A note on overtightening

[Keen]

Thr R9 has been a well behaved and generally easy gun to get along with. However over the last few months it has been annoyingly erratic. Same hole for 3 shots in a row, then suddenly a 1/2 inch up, down, left or right, which earned it a place on the gun rack. I'd "tightened" the stock screws a couple of times, the forward ones being a tad loose occ, but the rear trigger guard screws werealways tight.

Yesterday, after a session with the pellets dancing around the target, I was frustrated enough by this inaccuracy I'd decided to tear the gun down and inspect from the ground up.

I got as far as trying to remove the large trigger guard screw. Not sure If I was drunk, forgot how to read a torque wrench, perturbed off at the missus or what the last time I put the gun back together, but it took a remarkable amount of force to get that screw moving. Turns out it was tightened way above the recommended torque. So not wanting to needlessly dismantle the gun I re-torqued the trigger guard screw to 42'ish in/lbs. The fronts were good at 22 in/lbs and the small trigger guard screw at 16 in/lbs.

And order was restored to the world. 20 some pellets in a row pretty much where they were aimed.

My one concern was that I may have damaged the trigger cassette but it doesn't appear that way at the moment. Trigger let off is sub one pound and predictable.

Loose stock crews are no bueno, apparently to tight stock screws yield a similar result.

[Yogi]

Thanks for the information!

-Yogi

[nced]

With my .177 R9 I had "stock screw tension variation", mostly depending on the atmospheric humidity. This was especially noticeable when the gun was shot during low humidity days after zeroing the gun during high humidity days. Evidently the stock wood would shrink and swell with humidity variations and years ago I found that re-tightening the stock screws after they loosened up didn't solve the problem. Also, I found that excess tightening the screws to TRY maintaining the screw tightness only caused the wood fibers under the screw heads to crush. Here is the "official" stock screw torque specs directly from Weirauch.........
Here are the stock tightening specs for HW springers per HW in Germany..........
Front stock screws = 2.5 Nm is equal approximately to 22.1 in-lb, and Large trigger guard screw = 5 Nm is equal to 44.3 in-lb, and rear trigger guard screw = 2.5 Nm is equal approximately to 22.1 in-lb (I never tighten the small rear trigger guard screw tighter than 15 in/lbs).

Convert newton meters to inch pounds:
http://www.convertunits.com/from/newton+meters/to/inch+pounds 

Anywhoo, I finally solved most of the screw torque issues by machining some metal "piloted screw posts" like this.........




Clamping the metal inserts between the screw heads and receiver mounting tabs doesn't directly compress the stock wood so wood movement doesn't change the screw tightness.

As a side note, "screw cups" are available commercially however some designs have "flat bottom cups without pilot" which still compresses the stock wood fibers a bit..........






Testing the consistency of the grouping on separate days after installing the home made "screw cups and posts" without re-zeroing the scope. 18 yards shot sitting on a bucket resting the gun on cross sticks...........
 

[HectorMedina]

Hello everyone!

What follows applies to ALL break barrels, not only the German ones, but ANY break barrel that has at least 3 screws: 2 in the fore end and one at the rear, usually part of the trigger guard assembly. They MAY have a FOURTH screw at the rear of the trigger guard that also attaches the stock to the action, but that is almost non-functional for our purposes.

The POI depends heavily on the bedding, and not all beddings are exactly alike.

Stocks for these guns are DESIGNED to have a slight comb that puts some tension at the middle of the mechanisms tube/compression chamber

This makes the compression chamber work "under stress" that allows the gun to flex a little, but returns it to battery after every shot.

If the front screws are loose or wiggly, the gun shoots in an inconsistent manner.

If the front screws are too tight, the gun shoots outside of its optimum point of work-cycle and it also becomes inconsistent.

The addition of "pillar bedding" MAY be a solution, but may also cause some problems.

The addition of urethane strips for what is called "soft bedding" can also create problems. The TYPICAL problem they solve is that the screw wells in the stock are so worn out that there isn't enough compression applied upon tightening and so the gun is, essentially, shooting from loose screws.

So, how to cure all that?

When the gun is NEW, glue a flat washer of the right size to the bottom of the screw wells. And then use the star washers that the factory provided, OR (even better), use locking washers (Nord-Loc, or equivalent) to make sure that the original torque does not change with the vibrations inherent to the functioning of a spring gun.

Once the FRONT screws have been made an "unmoveable object" you can then "tune" the rear / front rear screw to its optimum point, which will be different for each gun and each caliber/power combination. Start tight at the rear, and test; release the tension by 1/12 ths or 1/16 ths of a turn and test at least three groups at each setting. You will see the group tighten, then start spreading, you can continue and see if there is another, better, "sweet spot", but if you do not want to, and usually at high powers there is only ONE sweet spot, you can then return to that point with some blue loc-tite or vibra-tite in the rear screw.
IF you can do this once in one go, then use Vibra-tite and once you have reached the sweet spot, let the gun rest and the vibra-tite settle. If you have to do it in several sessions, do it separately and once you have the position identified, repeat that position.

NOTE that using loc-tite can HALVE your torque readings; other liquids (oil, grease, sweat) and even using stainless steel screws WILL affect the torque readings, so IN GENERAL, torque recommendations for stocks, while good as a Maximum, are seldom the point at where the gun performs its best.

HTH, Keep well and shoot straight!





HM