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Discussion Starter #1
I just bought a NIB PS90 Triple Rail (black). I mounted an inexpensive NcSTAR holographic optic with 4 dialable reticles. At a mere 15 yards, I was struggling to shoot tight groups. The groups were walking on me, right to left. I tracked it down to me not setting the windage set screw properly. By the very end I shot a few tight groups, but I need to test further to build up confidence. Very fun to shoot, but I was expecting a tack driver.

When I field stripped, I noticed some play between the barrel shroud and receiver. A few degrees of rotatory play and a slight knock. I expected this to be more soild. Is it normal to have a little play here? This is not the cocking mechanism I am feeling. The barrel is tight to the shroud.

I was able to reduce 95% of the play by wedging a folded aluminum shim between the flat of the barrel shroud and the front of the receiver. I will test fire tomorrow to see if the shim falls out (I don't think it will) and to see if it improves accuracy (I think it might).

Like I said there are at at least two possible factors: sight drift (I think the windage set screw is now correct), and the play between the barrel shroud and receiver (now shimmed).

Edited: the barrel play is apparently a normal design feature and shimming is NOT recommended. I removed the shim before further test firing. My accuracy problem is probably related to my not tightening the windage set screw properly. I found that I need to push the rear of the sight body to the left against spring pressure while tightening the set screw. Once I did this, the last group was a one hole group at 15 yards, but then I ran out of ammo. More testing is needed. If this does not improve accuracy the next step is to invest in a quality optic.
 

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1) Do not shim the bbl/bbl shroud, that rotation & movement of the bbl is natural and a designed feature. You risk malfunctions by shimming.
2) NcStar is not noted for quality optics, if you want the best accuracy then mount a decent optic (SPOT Mk.III @$150 is the least expensive halfway-decent optic I'd mount on a PS90).
3) The PS90 was not designed for sniper-grade accuracy. The very best I've been able to manage from prone under ideal conditions is 1/2" 3-shot groups at 50yds using SS197 but most tend to be double that (figure 2"+ groups at 100yds).
4) When sighting-in or shooting for group size don't rest the bbl on your sandbag or rest as the direct pressure on the bbl will adversely affect accuracy.
HTH, keep us posted (and don't forget the pics!)...
Tomac
 

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Yes, you will damage the receiver if you keep the shims in there and keep firing the weapon. You do not want weight on the barrel when firing, and you do not want to stop the movement of the barrel. This would have been part of the cause of your accuracy problems, I would say.

Also - by not having the windage screw set right - that was part of your problem.

I've had my PS90 since Dec 2006. In Dec 2007, My groups were starting to suck. At almost the end of the session, after I kept having to adjust the windage - I finally noticed the mounting screws on my aftermarket rail (I have a standard PS90) had vibrated loose after 1 year. So, I went back home and rethread-locked them. I resighted in my optic again, and now I am back in business.

I can get 1/2" groups at 50 yards.
 

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Discussion Starter #4
I have not fired the weapon after shimming it. I guess I should remove the shim. Still, I cannot see how having play between the barrel and sight rail (I have a Triple Rail model) is condusive to accuracy. Have there been any reports of malfunction or damage when shooting with a shim, or is this theoretical? If so, I will remove the shim before firing it and concentrate on anchoring the inexpensive NcSTAR sight. I know it is a cheap sight, but it has so many other features that I like, that I want to make it work.
 

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I highly suggest removing the shim, getting a quality optic, and practice, practice, practice, your trigger pull.
 

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WildRice said:
I have not fired the weapon after shimming it. I guess I should remove the shim. Still, I cannot see how having play between the barrel and sight rail (I have a Triple Rail model) is condusive to accuracy. Have there been any reports of malfunction or damage when shooting with a shim, or is this theoretical? If so, I will remove the shim before firing it and concentrate on anchoring the inexpensive NcSTAR sight. I know it is a cheap sight, but it has so many other features that I like, that I want to make it work.
The barrel moves backwards a bit during the firing. Trust me - the gun is pretty accurate. We all wouldn't be raving about it all the time if it wasn't. U have a TR, so you won't have my issue of my optic rail coming loose. However - the play in the barrel must be there. People who hang overly heavy items on the barrel - or use one of those slings that actually attach to the barrel can damage their receiver over time (possibly) if the pressure remains on it during firing over the long term.

The gun isn't a sub MOA sniper rifle. But 1/2" groups at 50 yards ain't bad. My eyes suck to shoot decent groups at 100 yards - but some have claimed to be able to put 1" groups at that distance (it's really a close in gun, however - and' probably need magnified optics to shoot that kind of group at 100 yards).

You stated your groups got better at the end. Now that you have the optic tightened down - give it another shot.
 

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Discussion Starter #7
Since the barrel play is apparently a normal design feature, shimming is NOT recommended. I removed the shim before further test firing. So no harm, no foul. My accuracy problem is probably related to my not tightening the windage set screw properly. I found that I need to push the rear of the sight body to the left against spring pressure while tightening the set screw. Once I did this, the last group was a one hole group at 15 yards, but then I ran out of ammo. More testing is needed. If this does not improve accuracy the next step is to invest in a quality optic. Thanks for the advice.
 

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WildRice as you most likely know the PS90 is of a blowback action, But as the cartridge is fired the barrel recoils back for that short distance with the bolt allowing for the chamber pressure to drop to safe levels before the bolt leaves the barrel behind. so this is a little bit of a safety issue. Remember that this is more of a low powered rifle round instead of a pistol round. Hope this helps.
 

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Discussion Starter #10
Second Session, very accurate

For my second session I relegated the $50 NcSTAR Holo sight to my GSG-5 22LR and installed a $140 NcSTAR 2-7x32 "P4 Sniper" tube scope with lighted reticle. It has a variable intensity lighted cross hair ranging reticle that can be turned green or red. No power is a crisp conventional black cross hair ranging reticle, so no back up sight is needed. It has a quick release attchement that does not change zero. I know it is cheap, but it was very well featured.

I removed the shim as I was advised not to do so. I shot 100 rounds of the faster hollow point ammo. Sighting in was easy (no tools). I made a impromptu rest at the indoor range (no sand bags) and shot at 25 yards at 4x power. The magnification is very nice as my focus is not what it used to be. Another bounus is that I can see the group forming through the magnified scope without having to pull back the target paper.

There is a dial setting on the scope for 100, 200, 300, 400, and 500 yards). I set the dial to 100 and zeroed at 25 yards. The bullet will hit low at 10 yards due to the scope barrel offset, hit high at 50 yards and should hit close to zero again at 100 yards. For longer distances I can dial the range.

I shot several clover leaf one hole 3 shot groups, dead center on the bull's eye which is a world of difference from my first session. So it was not the PS90 that was inaccurate, it was the poorly set up Holo sight.

At one point I used the quick release and moved the scope back one notch on the rail for better eye relief. The gun still shot at exact zero. that means I can remove the scope (quick release, without tools) and replace it at will with no need to re-zero.
 

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Re: Second Session, very accurate

WildRice said:
The bullet will hit low at 10 yards due to the scope barrel offset, hit high at 50 yards and should hit close to zero again at 100 yards. For longer distances I can dial the range.
This isn't really so much of that offset - Even Ars do that - the bullet makes an arc. The best distance to site in the PS90 is 50-60 yards. At 1/2 that distance - it will be about 2" low - which is about how an AR would shoot at a closer distance too.

But - glad U are happy with the results of the PS90.

IF I didn't plan my setup for mainly CQC, I'd get a magnified scope for it.
 

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so i just got back from the range using the cmmg thread adapter. i didn't put in a shim either after reading comments about the stock sleeve shroud. there's probably a millimeter of play between the receiver and the top of the thread adapter when it is level.

i can't be sure without the right tools to take the short barrel off but from the little i can see it appears that the thread adapter left two notches in the receiver from either side. if i get some time to take a decent pic for review i'll post them.
 

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Good job and good news, indeed! :? Glad to hear it's shooting better for you now!
Tomac
 

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Were you really expecting a tack driver? The PS90 is more like a submachine gun than a rifle. If you can hit a man sized target at 100 yards that is about right.
 

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Discussion Starter #15
Actually, it did turn out to be a tack driver. With my impromptu wobbly rest (no sand bags) I could see my heart beat pulsations in the cross hairs of my magnified scope, i.e. it was not a very good bench rest set up. Despite that at 25 yards I shot several three shot groups with a tight one hole cloverleaf pattern, dead center on the bull's eye. By cranking up the mag to 7X I was able to see the one hole groups without drawing the target back. I don't know about you, but that is what I call a tack driver. I don't have the need to shoot people at 100 yards, in fact I will do 99% of my shooting at the 25 yard max indoor range.

My first session was disappointing but only because I had the set screw wrong on the cheap Holo sight. With the rigidly mounted tube scope, my PS90 is inherently accurate.
 
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