I have an old (1926ish) FN side by side 12 bore chambered for the 65mm cartridge.

I was recently told it was "no big deal" to expand the chamber to accept the 70mm cartridges.

Is it really that simple?

It is fairly simple IF the walls are thick enough. A bigger issue is the type of shells you use. Most of these old guns cannot handle today's pressure. You can buy 2.5 inch shells from RST (http://www.rstshells.com/index.htm rather reasonably if you are willing to buy a case or two at a time. I have used Mike Orlen to do some choke and chamber work in the past; however, some folks have had problems recently. Mike was always prompt with my work.

Stephen,

Contact gunsmith Danie Joubert in Pretoria, he has the required reamer. He has access to the required barrel wall thickness gauges as well.

If thick enough it shouldn't be a big deal - IF THICK ENOUGH.

And yes, I would stay away from any modern Magnum-loaded 70mm ammo, and for that matter anything marked "High Speed", "Long Range", etc. Old guns and modern high-pressure ammo are often not the best of combinations, but Danie should be able to give you proper advice in that respect.

It can be done, I have done it myself with a Brownells reamer.

Having said that, I would follow Dall85's advice and just buy the shells the gun was intended to shoot, 2-1/2". Even if the metal is thick enough, the 85 year old wood will suffer from the pounding and the action may loosen up. Nothing you can pick out of the air with 2-3/4" shells won't succumb to a 2-1/2" shell.

I believe the 65mm chambers typically were on European guns chambered for the old rolled crimp shells; the common one here was the old Browning 16 ga. auto. Lots of German combo. guns with 65mm chambers. I think the shells here were listed as 2-9/16 not 2-1/2. Anyway, what the others have said, if there is enough wall thickness you should be good to go. I don't believe the pressure standards were any different for the 65mm v.s. 70 mm chambers. The proof pressures may be stamped on the bbls.
C.G.B.

I think that it can be done, it has been done, it is a common modification. But...

Here in UK it then must have a mandatory Proof Test with cartridges loaded to 70mm over-pressure specification.

In SA maybe your law is different.

But even if all you ever shoot is "box marked" 2 1/2" shells it makes sense as most modern cases are actually 67.5mm and not 65mm long. Which can sometimes cause "chopping" of the case mouth with paper cases.

I'd stick with "box marked" 2 1/2" shells but if they all seem to be loaded in 67.5mm cases it may make sense to just extend the chamber enough to allow them to fully open.

As you know maximum standard load with a 2 1/2" is 1 1/8 ounces of shot. With a 2 3/4" it is 1 1/4 ounces of shot.

stephen,
I had my old Fred Williams opened up to 70mm by Vere Bressler in PMB, and then he insisted on having it re-proofed by SABS.
I used the lighter 24-28gram loads in it for years, but now the rib has shot loose so it has been pensioned off.
Too damn expensive to repair!

Thanks for all the advice so far.

I believe this gun is quite strong in design. It is still very tight. I posted another question about it and there are some pictures of it there so you can see it here:

Pics are on this old post

John, do you believe the rib shot loose because of the use of 70mm shells?
Maybe your Natal gunsmiths are too expensive. Come get it fixed here in the old Transvaal

Steve,
difficult to say, there is evidence of a bit of corrosion under the rib which I believe was caused by the re-browning of the barrels.
This certainly would not help.
I was thinking of chatting to Vince about re-attaching the rib next time I drive up to your neck of the woods.

John,
Vincent was who I would have suggested too.

In the "old" days the Greeks & Italians etc. would have no one but Bill Ritchie touch their guns, and as you know, Vincent had a VERY long "apprenticeship" with BR before taking over the shop, so he must be one of the best when it comes to shotguns.

I took my neighbour's gun there in the 90s. It was an English side-by-side and the barrels had parted. They did a super looking job, and the price must have been reasonable given that it was real "sikkel-tyd" for me & the neighbour then....

btw. If you're in that neck of the woods you will have to come have a steak & a drink with me as it's not much further down the road

now there's a plan! Hic!