Here is an example of vertical stringing of shots due to the unsupported portion of the scope base flexing as reported by Blaine D.
"When attempting to sight in my 338-06 barrel (at 100 yards), it was obvious that every shot was hitting approximately 1" higher than the last one. After about 6 shots, I had to spin the turrets on my scope to bring the bullet impact back on the paper. 5 more shot fully confirmed that the bullet impact kept hitting about an inch higher each time. Bullet impact also gradually strayed to the right about 2" during all of this. At this point, I noticed that the gap between the barrel and the bell objective was smaller than when I first mounted the scope. When you mentioned tension on the scope tube, everything clicked together and made sense. At first, I couldn't figure out how the base could move, but yet still remain tight and solid. Based on your remark, I reasoned that the when the base is flexing during recoil, the scope tube is probably shifting in the rings, but does not shift back after the recoil. This creates tension between the scope and the base, resulting in the base being held in a stressed state. This also results in the scope pointing downwards more with each shot. This would explain the upward vertical stringing. I took pictures of my scope, particularly the gap between the bell objective of the scope and the barrel. I measured the gap between the barrel and the scope itself (not the rubber cap) using feeler gauges. It measured 0.119". Then I removed the top half of one the rings to relieve any tension. I felt and heard a distinct pop as the rings shifted position on the scope as soon as I loosened the rings. I remeasured the gap, and it measured 0.150". The gap widened 31 thousandths of an inch! No wonder the bullets keep continually walking up the target with each shot. The tension in the scope tube was pressing and holding the tip of the base down more and more with each shot."
|The steel Burris base is potentially more rigid than most aluminum bases, but in spite of this, note the gap between the front bell of the scope and the barrel in this top picture, then note how the gap has closed after shooting.
See the inset close ups in the upper right corners of the photos.
|Now, note how much smaller the gap is below the bell in the photo below.|
Flexing the scope downward stresses the scope tube and the rings. The scope cannot return to its at-rest position. Poor design choice, and for no logical reason.
|Here is how I solve the problem by stabilizing the front tip of Weaver-type and EGW picatinny rail scope bases that overhang the tapered part of the barrel.|
|Note the small headless set screw through the far left end of the base. This screw just contacts the surface of the barrel.
The next screw from the left is threaded INTO the barrel.
NOTE: Locations of the two front screws varies from photo according to the application.
|Pictured is the Weaver #410 base modified so the front tip is anchored. We give the picatinny rails the same treatment to eliminate the "diving board" effect.|
Anchoring the front tip stops both the flexing downward as the barrel moves up in recoil, and if there is downward force on the barrel from a muzzle brake, the screw INTO the barrel opposes it.
The front tip of the scope base overhanging the tapered part of the barrel is held rigidly in place.
In the style of mounting pictured here, one more screw is added at the point just behind where the taper of the barrel begins..... what we refer to as "6-screw" attachment, 5 screws in the straight shank of the barrel, plus 1 into the tapered part of the barrel.
|Screws used are Weaver-style "oval head" screws, not flat bottomed "fillister head" screws that can shift.|
I have become pretty hard nosed over the years when it comes to using flat bottomed head screws simply because the base can move a few thousandths in any direction due to the clearance around the screw shank and nothing to keep the head itself centered.
At a minimum, if I do use a flat bottomed head screw I put a tapered seat in the base to center it. But I prefer to use the Weaver style screw, "V" head in a "V" seat so the base must stay centered and cannot shift.
This may be a minor detail that may or may not show up on paper, but fundamentally the security of a "V" assures the base cannot move in any direction.
Muzzle brakes that have the ports angled back DO in fact put a fore and aft strain on screws and scope rings, creating more potential for the base to move front to rear, rear to front with each shot.
|It's all about doing things as right as theoretically possible.|
|Do this simple test with Weaver style bases installed with just the standard 4 screws!|
Place one finger against the front end of the base at the gap where it overhangs the taper of the barrel.
Now, with or without a scope on the base put finger tip downward pressure on the front tip of the base with the other hand.
When you release that pressure, you will feel the base flexing upward
...... as obvious as a train wreck!
This is just finger tip pressure. Stop and think about the sudden force of recoil upward.
If the base is flexing, the scope has to be flexing with it, and you have all that leverage being applied to the rear ring..... and the scope tube, too, of course.
|Add to your barrel purchase from us either EGW picatinny rail or Weaver base installed with front tip of base stabilized $59, billed separately, base included. Picatinny base installed with front tip of base stabilized $79, base included.|
Note that hole patterns added are not "standardized". I vary the hole pattern according to the application.
|Picatinny bases are 3/4" longer & provide more eye relief. Click here.|
More eye relief and more places to position scope rings as well as locations for adding more scope rings to keep the scope from slipping in recoil.
|6-screw Weaver scope bases on UNtapered barrels still $49|
This applies ONLY to the UNtapered, straight, Contender S-14 type barrels, current production "G2" barrels, and UNtapered Encore custom barrels where the untapered shank of the barrel is AT LEAST 3 1/2" long so as to provide support of the base at all 6 screw locations.
Mike Bellm email: firstname.lastname@example.org
Phone: 541 956-6938
|SCOPE BASE INSTALLATION 101:
Just put the screws in the holes and screw the base on, right?
It is often a big mistake to just grab the base and screws and start twisting screws.
Here are some basic steps for intelligently installing scope bases and not jeopardizing a hunt with a scope base rattling around and while minimizing the risks of base screws stripping out.
1) First clear the barrel screw holes of any debris or thread locker, ie, "Loctite", that will interfere with turning the screws all the way in.
I grind all but about 1 thread of leade off the ends of taps and chase each hole with a tap. IF you are up to it and comprehend what to do, I take ALL the leade off of at least one tap and grind the very bottom end like an end mill so that it will actually cut flat at the bottom of the hole.
I find that many factory screw holes are too close tolerance to start a dead flat-ended tap into and must use a starting tap first before going to the dead-bottom tap.
Kurt supplies taps and tap handles on our website.
No taps? Acetone helps.
2) Common 6x48 scope base screws MUST have no less than 4 threads of contact. 5 or more is required for optimum strength where EXISTING hole depth permits. (I don't recommend drilling holes deeper than original.) Many scope base screws as supplied by the manufacturer have a minimal or insufficient thread contact.
Put the base on the barrel, run each screw in tight.... one at a time...., check for wiggle of scope base, then UNscrew it, counting the number of turns before it is free from the hole.
3) If a screw is too long, obviously, it cannot pull the base down tight to the barrel and screws may need to be shortened.
Repeating, install the base but only tighten one screw at a time, making sure that the base is pulled down tight by each screw..... NO wiggle.
After tightening all screws, remove them and examine the bottom end of each screw for a bright spot in the finish indicating the bottom thread and/or the end of the screw is being "munched" on the very bottom or the leading thread at the bottom of the screw threads.
If they are being "munched", shorten the screw by at least the amount of the bright surface damage you see, then go through the process again to be sure you have taken off enough. Screws showing a "munch" on the end will tighten down further with the "munch" removed.
4) If you do not have at least 4 threads of contact (5 or more is optimum for 48 pitch screws) get longer screws or deepen the contersinks in the scope base. Brownell's sells piloted countersinks for this purpose, but in a pinch a common drill bit can be used..... best done in a controlled manner on a drill press or mill for those of you who have that capability.
(Grind the unthreaded "nib" off the end of screws..... common to Weaver's screws. That nib wastes at least one potential thread.)
1) Make sure screws are not bottoming out in the holes and preventing the screws from pulling the base down tight.
2) Make sure you have at LEAST 4 thread of contact, ie., 4 turns of the screw.
I believe the rule is supposedly 20 inch/lbs. of torque.
HOWEVER, I put 'em on to stay, no doubt exceed 20 inch/lbs., and am prone to breaking screw driver tips. Allen, Torx, and slotted screws will limit you to how much torque you can apply.
Bases on barrels sent in indicate many of you hardly use enough torque. Go for TIGHT. If the barrel threads are in decent condition and you have enough thread depth, it is highly unlikely you will strip threads..... but use a little good sense about it.
Regarding open sights on TC factory barrels........
While open sights installed by TC are often only finger tight and must be checked for tightness. HOWEVER.... the open sight screw heads are quite thin and prone to breaking! Tighten them, yes, but don't get carried away!
Avoiding stripped screw head slots:
1) Use only a hollow ground screw driver in good condition that fits the slot. Meaning, do not use a wedge shaped common screw driver.
2) Bear down hard on the TOP end of the screw driver to minimize the tendency for the screw driver blade to force itself out of the slot. Use one hand to bear down, and the other to turn the handle.
Some general rules for Contenders and Encores:
1) Most Contender scope base holes are about .135" deep, except for .45/70 barrels which are only .125" deep. On most scope bases, screws will be pretty much level with the bottom outside edges of the base. More recent production barrels sometimes have holes .150" deep. On average, standard .135" deep holes will give you about 4 to 4 1/2 turns, ie, 4 to 4 1/2 threads, of contact.
2) Standard hole depth for Encores is approximately .185" with the potential for at least 7 threads of contact. Encore base screws will project 2 or 3 threads below the bottom level of most scope bases.
3 threads or less and a bit of recoil is almost guaranteed stripped out threads!
You would think that barrel and scope base manufacturers would assure adequate thread depth, right?
Don't count on it!
I had a an Encore barrel in the vise recently with only THREE, that's right 3 threads in the holes...... totally derelict and inviting "the unwashed", trusting shooter to either a disaster with the scope flying off, or if it made it to a hunt, the potential of ruining a hunt.
I put scopes on to stay and use only "permanent" red Loctite, but "your mileage may vary".
Is thread locker necessary?
Not necessarily, especially if you are prone to changing scope mounts from time to time.
Stuck screws, tips on how to remove them:
1) Simply smacking each screw with a flat punch and hammer will sometimes "break the seat" enough to free them.
2) Breaking the seat makes a tiny void for penetrating oil like PB Blaster to wick in and soften the thread locker. Smack each screw, apply penetrating oil, wait a few minutes, then try each screw. If a screw moves at all, give it more penetrating oil, wait awhile, and try again. You may have to repeat this a number of times. It works for me most every time.
3) Kurt is a proponent of using a soldering iron to heat each screw. 400 degrees softens thread lockers like Loctite. Makes sense, but I have yet to try it myself.
4) If the steps above won't free screws, you may have to resort to drilling off the head of the screw (s).
Use a drill bit #27 or larger, centered with the screw head, and drill only a little at a time until a black ring starts to appear in the hole you drill...... indicating you have gotten to the screw shank..... then STOP once the head is free. Don't get carried away and just keep drilling. Sneak up on it!
You want to leave as much of the screw itself as you can so that once the head is cut from the screw shank and the base is lifted off you have enough screw sticking out of the barrel to get ahold of with pliers to turn the screw shank out of the hole. Side cut pliers make their own flats to grip the screw and will let you get down closer to the barrel for a sure bite on the screw.
Stripped hole threads:
Best option is to chase the threads with Brownell's "over size 6x48" tap, aka .146"x48, then use their "Oversize 6x48" screws. The tap and screws are a bit spendy, but worth having if you have to remove many scope bases.
Broken taps and sheared off screws:
Most often either will result in damaged hole threads if drilling them out is attempted. Much of the time it is necessary to go a step larger with screw size, either .146"x48 or 8x40.
While not looking for such work, I do scope base and screw hole work routinely.
Costs vary and may require going up to the next larger screw size, #8x40.