I have an M70 with two barrels... .308 factory and 22-250 AI Hart. I enjoy this set up and it does save a bit on hardware although I use two scopes, one for each barrel. Mainly, I like the concept and have experienced excellent repeatability and accuracy switching barrels and scopes. I used to do the same with an M700 that I had two bolts, six barrels, two scopes and 2-3 stocks for, but I had an inside action wrench and portable barrel vice for. For the current M70 I am using neither, just two-hand tightening. I haven't had either come loose and I do check frequently. Any problem with with this? I seem to recall reading that BR shooters did the same thing.
If the face of the receiver and the shoulder of the barrel are perfectly square then locking a barrel on 2 handed should hold securely, but I wouldn't reccomend it. Even if your checking the tightness frequently, it's the one time you don't it will be loose. Not only will you have loose barrel that won't shoot you'll have a headspace problem. I would reccomend using a action wrench and a barrel vise to install any barrel. Then you'll know it's tight.
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I've never built a switch-barrel rig, but I've seen a lot of discussion and opinions on barrel tightness. I cut close fitting threads and torque up 1", 1.0625", and 1.1" barrel tenons to around 100 ft/lbs with a light oil lubricant. I believe it makes for a very stable barrel/receiver joint that eliminates any possible shifting under the stresses of firing. Should contribute to accuracy, and minimize the risk of fretting and metal fatigue. I think that large, high intensity cartridges shooting heavy bullets absolutely need this joint tightness.
It's not something that we see very often, but the Brits used a lot of left twist rifling and right hand receiver threads in their barrels, and those barrels had to be made up hard, so bullet torque wouldn't unscrew them. The hand tight barrels you used had to be right twist rifling which would tend to tighten a right hand thread when fired, so were not a safety issue if they were headspaced in that condition. I don't know if it would degrade accuracy, but I get a bad feeling about a barrel I could unscrew by hand.
P.O. Ackley did a ton of experimentation on all kinds of barrel / receiver / bolt configurations, but I don't remember If he dealt with hand tight barrels. I'm going to have to look again.
Remember, this is just my opinion, not backed by any data.
Good shooting, Tom
Texas State Rifle Association Life Member
NRA Endowment Life Member
A big fast bullet will beat a little fast bullet every time
Thanks for both replies. I guess I'll need to track down an action wrench for the M70. Everybody seems to make them for everything but. I think my old set up came from Sinclair.
BTW... what prompted this post was having my first ever (in 35 yrs of reloading) case head separation. I'm not sure if it was caused by an untorqued barrel's threads "stretching" when fired, causing excess headspace, but it could be, I guess. Could also be really old brass but I don't believe too much in coincidence. I measured fired cases and reloaded ones prior to firing and set headspace to about .002" - .003" so don't see how this could happen. Primers are not flattened as I'd expect from a stretching and springing back phenomena but I should probably play it safe.
Thanks very much again.
Probably fired 8-10; trimmed 3 or so. Also, although smith indicated that this barrel (factory .308) had less headspace than it had when the gun went in to have the 22-250 barrel installed, due to receiver face turning, lug and bolt facing, etc it didn't really seem that way to me as cartridges I had already loaded chambered with no feel whereas they had a slight resistance to closing before. That's actually what I think has happened but so many people have "looked at me funny" when they've heard I hand tighten the barrel that, combined with the separation, it spooked me. Cartridge guage agrees with my smith... fired cases show less headspace length then they did before rechamber, but still remember how the previously sized cases felt "loose." As regards accuracy, both barrels shoot very well; the factory .308 as good as ever or even slightly better.