6.5mm general chamber question

I maybe just thinking out loud. But need to get a confirmation.
I have a very nice Daudeteau carbine, and have started to try shooting it. Slugged the bore, which is very good shape, and it comes out to about .267/8.
At the moment I have some different boolits of Carcano style and a Lyman clone, not the 469 number, but the bore rider nose style of around 150gr. Brass is Buffalo Arms resized from 45/70.
So, I'm assuming that right there maybe my problem.
Only .264 140gr. Jwords chamber, but don't shoot worth a hoot. That was just a load of 30grs of IMR 3031. I tried Carcano dia. Jwords, bolt will not close but very hard. If I try a bigger boolit, they don't chamber well, but very hard to close the bolt if at all possible. I'm not even sure if I should try make it happen, as to possible high pressures? Maybe just pull the bullets and start over.

Being 45/70 brass is necked down that far, should I be neck turning the cases? Neck wall thickness by caliper is .015/16. Anything else I may be missing?

Thanks.

Neck diameter and OAL need to be right first. Make a loaded round and measure neck dia. with bullet in the neck, then check the chamber to see if it interferes. To really know OAL on something old I would guess you need to make a chamber cast. I would have these things figured out before I worried about bullets, proper loaded ammo should have some free bore.

Thanks.
Also, I did anneal the necks.
I'm going to check the ID of the neck from the .264 jword fired cases and see where that's at. These rounds I've loaded don't have the bullet out far, right about at the cannelure is where they are at. And with no camming action of a Mod 71 Mauser action, they will hardly close the bolt.
OAL, man I would think that should be fairly ok and a generous throat being military, but I will have to find out.

I don't think you can assume anything, like generous throat.

Per Cartridges of the World nominal neck diameter is .298, how do your various bullets compare when seated in a case. FWIW Donnelly's book on conversions recommends 7.62 X 54 Russian as a parent cartridge.

Ok.
Only loaded with .264 will it lay under that dimension. At about .295
.268 it comes to around .299
I had read the Russian case is used for some, 45/70 for the Mauser bolt face. But it maybe possible the the Russian case would work better, or at least to try.

After firing can you slip a bullet into the fired case, .264 or .268? That neck thickness shows up doing that, also if a case is too long and crimped by the chamber.

.264, yes easy. A .268 will go in part way. Not very far.
Id of the fired case is .266, close to .267.
But a cast boolit of .268 I would suppose may swage down a bit when chambered. The jacketed ones do not wanna close (.268) the bolt easy at all. If at all. I'm afraid to force it because if I don't fire it, you can hardly open the bolt. The basis of the rifle, 1871 Mauser, really has no cam action from bolt lugs. Just the handle. So you almost have to pound on the bolt handle with a wooded hammer.

I would expect a bullet slip fit in fired case no matter the bullet type. It is about release, and clearance to expand the neck under pressure of firing.
No radial room to expand the neck will cause start pressure to go up. Perhaps substantially.

Interference will also cause force upon chambering no matter the bullet construction type (re: your comment on camming). You might well be able to thin the necks on some brass for the 268/9 bullets. Given the stated bore dimension, I would think 268/9 would be a good choice. Thin the neck radially by 1/2 amount of bullet dia change, and the die should still give sufficient tension for bullet retention.

I will try it out and see.
Thanks guys. It's a pretty nifty little carbine, and single shot. Thee trigger pull is really good also.
When I get some things to work out, I will post some pictures.