Is the front ring marked Y1903 or M1903?

Either would have started life as 6.5X54 MS, with many of the military (Y1903) having been modified and 'sporterized' over time, sometimes to include a rechamber or rebarrel.

In some cases the rebarrel and / or rechamber would be to someone's own pet 'wildcat' load

As Louis advised, images would be helpful for identification, especially of all stampings, proofmarks if possible.

Your idea of 'slugging' the bore is, indeed, the best way for you to determine what's gone on in there.