Oversized and/or worn chamber?

[Mad Miles]

G'day.
I'm new to old lever guns with limited knowledge of them
I have an 1889 Marlin chambered in 38-40.  I shot 10 rounds earlier in the week and measured the cases after fired.  The neck of the loaded cartridges before firing was 0.4145" between the crimp and the start of the shoulder taper, the neck of the fired cases are 0.4475".  The fired cases look like straight cases not bottle neck cases.  Seems to me the chamber should be shaped with the neck a few thou. larger than the neck of the un-fired cartridge, but it's almost as large as the unfired cartridge between the head and the shoulder.  I realize 120 year ago things were not as standardized as they are these days, but to me it looks like the chamber is just worn/shot out.  The bore is in real rough shape. Any thoughts?
Al

[willy]

Sounds like you have a 44/40 The 44/40 has a neck  dia. Of .443 before firing.

[DeaconKC]

Also, when you shot it, did the bullets hit the target cleanly or did they keyhole?
Oh, and welcome to the place! 

[Mad Miles]

Thanks for the reply, Willy.
It's stamped 38-40 on the barrel and rounds go through the bore.  I cannot put a bullet into the muzzle.  The diameter of the bullets are 0.401" and the 44-40 bore diameter I think is 0.427"?  I'm quite sure it's a 38-40.  What is the diameter of the neck of a 44-40 after it's been fired?

[Mad Miles]

Thanks Deacon.  They went into the target with a nice round hole, which kinda surprised me with the rusted looking bore.

[willy]

Thanks for the reply, Willy.
It's stamped 38-40 on the barrel and rounds go through the bore.  I cannot put a bullet into the muzzle.  The diameter of the bullets are 0.401" and the 44-40 bore diameter I think is 0.427"?  I'm quite sure it's a 38-40.  What is the diameter of the neck of a 44-40 after it's been fired?

After firing the case neck will match whatever the chamber is..
I have no idea why the chamber is so large on a 38/40 . Even if someone thought about converting the gun to 44/40 the barrel would have been removed  and rebored or lined when the chamber was cut.

[Mad Miles]

That's right, the casing will fire form to the rifle chamber when fired.  Some of my other rifle cases in other calibers, like 260, take a couple firings to fire form to the chamber, but they are rather thick walled compared to the 38-40.  I then bump the shoulder back until the bolt will just close with no resistance when re-sizing.   Which leads me to think the chamber has been shot out, or maybe it had been left sitting and corroding for a few of the 120 years of it's existence.

I'm far from being a gunsmith and don't know if a chamber can erode  away by being excessively fired with a chamber that was perhaps a bit on the large  side when manufactured.  Thus my asking.  The rifle has a lot of wear and must have been fired quite a bit in the past, the top of the hammer has a good amount of wear from the bottom the bolt by cycling the action.

[Pettifogger]

I have several old .38-40s and the fired brass looks nothing like new brass.  Factory loaded shells are tapered apparently so they will fit into all the old poorly spec'd chambers.  Once fired they fire form into something that looks completely different.  Still reloads and shoots OK.