Re: Need advice from a rifle smith or others with rechambering experiance.
I would scope the bore and see what the bore looks like ahead of the chamber. If you take off 2.750" off the breech end, you will or likely will be able to clean up most of the heat cracking in the bore by cutting the new 308 Win chamber.
One thing you may have trouble with is getting a live pilot bushing that will fit the bore just ahead of the RUMs throat.
The throat will erode and be larger in diameter. IF you want true precision results, you will have to use a live piloted reamer and you will need a set of pilots that allow you to fit the bore properly.
When you start, you will be using a certain size bushing but as you progress with the chambering, you will likely have to step down one or two bushing sizes which are 0.0002" step downs.
If you do this, you will keep the chamber on axial center of the bore which will give you better results with the finished products. Will take slightly more time chambering as you need to pay attention to bushing fit but again, in the end it will be worth it.
Again, the only real problem is that likely the bushing diameter needed for the eroded start of the bore would be a custom order item. Not a big deal, you can generally get them custom made for around $20-$25 but to get it right you may need 3-4 different sizes unless you have a precision bore plug set that will allow you to accurately measure the bore diameter.
If the barrel is still shooting sub 1/2 moa, it should still do that when rechambered if you remove the entire chamber and use a piloted reamer that will guide off the bore axis.
As far as barrel life. I have rechambered my heavy 338 Allen Magnum twice. Starting with a 40" barrel, I put nearly 500 rounds down the barrel and I was not overly nice to the rifle during those 500 rounds. Accuracy started to drop off. I set the barrel back 4" and rechambered. Accuracy came back to near original levels and this time I really took care of the barrel but still accuracy started dropping off around 400 rounds down the barrel.
I set the barrel back another 4" leaving a 32" barrel length. Accuracy came back but was slightly off what it was originally and velocity was a bit less then before with same loads. This time however I have only 200 rounds down the barrel and accuracy is dropping off already.
Now, this is about as extreme as it gets for a chambering but it gives you an idea on barrel life. From what I have seen, you loose around 30 to 40% barrel life with each barrel set back you do.
For example, lets say you get 1000 rounds of good accuracy from the original chamber job. Second time around, you will likely get 600-700 rounds down the barrel and the next time probably half that.
It certainly does offer you extended barrel life for a relatively small investment but realize that once you rechamber the barrel, its NOT a new barrel again.
This of course all depends on how the original barrel was cared for and it sounds like yours was cared for well if your still getting sub 1/2 moa groups with a 1000 rounds of RUM ammo down the bore.
The 308 will also be much easier on the barrel and on bullets with the lower velocity range so your numbers may be higher as far as barrel accuracy life goes.
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