was looking at the WFT the above was the general answer given to me.

Measure your chamber using a sinclair chamber length gage, they come in whatever caliber your wish to use them in. They're only like $7.00 Whatever that measures, trim less .024".

I'd buy a Wilson chamber gage in the case you have (fired), and a sinclair/wilson trimmer. These will last two lifetimes. This will square up the case to the trimmer's head.

You don't have to use that trimmer, but the above product will tell you what the length of your chamber actually is.

I looked at the Sinclair CLG. Looks like a simple way and cost effective way to get that info.

Is there a reason behind the trim being .024"?


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I've used the Sinclair inserts for a number of years and I'm not sure what you guys are talking about as far as the .024" goes.
I do however deduct about .005 just to make up for variations in headspace and closeness of measurement.
I have never seen a factory rifle that doesn't have at least .015 more room for growth than Sammi specs and often more than that. They save me a ton of time.

Thanks for the info.
So just to verify, after you determine the chamber length with the gauge you are good to go with trimming your brass .005 shorter than that length?
Am I correct in assuming the brass stretches some upon firing so a slight trim is required after each firing? Or do you go with another reloading if you still have some room to grow?



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Once I determine my guns new and accurate trim length I can technically let my brass grow to up to .005 of that length before trimming. What really happens in real life is brass never grows identically the same amongst all your pieces of brass...so lets say I have a batch of 20 hunting brass. Once a couple get near that .005 threshold the whole batch gets trimmed.
I use redding body dies and Lee collet dies for a lot of my reloading so I never really get a lot of case stretch. With cheaper brass like Remington and Winchester I might trim it after the first firing just to get that case mouth nice and straight. With better brass like Lapua and Norma I probably wouldn't worry. In most cases for the life of the brass I might most likely only trim one time. And in some cases with belted Magnum brass I might never have to trim it before it's worn out after about 6 or 7 full power loads.

Alderman":33r2yuo8 said:

I looked at the Sinclair CLG. Looks like a simple way and cost effective way to get that info.

Is there a reason behind the trim being .024"?


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Click to expand...

This was the amount Fred Sinclair had listed in his Precision Reloading & Shooting Handbook and I've used that number for more years then I can remember. That number leaves enough plus or minus vs. .005" that I wouldn't want a case mouth coming into contact with the very end of the neck in the chamber? Wiggle room so to speak. But that's just my opinion.

Basically if you look at the SAAMI Specs for the chamber of the cartridge you're using, and the minimum chamber length vs. the maximum length of the case. Obviously there's a safety reason for that difference, and a chamber could be cut rather long and never need trimming, or short. In that case that's a problem.

Here's their video on the tool fwiw....

https://youtu.be/JKnXJxaAw2U

Alderman":31f0ppl9 said:

Thanks for the info.
So just to verify, after you determine the chamber length with the gauge you are good to go with trimming your brass .005 shorter than that length?
Am I correct in assuming the brass stretches some upon firing so a slight trim is required after each firing? Or do you go with another reloading if you still have some room to grow?



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Click to expand...

OK so I went to see where I found that number I've used for the last 17 years, and it's clearly written in the instructions that come with the Sinclair Chamber Length Gage Insert:

WARNING: Excessive case length will cause dangerous chamber pressures resulting in personal injury. We do don't recommend increasing case length beyond published SAAMI specifications. DO SO AT YOUR OWN RISK!! If you do increase your case length maintain the case so that it is a minimum of .024" shorter than the chamber length.

Almost all reloading manuals use a minus (.010") trim to length, but that's already taking into account the difference between the minimum chamber length and the max case length. So if I use the length I've found with the chamber length gage as an average absolute max length, I'll deduct (.024") as the trim to length.

I personally would feel more comfortable having a safety margin of .024" vs. .005" which by the way is about the thickness of a single human hair. The case needs to stretch and expand under fire and IMO .005" is way too close for a trim to length from the average reading one would get using this method. I for one tend to listen to the instructions from the manufacture and in this case Fred Sinclair, whom I've respected and learned from for at least two decades. So this is the method I use, and I would highly recommend that if anyone uses a chamber length gage that they maintain the recommendations using this product that's all.

Well.... that got my interest up and I know I have the original cards that came in the bags that the inserts came in so I just went and checked them and it says none of this. And that whole idea pretty much shoots the virtue of the insert pretty much down the tubes and makes the video look pretty useless too. I virtually never shoot without going over a chronograph and now have a lab radar set up.... I have never had a problem or seen an unexpected Spike while using these inserts over probably 15 or 20 years myself. So...... each one of us will have to decide how we want to handle it from here on out but I'll probably continue doing what I'm doing until I actually see a problem. The whole statement above seems a little bit lawyerish but who knows?

Another thought......"if" cases were to stretch out immensely and come in contact with the end of the chamber wouldn't they remain in that state after firing? they certainly couldn't contract afterwards.....so.... if we check our case length after firing and it's nowhere near our total chamber Dimension what would we have to worry about?

Not really if you pull out your calipers and look at the distance between the blades of what .005" looks like, and ask yourself that question?

If the shoulder or the belt from one case to the next isn't the exact same as every other round, and just so happens to run long into the end of the chamber neck, it won't open fully.......hence a pressure spike! But I don't hold my tolerances that close when it comes to neck length. I like a good measure of safety margin when it comes to that, but Hey...... that's just me!

Well...I may back my limit down to .010. I ALWAYS check my shoulder bump when reloading via the Hornady tool. I realize case expansion via belted magnum cases can be interesting.... I've seen virgin belted mag brass blow shoulders forward over twenty thousands of an inch on the first firing. But once they're fired formed I'm watching those shoulders there too. I have NEVER measured a fired case that was less than .005 from chamber length via the Sinclair tool.

Hey you may do whatever works for you, and whatever you're comfortable with?

On another note, this is why I don't use guns with tight neck chambers..... if by chance the neck is too thick, and can't fully open to release the bullet there's just another issue that can happen! Sure, the bullet will go down the barrel. It's the same basic principle, and frankly I seriously doubt that will make a difference in the accuracy, period!

The true purpose of why Fred Sinclair came up with this was Factory chamber's were sometimes cut too long, and the cases were being excessively trimmed too short.

I've used them to check the length of wildcat cartridges I had chambered, and make sure what I was doing was safe. One such wildcat was the 6.5/300 WSM I had made up years ago, and the chamber neck was cut so far out of spec. I will never need to trim the cases! Shilen even got it wrong the second time around, but they did reduce it by .0135" yet still too long at 2.145" vs. the 2.120" called out for the chamber.

The trim to length for a 300 WSM is 2.090" and a max length of 2.10" which still even at that max length leaves me .045" to grow into before the case would even come into contact with the end of the chamber's OAL. That's excessive, and I worried whether or not carbon would build up in that gap because the case neck can't chip away at it during expansion like you said in your previous post. I can't say since I don't have a bore scope, but as far as accuracy goes...... that gun shoots 1/4" groups, and it's nothing but a Montana Kimber with a Shilen match select ratchet four groove 1:8 barrel. Point being even with that excessive gap at the end of the case the gun still shoots bugholes, and way out there too!

The same could be said if the chamber length was cut too short. How would anyone even really know what the true length was? We check for head space, and case dimensions, so why not know what the chamber length is? Since I personally have never seen a degradation in accuracy by the case neck being too short in a chamber vs. too tight which I have witnessed causing serious issues I tend to stay on the side of caution, and a decent margin of error.

kraky1":1835rlp3 said:

Here's their video on the tool fwiw....
https://youtu.be/JKnXJxaAw2U

Click to expand...

Another way sacrificing a fired or new stick of brass.

1. Trim/square the neck of a FL sized case w/ .002- .004" shoulder bump, inside-outside chamfer.
2. Cut .100"-.200" or so off it off making a "ring" of brass. I/O chamfer the cut off end.
3. Re-trim-square the case an additional .100" or so shorter and I/O chamfer.
4. Seat a bullet in the case a tad short and just start the "ring" on the bullet major diameter.
5. Chamber and remove. When the end of the ring contacts the end of the neck portion of the chamber it will be seated (pushed back) to length.
6. Measure from the case head to the forward edge of the ring to determined the max length for that chamber.

Bill

That's pretty cool and certainly should be effective Bill...... but for guys like me who don't have the experience or tools to make those nice straight cuts I'm going to have to Fork out the whopping $6.95....lol.

What the heck. I ordered some up so I might learn something soon.
Thanks for all of the info.
Got my curiosity up now.


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