How to Accurately Determine YOUR Rifles Trim Length

[427Cobra]

I see lots of posts where people ask what is the trim length of......, most post book minimum when they answer, which is incorrect, but its the safe thing to do though at the expense of the rifles chamber, Sinclair International sells a handy mild steel plug(soft as not to damage the chamber) that is used in a modified case, this little tool tells us exactly where the chamber ends, knowing where the chamber ends and trimming back from there at whatever distance you choose, I did .010 on my 260, but I'm going to increase that number to .015 on my 6.5 Creedmoor, and use 1.922 as my trim length.

Sinclair Chamber Length Gage | Sinclair Intl

First thing is to clean your chamber then FL resize a case with the expander ball installed

Next drill out the primer pocket, the hole is used to push the plug back out


Now the instructions say trim off .100 but my .264 plug has more than that so I did .170, of course the lip of the neck has to be deburred in/out


Insert the plug


Make sure its longer than it should ever be and SLOWLY chamber your modified case, measure, push the plug out through the hole in the base and recheck until you have the same reading 3 times, in my case that number is 1.937


I now know where my chamber ends, and can determine when its time to trim a case, I'm changing from .010 back to .015 back because I'm planning on annealing every time, trimming is not about accuracy its a safety step, brass that is way too long can\will cause serious pressure spikes, brass that is too short burns up the chamber, my 260 Bartlein tube went 4000 rounds, I want my new Rock 6.5Creed tube to last as long as possible.

 

[Kirkd]

Thanks, i wonder if this can be done with a Hornady modified case with the oal stick attached?

 

[mtang45]

Thanks, i wonder if this can be done with a Hornady modified case with the oal stick attached?

Interesting idea! I'll have to try it.

 

[elfster1234]

awesome info! I might need to try that! thanks!

 

[downrange67]

Thanks for the info.

 

[427Cobra]

Thanks, i wonder if this can be done with a Hornady modified case with the oal stick attached?

Are going to cut .150 off your modified case, I'm not, good idea though, if it works please report back.

 

[Radhoagland]

Sounds like a lot of work to do just to get a few extra hundred rounds out of the barrel. I see gaining nothing by doing what you described in fact beyond 500 yards variable trim length's could cause accuracy problems unless you are trimming long but to the same length. In the 6.5 CM your throat will be shot out before your chamber wears out even if you trim long or trim to SAAMI guidelines.

 

[427Cobra]

In this game trim length has nothing to do with accuracy, it's just a time consuming step in the reloading process, even with a Giraud, and a "few extra hundred rounds" adds up, but in the interest of science I work for a Airline, I'll be glad to fly to Kansas and you can prove to me that trim length matters.

 

[milo 2.0]

It's been soooo long since I've ordered from Sinclair, and these gauges look like the ticket. In case a guy doesn't have one for your chamber, would going off the reamer print, and adjusting accordingly suffice? Most reamers go .010" over max trim length anyway.

 

[Radhoagland]

In this game trim length has nothing to do with accuracy, it's just a time consuming step in the reloading process, even with a Giraud, and a "few extra hundred rounds" adds up, but in the interest of science I work for a Airline, I'll be glad to fly to Kansas and you can prove to me that trim length matters.

Gunny thanks for your service. IMO trim length can affect accuracy on a crimped case. You say otherwise, fine. Please dont fly to Kansas the weather sucks here. LOL

 

[dondick]

Cool, I didn't know this tool existed . Thanks.

 

[427Cobra]

Gunny thanks for your service. IMO trim length can affect accuracy on a crimped case. You say otherwise, fine. Please dont fly to Kansas the weather sucks here. LOL

You bring up a very good point, rounds that are crimped trim length is extremely important, cases shorter than whatever length a crimp die is set up for will result in little to no crimp, cases longer will have too much crimp.

 

[FLIGHT762]

When you buy the tool and use it, get ready to laugh when you read the caliper. For DECADES, I was so anal about NEVER to let my 308 Win. cases get beyond the 2.015" SAAMI maximum. Both of my 308 bolt guns (one factory and one custom barrel) BOTH rifles confirmed readings from multiple try's. custom chamber 2.040", Remington 40XBKS 2.046", GAP 10 - 2.043". So much for the 2.015".

 

[bruddah]

That's pretty cool actually. Almost worth $6 just to find out what it is!!