R700 barrel setback question -- 204 to Tac20

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1stpitch

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Hi guys, I've been doing some "shadetree" smithing for years, but I've decided to try to get a little more serious. I have a little bit of experience with a lathe, and have use of several of them. For my next project, I want to setback the barrel on a R700 that's currently chambered in 204 Ruger, and rechamber it in Tac20. To get far enough back to clean up the throat, it looks like I'm going have to setback about 2 full threads. Here's the question, (at least the first one :)) is this too much to setback on a 700? I know I'll have a good bit of the undercut inside the action threads, and don't want to go too far. Is this safe? Have any of you done this?

TIA.
 
Barrel setback

One important thing to consider is the neck diameter of the new chamber. It is likely that a 20 Tac reamer will have a slightly smaller neck diameter than a factory remington .204. PTGs book of chamber prints shows the standard neck diameter to be .230 for the .20 TAC and .233 tapering to .232 for the .204 Ruger.If your new chamber has a slightly smaller neck, you need to set the barrel back far enough to cut the entire neck clean. This would be considerably more than two turns. According to my calculations, about .340 setback is enough to cut a clean neck in the .204 chamber.

Setting back factory barrels is ok for practice, but the results are sometimes disapointing due to the poor quality of the factory machine work, i.e. existing threads and chambers are prone to out of concentricity and there is usually not enough barrel to cut off the whole tenon and start over.

Scott Roeder
 
Thanks. That's the kind if info I was hoping to get. I don't have copy of the reamer prints yet, and was just going on case dimensions.
 
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