So we have the ammo

Yes indeed we had a box sealed and full of 20 rounds of 7mm Mk1z (thats nitro cellulose powder) and we had to open it. This caused consternation in some quarters. One forum even went so far as to call me a vandal.
My response? I cant fire it inside a box can I!
Yes my friend had won the argument we had decided to have built a brace of rifles in .280 British (as the Proof House decided it was called) and set about procuring a handful of suitable donor rifles.
We had looked at previous efforts and found that Lewis Potter and one other had rebarrelled at least one Ruger No1 in this caliber and they very generously shared their load information which unfortunately was mainly based on a 120 grain load but it was a very useful starting point and we were very grateful.
I will be reproducing the load tables here (later on of course) and also the test results even the ones which highlight all of the errors I made along the way. If I chose not to share my mistakes then you may end up making those yourself.
Somewhere along the way we decided that to save time we would work on ammunition production whilst the rifles were being built. Concurrent activity and all that!
Dies were sourced from Redding Dies in the USA and case conversions were investigated.
The obvious place to look was the history books.
As the initial offering to the Nato trials had failed due to anaemic performance the final version was based upon the same T65 case as the US were using in their 7.62 x 51 mm offering. Sadly the US wouldn't allow the improved .280/30 as she was known to be entered into the trials and the US offering was deemed to be the winner.

The image above shows the 30-06 Springfield on the left and the 7.62 x 51 on the right with the British offering of .280/30 or 7x43mm in the middle. The ammo all share one character, the case head dimension is sufficiently close enough on all three to remove any need for bolt alterations in our rifles.

So two Remington 700 rifles were acquired at auction. They were in calibres which would mean a shared bolt face. .22-250 and .243, Remingtons were chosen because of the range of spares and options available as we go down the road of building these rifles.

Next was selecting a suitable riflesmith capable of producing a decent job on these. We had a number of options but our pockets weren't sufficiently deep enough for the best so we looked at smaller businesses. A friend overseas had some work done and recommended a smith down in the south and my oppo Paul had ordered a 6.5 Grendel on a CZ527 action from Riflecraft. The difference in the end came down to cost.

I wish then I had spent the extra hundred pounds. Pauls CZ had a terribly tight chamber and the Smith wasn't as communicative or helpful as he should have been. All of these should have warned me but Pauls Grendel was impressively accurate.

We left the rifles (along with another project rifle) at Riflecraft, with four rounds of ammo for them, 2 for proof testing per rifle and also for reamer measurements and walked off.

Then we got stuck into brass conversion.

Brass conversion, now there is another tale.