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I'm looking at a friends No4 sporter with the front rifling worn away, the owner/farmer says he'd prefer it shorter for easier handling anyway, so I'm wondering is there a right and wrong place to cut off the worn section for accuracy?
The rest of the rifling is is plenty sufficient for the guns modest needs as a farm/animal management tool. Thanks
Removing the barrel from the action and parting it off in a lathe is the conventional way to shorten the barrel. But it may just be easier to counterbore it a bit until the rifling is strong. Drill and a reamer is all that's needed, but again, it's easier to do on a lathe. Why not just try countersinking it a little first? Or is the muzzle worn more than 2-3mm?
IF you are careful and know how to use a file to properly remove cut marks and square up the front, you can just hacksaw the barrel back. It's a lot more sweat producing than using fancy machines, but it can work perfectly well given some degree of mechanical aptitude. You'll still need to crown the barrel, lest the smallest accidental contact near the bore throw up a burr that'll move the zero off. That's the hardest part!
Note that the muzzle can be up to 7 degrees (maybe a little more) off plumb without reducing accuracy, but the zero will be so bad that you may run out of sight adjustment trying to zero the rifle. So, the closer to square, the better.
No burrs interuding on the bore, either, as a little one will throw off zero like a "crooked crown", at least for a while, and a big burr can lead to a small bulge, thus defeating your efforts.
Duplicating the rounded form of the old crown isn't required.
Basically, it just depends on how good the craftsman is rather than the degree of sophistication of his tools as to whether the shorter barrel will be servicable.
I sporterized a number of sporter #4's and #1mk3's some years back. All I did was to chuck the barrelled action up in the lathe and cut and crown the barrel where it came through. That gave me about 18 1/2 " of barrel. I turned the crown and turned down the outside diameter until I could tap the front sight on. I cross pinned them again and that was that. They worked for truck guns and the guys were all overjoyed to get them. BUT, I had a lathe to work with...
If the lathe has a REALLY short headstock, you don't have to remove the action. Note that the original sight base through hole is cut on a taper, though. So to do that right you MUST have some fancy tooling. Or a really good hand with a file. My first sight base barrel taper cut was done on a precision OD grinder, but that's overkill. (? Maybe...)
Brownells also sells barrel facing tools with pilots for different calibres you can use in a hand drill to crown a barrel after cutting with a hacksaw. Personally, I use a lathe.
Another simple way to turn a barrel still on the action with a short swing home type hobby lathe is to mount the muzzle in the lathe with the stripped bolt in the body. Then use a centre in the concentric to the bore striker hole.
Machine to diameter the section of barrel that is going to have the foresight block re-fitted, Start the external chamfer then with a parting tool just slice the required length off!.
You could successfully complete the bore chamfer with a chamfering cutter in a slow running pillar drill.
If you do cut the barrel with a hacksaw, you can then use a lee case trimmer with a pilot that fits the bore to crown it - sounds a bit rough but I tried it on a 2-groove barrel that ended up shooting very well.
A .308 Win pilot is actually nearer .304". Put the case trimmer in the drill & be very careful. This will give you a flat but recessed crown, you can then carefully file the outside edge true to the crown.
The worst case is that the barrel is still no good.
It was recommended to me that a redneck (ie cheap and cheerful) method of crowning was to put a round head brass screw in a drill bit and use lapping paste to recrown it. Funnily enough one of my ost accurate 303s is a winchester 1895 which doesn't seem to have a crown at all, just seems to be cut flat / straight across.
I am embarrassed to say that my first and only barrel shortening effort was with a hacksaw, file and a circular grinding stone on my m96 Mauser. Something went wrong and it shot way less than an inch at 100 and served as my varmint rifle for many years. I seriously doubt I would be that lucky twice but as a 19yo it was as good as I could do. As a mid 40 year old it's still as good as I can do :)
Ps, I hate autocorrect . I will edit this when I get home so it reads properly.
Is there any consensus about where to make the cut? I have a vague memory/idea that cutting in the wrong place could make a more inaccurate rifle, something to do with harmonics etc.
I would guess just after one of the pressure points would be the spot, depending on how short they want.
If it's been sporterized, which I reckon would have to be the case, then there's not much way to tell if you'll stray far from a vibration node. It's not fixed! when you shorten it, the harmonics naturally change. But shortening adds greatly to the rigidity of a cantilevered structure. IIRC, the length variable in deflection is "X to the fourth power", so, if you double the length it's gonna be sixteen times less rigid! (That only works exactly on a structure of uniform cross section- a tapered structure will be even more rigid.) So, little changes can help!
Most of the guys that want a truck gun can't hit a bull in the a** with a shovel. If the rifle (18"bbl) will do 4" at 100 then that should be good. They did anyway. There wasn't much difference. These aren't match rifles we're building...
I rather suspect that the location of the cut will have negligible effect on accuracy with a milsurp barrel. If it were a bedded and cryo-treated air-gauged match barrel, maybe... but it's not.