Successful experiment

[Boyd Allen]

A while back, I ended up with an otherwise good looking barrel, that was chambered for a Panda (6mm PPC). My main benchrest rifle is built on an early Viper that has headspace, cone and tenon thread that is nominally the same...but the pitch diameter of the action is a few thousandths smaller than a Panda, or a current production Viper. The early inserts were thread milled and a little undersized. Years back I remembered a conversation with someone about being able to use a die to adjust thread diameter, so I looked around and found one(adjustable), made in Poland, and a holder (Chinese) for the grand total of about $40. I already had a 1'-2" mic, and had made a clip-on measuring wire setup out of .032 music wire, so I was all set. The short version is that I was able to carefully work the thread down to the same pitch diameter as one of my Viper barrels, and everything else looks good to go, the headspace is within .001 of the other barrel, so all I have to do is take one off and put it on. This has made me thing that rather than agonizing about thread diameter, it might be easier to cut a generous relief, cut the threads about .003 oversized, and finish with a die. The finish is very good. Of course there are no chatter marks.

 

[xs hedspace]

OMG!!! Heresy!! Almost as bad as me finishing a crown with a round head brass screwhead, battery drill, and valve grinding compound!!

 

[Boyd Allen]

Well, it has become more interesting. Evidently all thread profiles are not the same. The reason that I say this is that although both threads over wire measurements were the same. I could only get the barrel into the action less than half way. (It may be that there was some intentional taper put into the early Vipers to help distribute thread load. I seem to remember something along that line.) In any case, the adjustment screw that allowed me to open the die till it would snugly thread onto the Viper tenon, to find a starting place to work down from, soon stopped having any effect as I backed it out to reduce the thread diameter, and it became evident that the die would have to be made smaller by using the coned bolt that secured it in the die stock to compress the die within the stock frame. So, I was left to adjust the thread diameter with a rather coarse, crudely made cone tipped (to fit into a similar recess in the edge of the die) die retaining screw. Not being clever enough to use my calipers to measure the combined dimension of the screw and stock frame (20-20 hindsight) , I plowed on, by trial and error, to a point where the over the wires measurement was .006 smaller than what had only gone in less than half way. Wonderful (not) With that additional clearance (.003 per side) the barrel was a mite wobbly screwing in, and since there was a wide relief, that went almost to the bottom of the newly reduced threads, the wobble remained until the barrel shoulder was tightened against the action face. This was not reassuring at all, but upon consideration, with the action thread taper being larger at the action face. the effect was exaggerated, and we were only talking about half of the measured difference in thread diameters. I calculated the depth of an 18 pitch thread, and looked at a .003 gap between the jaws of my dial caliper to reassure myself. It tightened reassuringly abruptly, so tomorrow, I am off to the range, primarily to take some pictures of a new high end front rest, but also to try out the barrel, and determine what the insert is for my Vari-Base die needs to be ( I can wiggle a fired case from one of my other barrels in this chamber), and sight it in. The headspace is tight on a fired case from one of my original Viper barrels (no click here), but I could close it, and the cone to bolt face fit is closer than the thickness of masking tape, but it did not scrape of black marker ink with no case in the chamber, so I will at least give it a try. Onward and upward. One thing is for sure. If this thing shoots it will disprove forever the thought that a close thread fit is a requirement for accuracy

 

[Pete Wass]

A number of years ago now

I purchased a rifle from an acquaintance and he told me he had bought two Stiller Rattler actions to use to make rifles (of course, silly me!). One was fine but the other had "Coned Threads" in it. Said he sent it back and they tapped it out and it was fine after. It should be possible to single point the threads in that action the same way they do when squaring up Remington and other actions, no?

 

[Boyd Allen]

Actually, with my other barrels, there is no problem. I just overshot the thread diameter a bit, and there was nothing on the tenon to create a fit where the first thread or two are cut away at the front of the action, because the relief is to the bottom of the thread. Actually, I like the idea of a little taper. Vaughn discussed it in his book as one way to achieve better distribution of load on the threads, otherwise the first three do most of the work, because of tenon stretch as the barrel it tightened. I think of it as an advantage, but one does need to remember that it is there when first screwing in a barrel, so that one takes ones time and is careful. If I do another barrel this way (a used Panda barrel) I will back off some and sneak up on the setting, using my calipers to measure the span from the top of the screw that secures the die in the die stock, to the opposite side of the stock. Once I have that measurement for the fit that I want, I should be able to hit my mark every time. This barrel looks very good, but we shall see what the targets have to say about that.