Calculating Trim Length

[geraldgee]

Using Sinclair's case length gage, I measured the chambers in my Dasher and 6.5 Lapua.

Is there a 'rule of thumb' formula to determine trim length once you measure the chamber?
Do I trim .020 shorter? More? I wouldn't think I would want to trim less than .020 from the overall chamber length but I am asking for some direction.

Thanks in advance for your help!

Gerald

 

[CH Luke]

Using Sinclair's case length gage, I measured the chambers in my Dasher and 6.5 Lapua.

Is there a 'rule of thumb' formula to determine trim length once you measure the chamber?
Do I trim .020 shorter? More? I wouldn't think I would want to trim less than .020 from the overall chamber length but I am asking for some direction.

Thanks in advance for your help!

Gerald

Last recommendations I picked up here was .010 less. For me that meant most new 308 Lapua brass was going to take a lot of firing to get that long. On my latest batch I have two firings and I'm focused on consistent length so just some touch up likely but nothing like I used to do.

 

[Scott Harris]

My .02, this is strictly a clearance issue....no "right" number other than making sure you have enough clearance for safety and all the necks are the same. Just listened to Jack Neary talk about it, he said he was using .035" and did not give the exact number much thought, BUT he did make sure all the necks were trimmed identical.

PS. if you care what Jack thinks, .010" or less clearance is "bad juju."

 

[Zero333]

I trim once they grow between 0.020" and 0.015" less than chamber neck length for my bolt guns.

If you're sizing brass with a 0.002" shoulder bump, 0.015" clearance is plenty. If you're full length sizing, 0.025" clearance is better.

Gus guns I don't let them grow more than saami max.

 

[Dusty Stevens]

You really need to find out your reamer specs for trim length. For now trim em all to the shortest one. Measure that and keep em all that short

 

[geraldgee]

Great video - thank you for sharing it.

I appreciate everyone's thoughts and advice. Thank you for the help.

Gerald

 

[fguffey]

Thanks in advance for your help!

To understand trim length is to understand measuring the length of the chamber from the end of the neck to the bolt face. It gets complicated for those that can not understand there are two lengths. There is one length that is measured from a datum to the end of the neck and there is another length that is measured from a datum to the case head.

I understand reloading manuals list maximum case length and a ‘trim-to-’length’. I have one chamber that has .016” added length between the shoulder of the chamber to the bolt face. I could mindlessly measure the length of the case, check the book and then trim cases that do not require trimming.

And then there are machines that are used to trim cases. Some set up on the shoulder of the case with total disregard for the length of the case from the shoulder to the case head.

F. Guffey

 

[Dusty Stevens]

Show us a video on what youre talking about guff

 

[mike a]

You really need to find out your reamer specs for trim length. For now trim em all to the shortest one. Measure that and keep em all that short

Hi dusty, I guess I'm in the (for now) catagory as I don't really really know my exact chamber dimensions. I'm using a Whidden fl bushing die and on my $new lapua brass I'm planning on (at once fired) to trim to the shortest one. You think.

 

[Dusty Stevens]

Trimming to the shortest then staying on top of it would be my way. Be sure to watch for any pressure

 

[JimPag]

Jim Bordon told me you should trim 0.012-0.015 shorter than your chamber length.

 

[Brians356]

You really need to find out your reamer specs for trim length. For now trim em all to the shortest one. Measure that and keep em all that short

He already measured to the end of the chamber neck w/ Sinclair gauge. That's reality and all he needs, vs a reamer spec, or even having the reamer in hand.

 

[savagedasher]

Without the reamer specifications. Is just a guess. How tight you want your cases to be is up to the reloaded My self I like .008 off the reamer specifications. I can get two reloading and still feel safe.
Larry

 

[NorCalMikie]

Best bet is to know the "actual measured length" of your chamber. My guess is the chamber is cut about .020" longer than the max trim length. Measure it, write it down and refer to THAT measurement before you start trimming. My bet is you probably won't need to trim. Depends on which type of die you use. Precision shoulder bump dies keep the OAL from getting longer. 25 to 30 reloads using bump dies and still no need to trim.

 

[savagedasher]

Best bet is to know the "actual measured length" of your chamber. My guess is the chamber is cut about .020" longer than the max trim length. Measure it, write it down and refer to THAT measurement before you start trimming. My bet is you probably won't need to trim. Depends on which type of die you use. Precision shoulder bump dies keep the OAL from getting longer. 25 to 30 reloads using bump dies and still no need to trim.

I never have see a chamber that was different then the reamer specifications. Larry

 

[Dusty Stevens]

How can a hornady gage tell how long a chamber is?

 

[Brians356]

How can a hornady gage tell how long a chamber is?

You mean the so-called Lock-N-Load Headspace Gage? I use mine that way, and posted on this forum how to do it (with photos) but it seems to have been lost, looking for it now ...

Found it, my post on 13 June 2015:

http://forum.accurateshooter.com/threads/headspace-gauge.3875237/#post-36585857

 

[fguffey]

A reloader with a little back ground in case forming can measure the length of the chamber from the beginning of the throat to the bolt face..

A reloader with a reamer in hand should be able to determine the length of the case neck in the chamber. That leaves that other length; the one that includes the distance from the shoulder to the bolt face.

F. Guffey

 

[savagedasher]

A standard 22 rim fire case with out a bullet Put in the Lock and load head space gauge works in a 6mm
.2704 and .272 neck. Though every one used that. Larry

 

[Brians356]

A standard 22 rim fire case with out a bullet Put in the Lock and load head space gauge works in a 6mm
.2704 and .272 neck. Though every one used that. Larry

Probably because they paid attention to my post on it. Thanks for the plug. It's more accurate if you use a 22 WMR case and square off the rim as I did. (See photos above.) The 22 LR case works best for 22-cal chambers.