I have a Winchester m70 lightweight. I want it threaded for a suppressor but in order to get enough meat for threading I'll need to have the barrel cut back.
What will this do to accuracy? That is my only hesitation.
Have to agree with this. Cutting a barrel down shouldn't affect the accuracy in and of itself, but you need to get the barrel re-crowned. I'd recommend using a professional, or at least someone with the tools to do this right. That is unless you don't care about the end result and just want the practice. I'm guessing this is not the case with a Winchester.
Shortening the barrel will alter your harmonics and may require a slightly different loading to achieve the same accuracy as the original length. A quality job, and AllenM will do a quality job, of cutting, crowning and threading will give you one less variable to figure out as you search for that most accurate load. Shorter bbls tend to "whip" less and the bbl may actually be more accurate when you've finished.
You may not know this, but I do this kind of work.
Only thing is if the barrel has to be cut shorter than 20" to have needed diameter for threading then the barrel would have to be separated from the receiver to fit in my lathe.
Which may or may not be desirable. I have pulled many Remington barrel but have not done a Winchester as of yet.