@MexicoMike
There are a couple of decision points that lead you to consider "New versus Vintage" in the double rifle world.
I'll start with the false assumption: People will tell you if you buy a new one, you get to use factory ammo, whereas a vintage double rifle requires a custom load. That is almost totally false. The new double rifle is regulated for a very particular load from a very specific brand. Within months to years, that manufacturer will change bullets, or change their powder due to supply chain shortages, leaving you with a worthless rifle that cannot hit the broadside of a barn. Factory loads are also excessively violent in their recoil due to the powders they select. So from the start, new versus vintage, get it in your head that you need a regulating load developed so the rifle is accurate and functional indefinitely.
Now on to the biggest thing between a a modern versus vintage double rifle. To be specific, I'm talking about a modern Heym because they are extremely durable with excellent metallurgy, not all new guns can shoot monometal solids so don't paint with a broad brush.
Generally:
A Heym modern double rifle (and a few others, but not most) can be regulated to use all-copper and copper-allow monometal solids. Their metallurgy allows you to do horrible things to the rifle with good chances that the barrels stay together and you do not irrepairably damage the weapon.
A vintage British double is superior to a Heym in every conceivable way. Aesthetic. Handmade quality, balance, fit and finish, etc. The one detraction for a vintage british double rifle is they must use traditional cup-and-core, lead core FMJ solids and their corresponding softs. They were designed and regulated for these bullets and these bullets are softer than the barrels which are not as good of metallurgy as a modern Heym. In short, a vintage British Double is like a Ferrari, you can't buy one and then bemoan that it won't run on 87 octane fuel. You'll need to seek out and hoard the bullets that work with a vintage double and manage supply chain concerns as they come in and out of production every few years. Hornady DGS and DGX are available in most calibers and serve this purpose, even better are woodleigh softs and woodleigh traditional FMJ solids that are a bit harder to find in some calibers.
That's the big difference. I've loved the Heyms I've owned, I've truly loved the British rifles I've owned. Both are wholly reliable and sufficient for dangerous game hunting if properly maintained. They Heym needs far less scrutiny to its serviceability because they are 1-30 years old and the vintage British rifle is 75-125 years old.
Some caveat emptor on modern Double Rifles. I know that Rigby will not authorize hand loaded monometal solids. I believe Merkel and Krieghoff as well, but double check on the latter two. Either way, I'm not spending "British Rifle" money on anything modern that isn't a Heym or something very rare with impeccable reputation. (e.g. Hartmann and Weis, Marcel Thys, etc.)
