Chamber length vs brass trim length clearance

[zepheris.hk]

Hey fellas,

I just wanted to know what the general rule of thumb is concerning brass COAL and chamber length. I already found my chamber lengths, just want to know what the recommended case neck length clearances are.

.223 AR:
1.783" chamber length
BOOK trim length: 1.760"

6.8SPC AR:
1.708" chamber length
BOOK trim length: 1.676"

I was thinking something along the lines of .005-.010" clearance from the chamber?

Any insight or feedback would be greatly appreciated.

 

[deadly0311]

Re: Chamber length vs brass trim length clearance

i trim mine to where its .005 shorter than what the book recommends as the trim to length

 

[zepheris.hk]

Re: Chamber length vs brass trim length clearance

I was looking to keep the brass as long as possible to aid in gas seal and consistency. I bought the Sinclair chamber length gauges to determine how long my chamber was and trim accordingly as opposed to going with the book trim lengths.

 

[-edgecrusher-]

Re: Chamber length vs brass trim length clearance

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: zepheris.hk</div><div class="ubbcode-body">I was looking to keep the brass as long as possible to aid in gas seal and consistency. I bought the Sinclair chamber length gauges to determine how long my chamber was and trim accordingly as opposed to going with the book trim lengths. </div></div>

the problem with that is you won't be able to fit them in your magazine without some serious setback on your projectiles, those gauges are great for bolt guns, long bullets and long cartridges in gas guns can be problematic very quickly...

 

[zepheris.hk]

Re: Chamber length vs brass trim length clearance

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: -edgecrusher-</div><div class="ubbcode-body">
the problem with that is you won't be able to fit them in your magazine without some serious setback on your projectiles, those gauges are great for bolt guns, long bullets and long cartridges in gas guns can be problematic very quickly... </div></div>

My bullet seating depths will remain the same, just what brass length clearance is optimal is what I'm trying to figure out. I've shot my bolt/gas guns with longer than book max trim lengths fine, but was never able to gauge it until now. Since I can gauge it accurately, I wanted to trim it to X length and help gas seal/consistency.

 

[sobrbiker883]

Re: Chamber length vs brass trim length clearance

I'd trim .005" shorter than the shortest measurement you get when you measure with the Sinclair gauge 5 or more times.

Then you should loan your Sinclair gauge out to people

 

[zepheris.hk]

Re: Chamber length vs brass trim length clearance

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: sobrbiker883</div><div class="ubbcode-body">I'd trim .005" shorter than the shortest measurement you get when you measure with the Sinclair gauge 5 or more times.

Then you should loan your Sinclair gauge out to people

</div></div>

Thank you for the confirmation. I figure .005" would be good to start out with.

As for the gauges, I figure since they were only a few bucks, it's be more trouble then it's worth to loan out and shipping.

 

[Clark]

Re: Chamber length vs brass trim length clearance

The load book's "one size fits all" instruction says:
Max 1.760"
Trim to 1.750"

My formula for case trimming is:
Max case length = chamber length from breech to end of neck - [chamber shoulder head space - brass shoulder head space]

What does all that mean?
The case can be as long as the chamber, minus how much forward the firing pin can push the cartridge.

So if my worst case 223 rifle [Bushmaster AR15] is 1.786" to the end of the chamber neck,
and the same rifle has a shoulder headspace 1.4716" [.008" over SAAMI minimum],
and my pool of handloaded .223 ammo for all my .223s has a shoulder headspace of 1.4626" [.001" less than the SAAMI max],
Then the max case length is 1.786" - [1.4716" - 1.4626"]= 1.777"

If I run 66kpsi and push the shoulder back .009", the case length is going to grow ~.009" per firing, and I would be trimming every time I FL resize the brass.

But if I shoot my Ruger #1V .223 instead, and Lee collet neck size instead of FL resizing, 66kpsi will get me ~ 20 firings between trimming.

 

[zepheris.hk]

Re: Chamber length vs brass trim length clearance

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Clark</div><div class="ubbcode-body">

My formula for case trimming is:
Max case length = chamber length from breech to end of neck - [chamber shoulder head space - brass shoulder head space]

What does all that mean?
The case can be as long as the chamber, minus how much forward the firing pin can push the cartridge.

So if my worst case 223 rifle [Bushmaster AR15] is 1.786" to the end of the chamber neck,
and the same rifle has a shoulder headspace 1.4716" [.008" over SAAMI minimum],
and my pool of handloaded .223 ammo for all my .223s has a shoulder headspace of 1.4626" [.001" less than the SAAMI max],
Then the max case length is 1.786" - [1.4716" - 1.4626"]= 1.777"

If I run 66kpsi and push the shoulder back .009", the case length is going to grow ~.009" per firing, and I would be trimming every time I FL resize the brass.
</div></div>

Your formula is very helpful and exactly what I was looking for. Thank you.

 

[mdesign]

Re: Chamber length vs brass trim length clearance

I also use Sinclairs chamber length gauge and trim .005 shorter than the actual chamber length. The thing you need to know is that when using this method your actual trim length may be very different (usually longer) than the "recommended" trim length for most cartridges. It is not uncommon to be running your brass .020-.030 longer than the "recommended" length.

Always wondered why it was this way. On a factory chamber, I can see it due to the accepted tolerences in the mfg process but on a minimum spec reamer used to make a custom chamber I would think the length would also be close to SAMMI.