If you are going to rebarrel a 12/15, you would best go for either a bentz or match reamer for chambering.  Diameter?  check how much space you have to the top of the action from the middle of the barrel shank.  Even with the standard 12/15 barrel, these rifles produce awesome accuracy, so...1
I'm afraid I'm not the giuy to talk about loctiting barrels though, but I have heard it is the best way to do it.  Also, for a .22 it is best to have a slight 'rim compression' when closing the action for accuracy purposes.  Other than that - have fun with the rifle!

If you are going to build a bench rifle for rimfire, have the largest barrel you can mount on the rifle. In ASSRA, barrel minimum length is 21.5". Therefore, I would suggest a large diameter, relatively short (23" - leave a little leeway in case something happens and you have to set the barrel back to re-chamber). A match quality barrel (get the best that a particular manufacturer offers) from any of the major makers will do just fine. You might consider a stainless barrel. Most manufacturers, if pressed, would admit that their stainless barrels MIGHT be a bit better.

Then, have it chambered with a match reamer. It would be best if someone who is familiar with head space requirements of a .22 rimfire did the work. Have the barrel drilled and tapped for the type of scope you prefer (for instance, I prefer a modern scope).

Make, or have made, a true bench rest stock and forearm at the maximum dimensions allowed by ASSRA.

NO ONE can predict what brand or choice of quality .22 ammo will work best in your new rifle. I have settled that question to my own satisfaction with lots of experience with the problem. You just have to try various brands and types - when you find the combination that works, buy LOTS of that particular brand, type, and LOT number. If you like to shoot, this is NOT an onerous task. It will be fun to find out what your rifle likes. It is flat amazing to see the wide variance in down range performance that the various brands, types, and lot numbers have with your particular rifle.

Good luck!
Dale53

Thanks guys,

I haven't had any experience with the bentz reamer although I am aware of them.   

I am leaning towards a full benchrest stock, and just happen to have a massive chunk of walnut set aside for the project.

As far as the barrel goes, cutting it longer than minimum is a good idea.  I probably can't go with stainless though.  I don't mind it in anybody elses gun, but it would hurt my conscience a little to have one on mine.  I will also probably go with a modern internally adjustable scope.  I expect to try different brands of ammo, but right now I am just trying to get the design to settle down.   

All ideas are appreciated.

Thanks 
Brad

I have a Martini set up as a heavy bench rest rifle but it is not in .22 rimfire.  The rifle started out as a 22 Hornet and is now a 22 K Hornet.  Barrel is 1" untapered with stock and fore end in proportion.  Odd looking thing.  I have been debating to rebarrel and restock this action.  If interested, I would sell the stock and fore end at a real reasonable price to get this project going.  Will send pictures via snail mail.  Send your address by PM.  Stock/fore end is unfigured walnut.  2520

Brad:

The recoil thing is not an issue. I fit barrels for .45-90 Creedmoor rifles in the same manner.  As far as better accuracy, that is very difficult to quantify. I do believe from my experience that a non stressed barrel/ action fit that is all square and flat combined with a concentric bore / chamber/ crown is easier to make shoot well and is a bit more forgiving of minor load and hold variations. Again, this is hard to quantify. Perhaps the bulk of the improvement is in shooter confidence. In any event, good workmanship is never going to be a detriment to accuracy.

Glenn

Good morning, Folks;

Glenn said, "In any event, good workmanship is never going to be a detriment to accuracy."  Truer words were never spoken!

As Joe said, I'd also like to hear your method.

Y'all be good.

horsefly

Gentlemens and Mr. Horsefly:

Now look what I done got myself into...


The way I fit a barrel is pretty much as described by Wayne Schwartz in 'The Modern Schuetzen Rifle' by Schwartz and Dell. My techniques vary a bit based on what tooling I have and things learned in a past life as a machinist, (the one after Shirley McClain and I ruled Egypt) but the method described in the book is very hard to improve on. 

The basic idea is that the receiver threads are the center of the known universe, the base line which everything else relates to. The action face and the breech face are squared to these threads. The bore, chamber, barrel shoulder and barrel face are squared to the barrel threads because when the barrel is set up in the lathe the bore is indicated to be parallel and concentric with the lathe spindle, and these are all cut in the same setup. With the action all square to the receiver threads, and the barrel all square to the barrel threads, and a proper fit between the threads, the whole universe is in harmony.

The key to this is the threaded bushing and mushroom lap described by Schwartz. An appropriate piece of stock is chucked in the lathe and turned and threaded to fit the receiver threads as closely as possible. While the stock is still chucked and the threads perfectly concentric with the spindle, a 5/8"  hole is bored into the the threaded stock. This hole is finished to size with a single point cutter so the hole is perfectly concentric with the spindle and hence the threads. This threaded piece is cut free and you have a piece of threaded stock about 1" long with a hole precisely down the middle.

This bushing is turned into the receiver threads, the result of which is that you now have a 5/8" diameter hole in the action perfectly concentric with the receiver threads. 

A piece of cast iron stock a little larger than the diameter of the barrel (say 1-3/8" diameter) is chucked in the lathe and turned to have an 1-1/4" long section that just slips into the hole in the threaded bushing. The end of this section is machined square, as is the shoulder up to the diameter of the stock. You end up with a mushroom shaped piece with a 5/8" diameter stem 1-1/4" long and a 1-3/8" diameter cap. The bottom of the cap and the end of the stem is machined square to the stem. This piece is parted off so the head of the mushroom is of convenient length to handle.

We are now ready to square the action. The threaded bushing is threaded into the  disassembled action until is it just below the action face. The mushroom stem is fit into the bored hole and so the bottom surface of the head or cap is unavoidably perpendicular to the axis of the receiver threads. A small amount of abrasive lapping compound is placed on this bottom surface and the surface is brought into contact with the action face. A few spins of the mushroom will cut a witness mark in the action face. The mushroom is removed and the action face examined. 

Typically the mushroom will only touch at one side of the action face. More lapping will cause this spot to grow and the resulting surface is perpendicular to the receiver threads. The idea is to keep at it until there is full contact all the way around. If there is a lot of material to be removed the action will need to be set up and machined, but ultimately it is lapped until the action face is flat and square with the receiver threads.

Now the breech block is installed in the action and the threaded bushing is turned in until it is tight against the breech block and puts the block under load as it will be when the rifle is fired. The squared end of the  mushroom stem is used to lap a spot on the breech block face that is square to the threads. The block may or may not require machining, but the idea is to get a flat, perpendicular breech face to support the case head.

So there you have it.  All the working surfaces are concentric or perpendicular to each other. There is no side stress, the barrel shoulder is equally loaded around its entire diameter. The breech face is perfectly square to the chamber so nothing moves when the cartridge is fired. All is good and right in the world.


Respectfully,

Glenn Fewless