Will a Weatherby Mark V American (6 lug) barrel fit a Vanguard (Howa) action?

The HOWA is a metric thread 26MM X 1.5MM and the MK5 action is a 1.060" X 16 TPI. So no, they don't interchange. Close,but no cigar.

Am curious. A few months ago I had a Howa 1500 rebarreled. Will not bore you with the details, but essentially the action was rethreaded to Winchester thread by simply by running a tap into it. Does the Weatherby Mark V have a Winchester diameter and threads?

Personally, I would be wary of rethreading the action to Winchester's threads. They use 1"-16 tpi while the Howa 26mm-1.5mm is approximately 1.026-17 tpi. Several years ago, I did an experiment by rethreading a Howa barrel to 1"-16 tpi, the mirror image of running the tap in your action. The results were Ugly! I feel sure with enough red Loc Tite it would be safe, but I sure would not want my name on it.
Most lathes can be changed over to metric threads by changing a couple of gears so why take a chance? I do charge extra for a Howa because it does take time to change the gears but I am happy to put my name on the finished product.

Sounds like the guy that did the job didn't want to, or couldn't, thread it for a metric thread.

Would like to know this myself. Would Win. and Weatherby barrels interchange?

I answered in the 2nd post but I'll do it again and elaborate here.

The shank O.D. of the Howa specs at 1.023"
The Weatherby is 1.060" (Old spec was 1.0625 or 1-1/16")

The Howa is a 1.5 MM Pitch or .059" per turn
The Weahterby is 16 TIP, or .0625" per turn.

So the Howa action has a Shank Well that is .038 smaller then the MK5 and the threads are off 31 ten-thousandths per revolution.

If your barrel turned into the receiver 13 turns it means the thread will be off a total of 49 thousandths. Far too much to work. The shank of the Howa is too small for the MK5 and the MK5 is too big for the Howa.

You could cut the shank off either barrel and re-shank and re thread to make either work in either action, but it's no different than starting with a contoured blank in labor. You would be ahead in the cost of the barrel however, if you already own the barrel. In either case you will loose about 3/4" to 7/8" of barrel length.

Winchester M70s have 16TPI like the Mk5 but the shank is longer, and only 1" in diameter not 1-1/6" like the MK5. So it would have to be cut off and re-shanked to work, which also means deepening the chamber when it's fitted. I have put M-70 barrels in to MK5s before.

Lastly I would point out the old "Varmintmaster MK5s has a totally different shank and thread, so if you have a 6 lug action it may be the same as the old Varmintmaster.

Those actions had a shank that was shorter buy .030 than the standard MK5, and were 7/8" in diameter with 18 TPI. So again both the regular HOWA or the regular MK5 could be machined to fit, but the work is the same as starting with a contoured blank but for the fact that the chamber would be about 95% done so it would only need a small amount of reaming.

KP231, I'll give you a quick-tip you may like.

Make a die stock to go in the tail stock of your lathe. Make it to hold a 1.5" round die.
When you need to thread a metric shank buy class 2 thread dies in the correct spec for the job and it's fast and easy. Just turn the shank to the correct OD and put a 7 degree taper on the leading edge for about 1/16" back. Insert the die and run the tail stock up to the shank and turn the chuck by hand.

Works like a charm and the cost is only the price of the die and about 1 hour in making the die stock I used 2-3/8" bar stock to make my own.
It's very easy.

In some cases you can't get class 2 dies. It's OK to use class 3 or 4, but you'll want to put an 85 degree shoulder on the barrel with looser threads to make it center up better. It does work, and presents not real problems, but always seemed to be a bit "sloppy" to me, so I always try to get class 2 spec dies. Class 1 is too tight and can gaull the action threads, so don't use them. A class 1 thread is a fight to get on and off. If offers nothing to the job and can create problems.