I have a Lee Enfield 303 British and was thinking of upgrading the barrel to a longer one . I would like to know if I can use a barrel from a Mk4 to do the swap??
FWIW, It won't be a drop-in task, the math will have to be dealt with....plus some of the other parts that hang from the barrel or sit atop of it...clock position of sights is an example. The only numbers I have on any of the .303 Brits is for the Short magazine Lee-Enfield(SMLE) Rifle. The Barrel diameter at the joint of BBL to Rec is 1.215" the shank steps down to .997" and is threaded at 14 threads per inch for a length of .688", then there is a short unthreaded portion the steps down to .900" for a length of .100"
That said, You must remember that this British service firearm was made in several countries and in several arsenals in each country for about 100 years. There are more than a few variations. Our hosts paper catalog shows 2 of them, while the Standard Catalog of Military Firearms shows in excess of 30...more if you count the .22 caliber versions. They also came in single shot .410 shotgun and the newer and stronger versions are chambered in 7.62 Nato(aka .308 Winchester)
To be honest with you, swapping barrels is NOT going to give you a gun with a long barrel on the cheap. If you are not already tooled up to do barrel work, that tooling alone will exceed the cost of buying a complete rifle with the longer barrel. About the only thing this task will do for you is give you some homework in Gunsmithing 101. I would gues that somewhere on YouTube there is a guy showing you how to pull the bbarel and install another one and showing all the tools(both homemade and factory). An example of something that can go wrong,if the hedspace is excessive(and almost all military barrels will border on it) then you will have to do a barrel setback...starts with facing off the front of the reciever(to remove and scarring from the prior bbl torquing) then the shoulder of the barrel will have to be cut forward. The tricky part here is..you have to do the math on how much metal removed will allow for one complete revolution of the barrrel(to bring any clock position thins...like sights...back to clock postion) depending on how much the headspace is out, you may have to cut enoung for more than one revolution...then the threads will have to be cut forward, the OAL of the shank adjusted, including the short unthreaded portion, then the chamber recut to proper headspace. All requiring specific knowledge and tools.
That said, If I may, why do you wish to install a longer barrel? Has it been cut shorter than the legal minimum (16" from the bolt face in lock up)? If your looking for a "lok"..it would be cheaper to install a muzzle brake...that will add about 3" and can be done(by a pro) for under $150(William Gunsight Corp)...if your looking for more horsepower and velocity...then reloading will do it cheaper.....
If you have a no5 marked receiver and a no5 barrel it is worth too much to bastardize.I have put the 7.62 NATO Indian No.1 barrels on to no4 actions so they are interchangeble.