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Not sure about others but I have a Marlin '94 w/26" barrel and the same ammo with the .401 bullets did this---remember ?What you are saying would make sense that it started to lead due to poor bullet fit. Started key-holeing. I think matching bullet to your groove would be a great first step. Your going up in diameter is in the right direction. What diameter are other b38-40 users using in vintage Marlins?
Good to know Lou--Thanks I'll keep that info. handy in case I need to go that route. I'm hoping the .403's will solve the problem though.No problem. They are good bullets and I rechecked they are 8-9 Brinell. I think pure lead is 5 so they are soft!
I used these in my 38-40 that measured .409 on the groove.
PSB and IMR-4227 gave me clover leaf groups at 50 yards!
Thank you for the pm. That is a nice vintage Marlin you have. :biggrin: I would bet that the barrel is just as it left the factory. Your rifle was made just prior to the advent of smokeless powder / metal patched (jacketed) bulleted ammunition and thus, the barrels produced had wider tolerances .....some of which, like yours were better adapted to the black powder / soft lead bulleted ammunition of the day.
If it were my rifle, I would load up some b.p. ammo with soft lead bullets and give it a try. Based on my experience with my '73 .44 W.C.F. and it's .009" oversized barrel, I would expect the results to be good. If you would like to try some b.p. ammo, let me know and I'll load a dozen or so cartridges and send them to you.
Buffalo Arms offers .38-40 b.p. ammo but I think that the bullets are likely a bit too hard to bump up to totally fill the groove.
Ammunition|Buffalo Arms Company
If your .3095" slugs will fit into a fired case (crimp removed), then you could try bullets of that size (.41 Mag).
If not, what is the approx inside diameter of fired cases?
For smokeless, using PSB may be pretty much your only other option with the possible exception of using Trail Boss with a soft, almost pure lead bullet.
Trail Boss has a pressure spike a bit faster than b.p. although the total push would be less. Might be something to try......
Don't lose heart! I'm sure you can get it to shoot decently as my most recent 1894 adventure with a much newer and poorer condition shows. http://www.marlinowners.com/forum/1894/105759-1894-basket-case.html
I think your bore was just oversize from the factory. I've heard the 38-40 was particularly bad about that, even in pistols. That's probably why the bore was in such good shape. It may not have even been shot very much since accuracy was so iffy with factory ammo it would have been an exercise in frustration. I would suggest a chamber casting and if the chamber will handle it, try a .410 or .411 cast bullet.
Mike Venterino had some interesting articles on various leverguns and gets in to tweaking the ammo for over-sized bores.
Chin up! I'm sure it will shoot eventually!