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As above, anyone have any spare?

You can make them from 32-20

I’m holding out for some ‘proper’ ones (it’s the OCD in me) but my backup is the 32-20 route.

Check your chamber length. BSA seem to have made a habit of running the chambering reamer in too far. My cases are about a tenth of an inch longer than standard. The same applies to the rim thickness. Some 310s need the 32-20 cases to be thinned down, some don't. Mine doesn't.
As a result of the above, you will probably be better off with cases made from 32-20 brass.
Fred

How many are you looking for?

I have some spare Bertram cases.

I have a box of new unused Bertram cases they dont chamber in mine I have all ways used 32-20 cases with out any issues.

FredB wrote:Check your chamber length. BSA seem to have made a habit of running the chambering reamer in too far. My cases are about a tenth of an inch longer than standard. The same applies to the rim thickness. Some 310s need the 32-20 cases to be thinned down, some don't. Mine doesn't.
As a result of the above, you will probably be better off with cases made from 32-20 brass.
Fred

What’s the easiest way to measure the chamber length?

I have been given some .310 cases, which look unused.

Probably using cerrosafe to make a cast of the chamber.

You don't need to. If you are starting with 32-20 brass, first reform it in the sizing die and then progressively reduce the length until it just chambers. Knock a few thou extra off to be sure and that is your new case length. If you have access to a small lathe, make up a split sleeve. Take a piece of bar slightly larger in diameter than the case flange and cut it to your desired case length. Bore it through to the neck diameter of your new case and then bore all but the last 5mm or so through to the case base diameter. Then, make a longitudinal hacksaw cut through the sleeve.
Insert a formed case into the sleeve and chuck it in the lathe. The longitudinal cut will allow the chuck to grip it tightly. Run a slightly angled parting off tool across the case neck and your reformed case is now your new 310 case. You will probably need to use the dies for the first reload, but after the first firing, do not resize unless you intend to use the cases in a different rifle. Just re-cap, add powder and thumb seat the bullet up to the end of the heel section. There is no need to crimp. Crimp is to hold a bullet in place through handling in the field and to stop a conventional, parallet bullet from being seated to deeply when fed from a magazine.
I won the 100yd Rook Rifle comp at the last Trafalgar meeting and quite a few previous ones using ammo prepared in this manner.


Fred

If you want to try the 32-20 method I can pop a couple of new 32-20 cases in with your order.