FWB sight random malfuntions

[badwrench5150]

Over the past month, my son has reported "sight doesnt work" several different times. We have 2 FWB 800x rifles and a 2700 Supermatch SB rifle all equipped with the factory sights. While at Anniston's CMP Monthly Match in March, in the 3x20 Air Rifle match, he broke position in prone to tell me after 16 clicks left, the shots are not following sight adjustment. I watched him turn the elevation dial 3 revolutions and take a shot while in Kneeling and the shot stayed at the same place stacking them together high and right. Needless to say his score was way off his normal. After we got back home I inspected/cleaned with canned air, and lightly applied gun oil to the adjusting screws/sliders and sight shot a box test of 20 shots and it passed all tests.

The next weekend on our 50m range (Sius Target), in prone, he was sighted in and random shots went in to the 6's. Ammo (Lapua Center x)tested for this rifle will shoot 18mm groups outdoors in the wind. Also shot a box of tested RWS R50 match ammo (10mm groups) to verify random shots. I swapped out the sight from the newest 800x (freshly cleaned/lubed from above)onto the SB rifle and shots followed the adjustments to produce repeatable box test with both center x and r50.

My thoughts are this: The factory sights are cheap enough to just make do. Has anyone else had issues with the FWB factory rear sights shifting zero or hanging up?

I bought an Anshutz Universal 7020/20 from Champions Choice. I had my son perform several box tests with it and all is well, so far. So is it him or the sight?

[pdeal]

The fwb rear sights are of a very good and reliable design. The best way to evaluate a rear sight is to have it tested with a dial indicator to determine if there is a problem with it. Also, look and make sure nothing is loose or cracked. One problem with the fwb sights is if the mouting screws are tightened without the sight installed on the rifle it can crack the die cast mount of the sight. I have seen this quite a few times. Also, trying to install one on an anschutz rail can crack it. Otherwise I think it is more likely a problem with the shooter than the sight but anything is possible.

[TenMetrePeter]

I am fairly sure the fwb 800x has the sliding plate design where the knobs push left and down on a sliding plate against a diagonal spring. If you cant get any movement with 3 rotations its possible the plate is stuck. You should be able to grasp the iris and physicslly and easily push down and left with only spring resistance. It must spring back to contact the adjusters. If not it may have grit in.

[badwrench5150]

TMP, your description of the design is spot on. I feel like there may have been something binding in the sight. After pushing the iris back and forth several times the sight began to respond better. I sprayed out the housing with computer canned air and lightly lubricated the sliders inside the rear housing. But this kind of malfunction/concern is not acceptable at a match. I personally don"t trust the sights that came with the rifle to befailure free even though one of these rear sights are only 3 months old. I bought an Anshutz 7020/20 universal sight last week. At least itseems to be a better desing by looking at it.

[TenMetrePeter]

You have me wondering if that is what was wrong with my Centra pro 57 sight last week (same design). First two shots beyond the 0 ring at 9 oclock and not how I would have called them. Gave the sights a bang then back to normal. I have the additional burden of a MEC swap hanging off it. Probably got knocked on the club rifle rack!
You and I have sights that would retail for under $200 so yes a £500 screw-both-ways Anschutz sight would be more reliable I guess.
EDIT
I'd be interested if anyone has taken off the backplate from this type of sight if stuff falls out or if they take apart nicely!

[pdeal]

No not a sliding plate design. They are pretty unique. They use a sort-of parallelogram with little leaf springs. Below is one in pieces. This one has a broken mount as i described. The spring assembly is shown as it screws into the main housing. Back where the eyepiece mounts you can see two little ball bearings. These are where the adjustment screws bear against the assembly. Not much to go wrong unless you crack them or strip the adjustment screws.

[TenMetrePeter]

Thanks for that. (a bit wierd!) Presumably though they rely on the springs holding the carrier against the two adjusters?

[pdeal]

Presumably though they rely on the springs holding the carrier against the two adjusters?

Right- the springs keep the whole assembly pushed up against the adjustment screws. I think it is a very clever very low friction (just the ball bearings contribute a little friction I suppose) zero backlash design.