What is it you are reloading? .223?, .308? 22-250?
One thing I highly recommend to everyone who is reloading for their own rifle, is to buy some Cerrocast I think that's correct.. it's a metal that melts below 200*, you can melt it in a double boiler.
Use a cleaning rod to plug the rifle's bore about 1" ahead of the chamber, Make a pouring ladle from a soda can. with a sharp pouring spout..
Heat and melt the cerrocast metal, pour it carefully into the chamber, stopping before the chamber is full, you don't want to overfill and have to whittle the excess off the cast you just made.
The casting of your chamber you just made is good for looking at the rifling, the wear on the rifling, how centered the camber was cut.
But relative to your need for a case trimmer, what I look for and measure is just how long the neck of the chamber is, and just how long the neck is on a 'correct' length case. Often the SAMMI spec is VERY generous about providing a lot of clearance between the end of a SAMMI spec case and the actual neck length of your chamber.
I've found that most non-custom chambers can accomodate a lot longer than spec case. Safely.