Greetings. Brice here. I sent two responses to your last, but they came back undeliverable. I didn't want you to think me an ingrate. Your help is appreciated.

You are certainly correct that the 7 x 57 and 257 are the same length. Maybe I forgot to put my glasses on when looking at the drawings. I do have some 25-06 brass, so will give that a try. I enjoy making the occasional tool or gauge. Sounds like you do, too. It would be handy to make a chamber length gauge using a case with a friction slip fitting. Will have to give that some thought.

I think my problem will be finding some 257 Roberts brass long enough for this chamber. I have 100+ cases, and am pretty sure I bought them in one purchase. That is my usual practice. That said, they fall into two groups for length, about 30% being about .015 longer than the rest. I keep records of brass prep and history, but might not have done so back in 1997 when I built this rifle.

Yeah, internet experts. I prefer loading manuals (I have at least 15), Precision Shooting Magazine, etc. I am continuously and incrementally reducing my ignorance. Unfortunately, the bastards keep inventing new info. Have a nice weekend.

Brice
Thanks for your response. Sinclair or someone like them once sold chamber length checking plugs. I don't need them but one time a rifle so I use the method in the PMs.

Your chamber and brass is probably ok for all known uses with jacketed bullets. The people that are concerned about chamber length are usually cast bullet shooters or some one that shoots odd ball military rounds. Shoot both.

An example of chamber lenght checking
I shoot the 6.5X53R Dutch Mannlicher round.
The Brits also call it the .256 Mannlicher.
In some places it is also called the 6.5X53.5R

So which is it. The few manuals call for a case length of 2.110.
53mm = 2.0866"
53.5 = 2.1063

Neither match 2.110.

Since I have to make my brass from .303 Brit brass I have to trim it back from 2.222.
I started out with long cases anyway so I checked one as I cut it back. I found the actual chamber length to be 2.150".

Later I found an ordinance case section drawing in French on the net dated 1923.
It listed the case length with the following minimum and maximum length.

53.6 - 53.7

Those numbers correspond to 2.110" to 2.114"
I also found a Dutch reference that listed it at 53.65mm which corresponds to splitting the difference at 2.112".

Anyway now I know what the original dimensions and tolerances for the case were.
I also know the chamber was about 1 mm longer than the minimum case. I trimmed my first cases to 2.150" and fired them. They got shorter by about .004" to .007" when they expanded.
The Dutch round is like an AI round compared to the skinny tapered .303 Brit.

When resized the cases went back to about 2.148" to 2.151" I trimmed them back to 2.145"

This minimizes the amount of trimming since they are not cut back to 2.110" per the manual.
It also minimizes the gap at the case mouth for whatever that is worth.

PS your .257 cases will stretch a little the first time they are fired. When sized they will be longer. Check them then and decide how long to leave them after checking your chamber.