The Tubegun in F-Class - Part 3
by Germán A. Salazar
And now, for the rest of the story...
This should have been the simple part of the project, drill a couple of holes, pin the scope base, mill a few spots for weight savings and call it a day. Nope... it didn't go that way - but on the positive side of the ledger, John really put his talents to work and we solved some problems that we hadn't even anticipated. Our thanks to Gary Eliseo who made himself available by telephone and gave us some very valuable advice. I should mention that John is the owner of a construction company and this kind of project is hobby time for him, he doesn't do machine work or gunsmithing on a commercial basis.
The day began early for John who showed up at the shop while I was in the dentist's chair 20 miles away. By the time I made it down there, he had already made the first item on our list, a stubby and hollowed out adjustment bar for the butt assembly. John did a very nice job on the bar, he even duplicated Gary's circumferential reference lines to ease adjustment repeatability.
The back side of the bar is solid and is drilled and tapped for an attachment screw as well as drilled for a roll pin that comes through the butt plate to prevent rotation. All of this was duplicated exactly as on the original.
The new bar saved us 2.7 oz in comparison to the original. In the picture above, you can see the new bar on the butt assembly, with the original bar underneath. If we had to find more weight savings, the bar could be thinned out some more, but we're in good shape on weight for the time being.
Our attention then returned to the scope rail. When we finished yesterday, the rail had been cut with a taper to allow the scope to be adjusted for 1000 yard shooting. However, the tapering process means that the screw holes and their counterbores on which the screw heads bear were no longer vertical, but at a slight forward angle. A check of the holes showed that they were large enough that there would be no interference with the screws. The counterbores, however, had to be straightened out to allow the screw heads to seat squarely and apply clamping pressure evenly.
If life were simple, we would have simply clamped the scope base down to the mill table, and brought an end mill down to countersink the hols in a vertical plane. Life is not simple.
You'll remember that in Part 2 of this series, I mentioned that the slot for the scope rail that is cut into the tubegun sleeve is actually angled; in my rifle, it is 0.020" deeper at the front than at the rear. The scope base attachment holes are drilled on that same angle for ease of manufacturing. Therefore, we would have to account for that angle in our setup.
We shimmed the front of the scope rail the correct amount, clamped it down and then countersunk the holes just enough to clean up the screw head seat. The picture above shows the setup, the one at left shows the re-cut seat. Once all three screw holes were re-cut, we were ready - or so we thought - to pin the base to the tubegun sleeve.
Windage Alignment
Apart from the elevation issues with my Leupold scope, I also noticed that the windage was fairly far over to one side. When I spoke to Gary about this, he explained that the slot in the sleeve is cut just a little wider than the scope base to allow the user to center up the scope fairly accurately. The procedure is to install the rail with only the middle screw, and it should be a flat head screw with a tapered seat, not the regular cap head screw. Tighten it snugly, and you should be able to rotate the base a little bit. Center the windage travel on the scope, install the scope on the base, bore sight the rifle on a reasonably distant object and rotate the base to align the vertical crosshair with the object. Remove the scope carefully to avoid disturbing the rail's alignment, put the tubegun in the mill, drill the holes for the alignment pins, install the alignment pins and then the screws and you're done. That's the theory...
Weaver vs. Picatinny
I'm not a scope guy - really. The truth is, I'm a die-hard iron sight shooter forced into F-Class due to shoulder surgery. Now, I'm not saying I don't like F-Class, I do and I'm having a ball. What I'm saying is that there are subtleties that I was not previously aware of and they can bite you. The Weaver vs. Picatinny rail distinction bit us a little this morning. I'll explain the problem, hopefully it won't be too tedious.
Windage Alignment
Apart from the elevation issues with my Leupold scope, I also noticed that the windage was fairly far over to one side. When I spoke to Gary about this, he explained that the slot in the sleeve is cut just a little wider than the scope base to allow the user to center up the scope fairly accurately. The procedure is to install the rail with only the middle screw, and it should be a flat head screw with a tapered seat, not the regular cap head screw. Tighten it snugly, and you should be able to rotate the base a little bit. Center the windage travel on the scope, install the scope on the base, bore sight the rifle on a reasonably distant object and rotate the base to align the vertical crosshair with the object. Remove the scope carefully to avoid disturbing the rail's alignment, put the tubegun in the mill, drill the holes for the alignment pins, install the alignment pins and then the screws and you're done. That's the theory...
Weaver vs. Picatinny
I'm not a scope guy - really. The truth is, I'm a die-hard iron sight shooter forced into F-Class due to shoulder surgery. Now, I'm not saying I don't like F-Class, I do and I'm having a ball. What I'm saying is that there are subtleties that I was not previously aware of and they can bite you. The Weaver vs. Picatinny rail distinction bit us a little this morning. I'll explain the problem, hopefully it won't be too tedious.
Leupold Rings - Compatibility Problem
As I was mocking up all of the pieces, I noticed that the bottom of the Leupold QRW rings were not truly level with the scope base. This meant that the scope wasn't really centered over the base and thus would require additional windage to zero properly. Aha! Well, it's one thing to find a problem, but it's a whole different exercise to solve it.
A call to Gary led to the Weaver vs. Picatinny discussion. It seems that for all their similarities, the two systems have some significant differences. That's probably not news to any experienced scope shooter, but it was news to me. Fundamentally, the required scope rail dimensions for each are slightly different. The CSS rail is dimensioned to work with either type of ring, but is closer to the Picatinny spec. Accordingly, my Leupold rings, which are a true Weaver type, were a bit too narrow at the clamping surface to come down far enough to level out. The best solution would be to use Picatinny type rings, but we had the Weaver type and there's a match tomorrow.
We considered milling the rail to narrow it up a little, but that is actually a very big job that would have required milling all four angled surface (top, bottom, right, left). Gary suggested mounting the two rings on a 1" bar and milling the contact surface to allow the rings to come all the way down. We did just that and after a few passes of the end mill, that removed about 0.007" total, the rings were perfectly level on the rail.
The picture at left shows how the base of the ring is now level with the scope base. It's an odd picture because the camera angle needed to show the light between the ring and the rail makes the ring look narrower than the rail, but just direct your attention to the even gap - that's what we were after.
Here's another picture of the modified rings with the crossbars and clamping levers installed. By mounting the rings side by side on the bar as Gary suggested, we were able to ensure that they were both cut the same amount and on the same angle. This setup took a long time to accomplish, but it resulted in a perfect cut.
Pinning the Scope Base
After all of the unanticipated work with the scope rings, we were finally able to boresight the rifle, and rotate the base to allow the rifle to be zeroed with the windage travel reasonably close to center.
The picture at left shows the tubegun clamped into the mill vise and the drill bit following the pin hole in the scope base and into the sleeve.
There are two pin holes, one at each end. Once they were both drilled into the sleeve, we installed the pins, rechecked the scope alignment (for the 20th time it seemed) and wrapped up for the day. Here you can see the roll pins already installed and the flat head screw still holding the base down. That screw was then removed and the regular cap head screws re-installed.
Making Weight
There was, of course, one not so small item left on the agenda: weigh the rifle. The F-TR rules call for a maximum rifle and bipod weight of 8.25 kilos (18 lb. 3 oz.). We made it with nothing to spare. I might look for a few small weight trimming opportunities, but for now, the rifle is complete and legal. We'll give it a trial by fire tomorrow at the club match at 500 yards.
Success!
John says it's always a good day at the shop if you leave with the same number of fingers you came in with. We took inventory and it looks like it was a good day.
Click here for Part 4
As I was mocking up all of the pieces, I noticed that the bottom of the Leupold QRW rings were not truly level with the scope base. This meant that the scope wasn't really centered over the base and thus would require additional windage to zero properly. Aha! Well, it's one thing to find a problem, but it's a whole different exercise to solve it.
The picture at left shows how the base of the ring is now level with the scope base. It's an odd picture because the camera angle needed to show the light between the ring and the rail makes the ring look narrower than the rail, but just direct your attention to the even gap - that's what we were after.
After all of the unanticipated work with the scope rings, we were finally able to boresight the rifle, and rotate the base to allow the rifle to be zeroed with the windage travel reasonably close to center.
The picture at left shows the tubegun clamped into the mill vise and the drill bit following the pin hole in the scope base and into the sleeve.
There was, of course, one not so small item left on the agenda: weigh the rifle. The F-TR rules call for a maximum rifle and bipod weight of 8.25 kilos (18 lb. 3 oz.). We made it with nothing to spare. I might look for a few small weight trimming opportunities, but for now, the rifle is complete and legal. We'll give it a trial by fire tomorrow at the club match at 500 yards.
John says it's always a good day at the shop if you leave with the same number of fingers you came in with. We took inventory and it looks like it was a good day.
Click here for Part 4
