Any large Martini action I have will take the standard thread used on a No1 or No4 Lee Enfield. Why change something that works well.
As far as Khyber Pass Martinis go, good luck in finding one. I've been looking long and hard for one for over 40 years.
A few years ago, I thought I had found one. Turns out it was made in the Royal Afghan Armories. No, I don't remember the name of it. When I talked to the lady I bought it from, it turned out her son had it shipped home from Afganistan, where he picked it up in a bazaar, along with a scabbardless bayonet. To bad, but still lots of history behind the rifle and it's in 90% condition inside and out.
OP, the big issue with these rifles is condition. Many of the rifles we come across today are cobbled together from parts bins. I can remember in the late sixties when these rifles first came in. I helped clean up thousands of them. Most went across the border, into the US. Surprisingly though the best ones stayed here in Canada. They were in 577/450, 303 with both Enfield rifling and Metford style rifling. They also come with different style bayonet lugs and some, have the slip over the barrel type bayos. Lots of differences as smellie mentioned.
The 577/450 rifles had the worst bores overall. We took close to a thousand of those and welded them together in a triangular jig. Took out one of the falling blocks, threaded the internal muzzle and attached a lamp fixture. The bores in those rifles were rusty holes, devoid of rifling, but still recognisable as rifles. The parts on them alone would be very desireable now. Back then, we got $30 per lamp and still made good money.
The 303 rifles, were generally in better condition. Many had the finish worn off completely and came either shiney, brown patina all over or well blued. Many of them had the sides of the barrels worn paper thin beside the last 6 inches, to the muzzle. We were told this was from being taken in and out of scabbards. When the rifles were in scabbards, especially in the middle east, sand would get into the bottom of the scabbards and wear them down. Needless to say, when they were worn in such a manner, if the bores were good, many got docked back to where the barrels were round and recrowned. The stocks, suffered all kinds of abuses.
These actions, if they are tight are very strong. They handle surplus or commercial ammo with ease.
I've seen them converted to 308Win and one in 257 Rob, by a very crafty fellow in Penticton BC. He split the extractor he made up and installed springs and a wedge so that the extractor would separate when it came to the full extraction position, releasing the case. This also worked well for loading. To my knowledge, he used full pressure loads in both of those rifles. He got the idea from a fellow in the UK, so it isn't that unusual.
Like a fool, I bought the 257 Rob from his estate, used it for a couple of years, after mounting a scope and leather cheek pad, then tired of it and sold it. That is one of my regrets. Fantastic rifle to shoot, comfortable and accurate.
I've also seen them converted to 30/40 Krag and 7.62x54R.
The 30/40 Krag was done because the rifles could be picked up for less than $20 and when 303Brit surplus ammo wasn't available, there was always lots of 30/40 Krag brass to fall back on. The chamber, because of the similar rim would accept both cartridges and shoot them well.
I still have one chambered in 30/40 Krag. I bought it that way for the action but it shoots so well, I've stripped it for refinishing and it's sitting in Limbo. This winter will show some progress though.
Like mentioned, look for one with a decent bore, that locks up tight. If it's loose and sloppy, they will often pop open after being fired, on their own. Not dangerous but disconcerting to say the least.
They will be found with Australian, British, Nepalese, Turkish, Arabic and other African nations markings. Probably many others as well. They were popular because they were strong and efficient. When an operator was trained properly in their use, they could get of five rounds as fast as most troops with bolt action rifles. Much faster than the single shot bolt action rifles that were fielded in the day and just as fast as a Mauser.
The No1 and later No4 rifles had the edge on them though. The big advantage the bolt action repeaters had on them, was the ability to be loaded with chargers. I did see a special leather bandolier made up by hand that had been sewn with the ability to extract four cartridges between the fingers and thumb of one hand. It worked well and with such a rig, fast, sustained fire could be kept up until the ammo ran out.
There is a site in the US that has them for sale. IMA. They aren't cheap though and are of Nepalese and UK production.
I like Martinis. The most powerful cartridge I have one rebarreled to is the 7x65R. Powerful and accurate.
