Need help with trim length for 243 Ackley

[Alabama556]

I have a Winchester model 70 with CRF that bug holes installed a bartlien 3b barrel on.

I just got through fire forming all the lapua brass I had on hand and I went through and measured each case. This was brand new brass. I loaded the rounds where the bullet was touching the lands using close to max loads for 243 Winchester.

My case length are from 2.010 - 2.030. Some brass has 2 firings on it.

The saami trim length is 2.035. Is it a mistake to trim everything to 2.010 or just keep shooting the brass until it grows more.

Thanks

 

[RegionRat]

The reamer they used is one way to know what your Ackley should have when all is said and done.
If you don't know the reamer print, there are two easy ways to know the max case length without too much fuss.

There is a case length tool that requires a piece of brass to be cut down, then the tool and that case are shoved into the chamber such that the chamber will move that tooling to the max length. You then set your max length and trim length from there.

Another way, it to measure some of your cases and place them into the chamber while taking a look with your borescope. You can then see the approximate clearance to the case mouth and back into your trim length margin.

The typical margin for a SAAMI chamber for 243 is 0.010", so for the 243 WIN the chamber length is 2.055 and the max case length is 2.045" with a tolerance of 2.045" - 2.025".

You AI chamber reamer should be the same, but if your builder doesn't verify the print for you, then one of those tools or a look with your borescope will answer the issue.

The saami trim length is 2.035. Is it a mistake to trim everything to 2.010 or just keep shooting the brass until it grows more.

I would verify the reamer print or run the inspection. You probably have lots of room left such that you don't need to trim for a while, but knowing the dimensions is better than guessing. YMMV