03A3 Caliber Mystery it has been modified.

I don't know but the expanded case neck looks bigger then .30 cal to me.

Maybe a .338 or .35 cal wildcat of some sort.
I'd measure the I.D. of the fired case and see.

If thats what it is, the lower pressure of the .308" bullet in a larger bore is probably all that saved you from a blow-up.

rc

Looks like a .30 Gibbs; an easy rechamber of a .30-06.
If you are not a handloader, the barrel is useless to you, it will not safely fire factory loads like an Ackley Improved would.

If it is not marked and you were not informed, you were swindled.

It could be .308 Norma Magnum, a very popular caliber for converting Springfield and Enfield (M1917) rifles to a more powerful round. I strongly suggest doing a chamber cast or having a gunsmith do it and try to identify the proper cartridge. And don't shoot that rifle until you find out what it takes.

It was extremely careless of whoever reworked that rifle to not indicate the new cartridge, but some folks don't care. Does your rifle have the military stock and handguard? It is possible that the proper cartridge is marked on the barrel, but someone "restored" the rifle and covered up the marking. It seems unlikely, but it has happened.

Jiim

It looks like a .300 win mag. to me, minus the belt.

I agree with RC, it's bigger than .308 caliber from looking at the first picture. From your measurements, it comes very close to the 35 Whelen which also has a sharp taper neck and short neck.

It is almost certainly...

a .30 Gibbs. It is easy enough to check the caliber: unless the bore is obviously larger than .30, or a .30 bullet inserted into the neck of the fired case is 'way undersized, there is no reason to think it is of larger caliber.
As for the fired case, it shows a near-separation of the head due to grossly excess headspace in the longer-bodied Gibbs-type chamber, but there is no evidence of expansion of the body or head diameter sufficient to indicate a magnum chamber, nor any evidence of the case head expanding into the recess for a magnum's belt, which WILL be present on an '06 round fired in a .300 Mag chamber.
PRD1 - mhb - Mike

35 Gibbs or 333OKH???????

Get a chamber cast done and solve the mystery.

In my opinion the chamber is a Gibbs, as to the barrel not being stamped, the Gibbs family of chambers when reamed removed all of the original chamber, I would suggest necking 30/06 cases up to 35 Whelen then size the neck down to create a second shoulder for fire fire forming. the formed case can be formed to match the length of the chamber, but, that would require adjusting the die off the shell holder, to accomplish that, the reloader would first be required to determine the length of the chamber from the bolt face to the shoulder of the chamber.

Just a side note of no value, your rifle is a Springfield, Hatcher moved the shoulder forward .080 thousandths on a Springfield thinking he would get case head incipient separation, instead, he ejected a 30/06 Hatcher modified +.080 thousandths fire formed case.

F. Guffey

The only way to really tell is to have a chamber cast done. Measure the cast and find it in a cartridge book. Verify with headspace gages. When the caliber is absolutely known, stamp it on the barrel. This happens a lot with old surplus rifles and most don't have the caliber on the barrel.

The Gibbs family of chambers had/has short necks, the 30 Gibbs neck is .215, my 30 Gibbs neck is .245 thousandths long because I am the fan of bullet hold, the neck on the 30/06 is .385 thousandths long, that is a difference of .170 thousandths.

And Gibbs beat Hatcher’s creation by .032 thousandths when he increased the length of the chamber from the bolt face to the shoulder of the chamber, when forming and fire forming 30 Gibbs cases my cases shorten .035 thousandths +. not a problem, I want all of my chamber covered with the case so I find long(er) cases.

F. Guffey

Because no one mentions unsupported case head when changing/chambering barrels, I check, unsupported case head on the 03 is .090 thousandths when measured from the bottom of the extractor groove to the head of the case, seems safer than the Mauser, unsupported case head on the Mauser 98 type is .110 thousandths + the difference in length between the case and chamber, again, when measured from the bolt face to the shoulder of the chamber, and, when compared with the length of the case from the case head to the shoulder/datum.

“The only way to really tell is to have a chamber cast done” The only way? In my opinion there is no substitute for transfers, standards and a good understanding of verifying.

Head space gage? Head space gages are fixed standards, except my gages, I make them in thousandths, and, adjustable, and, variable like modifying a go-gage to a go to infinity gage, but most agree with you “The only way to really tell is to have a chamber cast done”, then there is the press, die and shell holder, all of my presses have threads, all my dies have threads, threads make it possible for me to adjust the length of the case from the head of the case to its shoulder, and to me that is just another way to determine the unknown, and micrometers, I took a picture of my gages and micrometers, the picture weighed 400 lbs..

F. Guffey

I'd like to point out the "magnum" belt that appears to have been blown "into" of the case as it expanded.

It might be from case expansion to fill what was missing, but the growth of the brass in length doesn't look anywhere near long enough to justify that sort of impending separation.

The belt end position on 300 win mag is 5.59mm, I'd measure it and see if that corresponds.


Still going to have to wait on a chamber cast, but that measurement right there might do it.

could be a 308 norma mag

To tell if it's a belted mag or not?

Try one of any caliber in the bolt face under the extractor.

A belted mag anything won't fit an unmodified 30-06 bolt face because it's bigger.

rc

Looks like the bolt face has been opened up for a belted mag to me.

Are the firing pin dents off center?

If they are?
That would indicated the extractor is holding the 30-06 case to the left in the larger chamber and bolt face.

rc

That would be a belted magnum, I would suppose. It appears to be blowing out where the belt cutout in the chamber is, looks like.

Got any 300 win mag lyin around ?

my vote goes to the 308 norma mag

The bolt face is leading me towards the .308 Norma, which is about the same case length as the 7mm RM shown. It was specifically designed to rechamber .30-06s, including Springfields, without cutting the chamber wall too thin for comfort.

I still don't see the fired cases expanding out into a belt recess, just separated from being fired with greatly excessive chamber headspace. The case web is likely too thick to show it.