Chamber Length Gauge


Sep 17, 2013
A couple of weeks ago I heard about a "Chamber Length Gauge" that one uses to measure how long the rifle chamber's neck is so that one can trim the brass to the correct length. I can't remember now where I heard of this tool. I suspect it was on this forum, but I can't remember. This is the tool:

I bought a couple for the calibers that I most load, .308, 7mm, and .277. I used the .308 one today on my new 300 Win. Mag. This chamber is tight for a hunting rifle in most dimensions. The loading manuals give a maximum case length for a 300 Win. Mag. of 2.620" with a trim length of 2.610". Using this gauge, I measured an overall chamber length of 2.668", .048" longer than the stated maximum case length. Or, in other words, the neck length of the chamber is .048" longer. I was very surprised. I guess the only reason I will be trimming brass for this rifle is just to try to keep them a uniform length. Theoretically, I could let the necks grow long enough to have a bullet's diameter length neck.

Has anyone else experienced this?

Dan, I have a number of these gauges, and I have used them in the past. They can be quite revealing. What you describe is not as unusual as you might expect. It does allow one to trim to fit the rifle rather than cutting to the printed standard.
Just a thought, leaving a gap above the neck would accumulate carbon leaving a carbon ring, would allowing the neck trim to be longer than SAMMI specs help with this issue?

You are correct. As I said in that post, "This is why I love this forum. There are always things to learn. I never heard of this solution before. Thank you for bringing this up. Dan"

I also learned about making false shoulders on this forum and am using that technique on the first firing of my brass for the 300 WM. In fact, I have learned many things here. This forum has been an invaluable resource for me. There are so many helpful people who are willing to share their experiences and help those of us who are less knowledgeable.

In addition to the knowledge and techniques learned here, I have gained some very good friends. Many thanks to all of you who post here and thanks to Nosler for hosting this forum. Dan
I never used these , as for trimming I have Lee case gauge for each cartridge I own. They are a simple design , and I never have worry about adjusting a trimmer for different length cases.
I use Hornady headspace measuring tools. Used to measure fired case OAL, they're fine for determining the trim length to use when customising your brass to each rifle. If cases are trimmed to SAAMI spec, essentially you're trimming too much off your brass & in my view, trimming to SAAMI spec is for those who want to use the same brass in several rifles of the same calibre. So, if you're gonna use your brass in your same rifle, the Headspace dimension is what you're looking to keep consistent. So you can set your sizing die to consistently trim only .002"/.003" off each time.
Cheers & good luck
here is a picture of the chamber length gage . clean the bbl real good before using . otherwise you could be measuring to the carbon ring . this tool measures your maximum case length . I keep trimmed to .010 shorter than my max length . the tool is reusable . drill out the primer pocket so you can push the tool out after use . trim the neck short , I'm guessing .150" . resize the neck . when you chamber the case with the tool slightly started into the case , the tool gets pushed into the case . measure and this is your max case length . it's a very simple tool to use .

just remember ; you no longer have a built in grace range . when you exceed this length you could be in trouble .

Today, almost everyone has a $50 Teslong bore scope. If you put a piece of brass in the chamber and then run the scope in from the muzzle, you can see exactly how much room there is between the brass mouth and chamber end. There is probably a lot more room than you think. Chambers are always cut longer than the "trim length" listed in the reloading manuals IMO. I never cut my cases back .010" anymore. It just leaves more room for hard carbon to build up.

Using a bore scope will also give you the opportunity to see if there is a problematic carbon ring at/near the case mouth. Using the gauges listed above, you won't know if the gauge it is hitting a carbon accumulation or the end of the chamber.
I trim my brass when I start to get resistance closing the bolt. One of these days I'm going to get a bore scope. Then I ll probably wish I hadn't!
Good stuff fellas.

I don’t allow a carbon ring to build up. Kroil or fuel injector cleaner will usually wipe any carbon out of the chamber and bore with just a couple patches. I keep it simple. I’ve always used the LEE case length gauge/trimmers. I use it after each firing. After a few firings all brass is the exact same length and may provide a small degree of consistently. With fresh brass not all will be trimmed to begin with, but after a few firings cases stretch and all are the exact same length. I think it keeps neck tension uniform. I guess it’s 6 one way and half a dozen the other.