Home Forums Hunting & Shooting Gunsmithing How to correct off-center scope mounting holes?

I've got a pre-64 Winchester Model 70, .270 standard rifle, that requires the use of a windage adjustable rear mount in order to mount and boresight a scope without using up most, if not all, the scope's windage adjustment. I am currently using a windage adjustable rear base; but doing so has required that I crank the left adjustment bolt nearly all the way in before I could boresight a scope with the adjustments centered.

I do not like windage adjustable rear bases, for both practical and aesthetic reasons so my question is this: what is the best way to correct off-center scope mounting holes? Can they be centered by drilling and tapping them for 8x40 screws?

Or is it possible that the problem is with the barrel but not the action? The gun shoots consistently 1.25" 5-shot groups at 100 yards, and the groups move in a straight line vertically and horizontally when I adjust the scope.

Drilling them 8x40 should correct the mounting holes if that is the problem to begin with. You could get a set of talley LW or some leupold dd and mount them. Lap them and mount your scope and see how it aligns.

Millet "Angle Loc's"-Muddy

Rather use steel Talley detachables like the rest of my rifles

Weaver slot bases and Burris Signature Zee rings can correct for a lot of windage error.

Tried them, it took the +10/-10 inserts to center the scope. It worked, but I prefer a more permanent solution.

It you don't take it off it is a permanent as redrilling the holes.

Redrilling the holes and tapping larger will only solve the problem if the holes are only off by a few .001's of an inch or at least a full hole width so you can drill all new holes. Each solution comes with it's own set of problems. How far off is the center adjustment of the scope at the target with the scope adjustments centered? Rule of thumb is 1" = .001" of adjustment at the base- give or take a bit.
I would go with the Burris Zee rings and call it a day. Not all scopes are centered in their tubes when the adjustments are centered, so if you have enough adjustment to get to your target and some left over for field adjustment, you're probably good to go.

Bob

Carefully tig weld up the holes, This can be done and the weld will not be so hard as to be un able to tap. it also can be done so that no discoloration will occur on the action and destroy the blue

Welding up holes that haven't been measured to be off seems a little silly. First thing to do is determine what is really wrong.

I have corrected many poorly aligned base mount screw holes by silver soldering plug screws in the old holes and simply drilling and tapping correctly located replacement holes. Using a proper locating fixture or a mill and edge finder is essential for getting the holes centered to the bore. The same fixture or mill can determine if your holes are off center, as well. Take it to a good gunsmith and have the problem diagnosed and repaired by a pro. If your holes are not top dead centered, no amount of shims, goofy inserts, or "fix all" rings will solve the problem.

This is the best way to go if you want a complete and professional fix. However, keep in mind several of the professional fixes suggested above will also require some filing and finish work to include a reblue of some sort. At least the Burris Offset rings will get you in the ball park and then you can determine if you need to take the next step, depending on the severity of the problem and how anal you are about such things.
Keep in mind, a bore sight is a poor way to tell if your bases are aligned or not. Best to put your rifle in a good solid rest, pull your bolt and align your rifle bore with a target center (50 yards should be sufficient) and see how your scope crosshair aligns with the target center.
I've used a Bore sight for years to get my scopes roughly aligned on my rifles, but they have never been even closely centered enough for target or hunting with my best efforts.

Bob

I just assumes that Oregon45 wanted to do it right. If the holes aren't aligned with the bore, and with each other, no amount of shimming will cure the problem. My grandfather once said, "always start out with what you want to end up with". There is wisdom in those words. Buying self aligning rings that don't fix the problem just make the right fix cost that much more. Very little refinishing needed with the abandoned holes, as they will be covered by the bases. I would probably charge $25.00/hole to abandon and drill/tap new holes. That's $75 -$100 for peace of mind, knowing that the job is perfect, instead of jury rigged.

I had a problem like that on one of my rifles.I'm using Leupold rings and bases.My scope never looked square to the rifle barrel even though my rear windage screws were screwed in an even amount on each side.The rifle shot great so I never messed with it.I changed the scope on this rifle and it required different ring heights.After I installed the new rings and scope,the scope was square to the barrel.So it appears the problem was the rings and not the bases or misaligned mounting holes in my case.

I face this problem most often with cheaply priced partially sporterized Mausers at gun shows.

I have TIG welded up the the hole and re drilled. There can be problems. The added metal can be very hard, extra material is added in the wrong places.

I now often take a scope base and waller over the hole and countersinks with small end mills. Then I glass bed the base to the receiver with loose screws while fixturing with a scope and bore sighter. I adjust the reticle to the center of the scope tube. Tighten the screws when the epoxy is hard. The fixturing and bedding is the same technique as used to avoid ring lapping.

Sometimes the hole are so far off, I need to change the concave radius center point on the base. I put the base in modified V blocks in the mill and with a boring bar, move over the center. Steve Acker wrote an article about this ~ 10 years ago in Practical Machinist.

If you're simply drilling them out and retapping them to 8-40 then it won't help much. Someone extremely competent with a mill could place the holes correctly without drilling them out.

The one guy that comes to mind who I know could fix it correctly is Mark Penrod in North Manchester, IN. I'd recommend calling him and telling him your problem, he'll be able to do it right.

Simply drilling out the old holes to #8's is a waste of time & money. A drill will just follow the old hole leaving you with a bigger hole that is still misplaced. Mill the hole instead.

That said, until the OP determines what the problem is, all this talk about fixes is premature.

I've seen the same problem with 5 or 6 different causes, ranging from off center scope base holes to rings improperly installed, to bad scopes, to crooked, bent or off center barrels. And some with everything the way it should be but the barrel still wanted to throw bullets to one side or the other. Sometimes even up or down. Doesn't happen often but you work on enough rifles and you see it all, just about.

The fix will vary depending on the problem. There may be multiple viable fixes.

I have three receivers in the shop for repair of off center holes. Two are already completed rifles so welding isn't a viable fix for those. Instead, they will have custom bases made to fit. The other, will be welded and new holes drilled & tapped since it will be rebarreled.

Sadly, as a previous poster stated, too many home built Mausers have off center or misplaced holes.

Being a tinkerer without access to a machine shop, I solved such a problem by modifying the tops of (steel) bases by adding (MIG) metal on a side and filing to reshape. Lots of patience, but I like to tinker. It was originally drilled by a "gunsmith" using a jig, all the holes are off equally. So much for the magic of jigs. Measure, measure, and measure.

If you install just the screws part way and lay a good yardstick against them you get a good idea of just how far the holes are off in relation to the axis of the bore.

There are several ways to skin this cat with good results. Including locating the holes where they need to be on undrilled bases.

No the question was how to correct off center holes, Not how to find out if the holes are off center

Is that a custom piece you made?

Very interesting jig.


Mike