If you really want to be frugal, this is my idea for a "Poor Boy" .303 British.

Find a couple of take-off barrels chambered in 7.65 Argentine Mauser. There are scads of them around and most are virtually new condition. The rifle version is approx. 29" long - plenty to play with.

I have one in the back of my closet that is brand new condition. It's been there 25 years. I always envisioned a Martini project in .303 . . . .

The .303 Brit. is a superlative cartridge and definitely a traditional one for a double.

Just my $0.02!

Curl