Month: March 2006

Free Floating

Posted by on March 16, 2006

I have just free floated two barrels. Those are by no means the first I’ve done, but confirmed a couple of things that might be of passing interest to some.To start with, free floating a barrel is about the simplest thing in rifle work, can easily be done by anyone, and doesn’t need much in the way of tools. Only a screwdriver in fact. That’s probably the only minor glitch. The screwdriver is for removing the guard screws and should be hollow ground to fit the screw slots exactly. Anything else will certainly damage the screws if they are tight as they often are. Otherwise the only other items needed are some wood dowel, sandpaper, and a bottle of Tru Oil.Although the work is very simple, there needs to be a clear understanding of what is to be done and why. But it is surprising, considering all that has been published about accurising, how many rifle owners seem not to be aware of the basic principles. The owner of these two rifles wanted them floated because, he said, they were not grouping decently. He insisted despite being told that free floating has to do with consistency not grouping. I suppose I’ll be blamed if he doesn’t get the results he expects and he’ll demand his money backIt is possible in some cases that free floating might improve grouping by removing contact between stock and barrel and thus the effect of that contact which might vary from shot to shot. But not often. In light sporters the slimness of the barrel is one reason that absolute accuracy ie tight grouping is limited. That’s because barrel vibration is greater than with a heavy barrel because a thin barrel is less rigid than a thick one.A close stock to barrel fit provides some extra rigidity which is why, all else being equal, such a rifle will group more tightly than one with a free floated barrel. And why free floating will not usually improve grouping.The purpose of free floating is consistency. It is well known that wood moves with the weather because it’s moisture content fluctuates. No stock finish yet invented prevents it. It can make a rifle print to a different point of aim in summer and winter. A few years ago Col Charles Askins wrote about one of his rifles that would group in half MOA on any given day, but would shift it’s point of impact as much as six inches in one hundred yards from one month to the next. In such a case a slight sacrifice in grouping is a good trade off for eliminating that problem.Most factory sporters have a close stock to barrel fit. Climate variation causes warping of the stock. Because the grain of the wood is seldom perfectly straight, the stock bends and the forearm exerts sideways pressure on the barrel. Those who imagine it can’t have much effect should try shooting with a business card inserted between the forend tip and the barrel. I have seen it lift point of impact six inches at one hundred yards. The point of free floating is to leave a small gap, usually no more than half a millimetre, between stock and barrel, to allow the stock to warp slightly back and forth in the seasons without exerting said pressure on the barrel.Which brings us to this particular Sako. The first move was to see whether a piece of copy paper could be slid between stock and barrel at various places. A strip of paper could be inserted at the chamber end and fed down and around the underside of the barrel and out the other side. So it was already “free floated” at that point.It was equally loose up to about 100mm short of the forend tip, but very tight at the tip. I loosened the guard screws so the barrel could be lifted slightly out of the stock, inserted the paper between them at the tight spot at the forend tip, and tightened the screws. The paper was gripped so tightly that it couldn’t be pulled out, it just tore off.So the obvious next move was to sand out the high spot. It took a lot of sanding with coarse paper. So much in fact, that between cut and try sanding and regular fitting of the barrelled action into the stock, it took three hours. Bear in mind that it was a customer’s Sako and I didn’t want to end up with unsightly gaps from over sanding, so caution was the word.A lot of wood came out of the channel near the tip. Eventually I reached a point where the paper was a consistently easy fit from the chamber to the forend tip, except for a high spot at the last 20mm or so. That spot took a long time to sand out, and when I finally achieved it, I ended up not with a free floated barrel but one with a uniformly tight fit along the full length.Remember that I started with a very loose fit at the chamber and a very tight fit at the tip. Now it was uniform full length. I have the feeling that, as well as leaving a high spot at the tip, the factories cut the barrel channel deliberately low at the chamber, so that when the guard screws are tightened the forend tip exerts a lot of upward pressure against the barrel. As I said earlier this is not the first barrel I have floated nor the first time I have noticed this. It has the same effect as the business card trick, and although I haven’t tried it I’m sure that tightening the front guard screw would raise the point of impact.At any rate, I had now done no more than arrive at a situation in which I had close stock to barrel fit full length, exactly what most owners would expect to be starting with, and I now had to do the actual free floating.The other thing to watch for is the stock warping as mentioned above. After I had deepened the channel to where I wanted it, I needed to clear the sides a little. The high spots are identified by the paper insertion and later by business card insertion. The channel was a free fit on the left but tight on the right. After quite a bit of sanding on the right it was as tight as ever but the gap on the left was bigger. That showed that the forearm was warping to the left and was shifting to the left as wood was being progressively removed where it was pressing against the barrel on the right. It took quite a bit of wood removal to fix, fortunately not so much that the difference in the stock wall thickness either side of the barrel was noticeable.So you have been warned, it’s not always as straightforward as might be imagined. It’s not more difficult, just that one needs to be aware of what to expect so as not to make any mistakes from wrongly identifying what’s happening.When the sanding is satisfactorily completed, apply three coats of Tru Oil.But that’s not the end of it. I feel that free floating is only one side of a coin, the other side being action bedding. As noted earlier, floating is for consistency, not accuracy. Perfect action to stock fit is not possible, even with the most precise custom inletting. The slight movement that always exists between wood and metal prevents best accuracy. As also noted earlier, stock barrel fit is tight on most factory rifles. After I had floated the Sako barrel the looseness of the action in the stock was apparent. Previously the tightness of the barrel in it’s channel had also prevented the action from moving. The looseness was quite a lot, the side to side movement was clearly visible. That’s on a Sako which is a quality rifle.A loose action/stock fit is also disguised when the guard screws are tightened. Although it remains to be tested, I have the feeling that this Sako will not shoot any better than before. Indeed I suspect that it might be worse. In my opinion the job will not be complete until the action is bedded.Those who have other opinions are welcome to share them. In the meantime I offer these gems of wisdom for whatever they may be worth.[Originally posted to SATalkGuns -- Admin]

Scope Mounts

Posted by on March 14, 2006

Because of tolerances in rings, bases and receivers, plus slight but unavoidable errors in drilling and tapping, dovetail type rings will not be a perfect fit to both the dovetail and the scope tube. However, the dovetail of the ring will be a good fit to the grooves because one side of the ring dovetail is fixed and the other side (the clamp) is movable. Tightening the clamp pull the fixed dovetail into close engagement with the groove in the receiver or the base. Assuming of course that both are straight as they should normally be. But the rings will not closely fit both dovetail and scope tube at the same time. The trick is to tighten the rings to the dovetail then lap them, which will produce a precise fit to the scope tube.There will then be perfect fit to both dovetail and scope, which will be repeatable if the scope and rings are removed and later refitted as a unit. Of course Weaver rings cannot be slid off the bases unless the clamp screws are completely removed because they engage in a cross groove in the base. The same principle would apply to rings which attach to Brno and similar rifles and which have a recoil stud engaging the receiver. Such rings thus cannot be used as a “poor man’s” QD mounts.The answer is to fit a separate recoil stud outboard of the rings. As the recoil is transmitted from rifle to scope the right place for the stud would be in front of the front ring. It should then be possible to slide the rings and scope as a single item off the rifle to the rear. As I am not familiar with all the available direct mounting rings I can’t say which if any would work in that way, so the rings might be a custom proposition. With a pair of big pair of coarse knurled clamp screws that should allow fairly quick removal and replacement of the scope with no loss of zero. That’s as close as can be got to real QD capability without the cost.But the key to precise fitting is lapping as it is with all scope rings.[Originally posted to SATalkGuns -- Admin]