GTA
General Discussion To Gateway To Airguns => The Shop => : MIKE I November 02, 2008, 03:51:52 PM
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New to the forum, hope I'm in the right one. My Gamo shadow 1000 is shooting low and right, approx 3" low (at the eleveation limit) and windage is only straight with the scope at it's adjustment limit at 20' from the target. I have tried two different scopes, reset and reversed and rotated the mounts. I have also noticed that the barrel is contacting the body of the gun on the left side at the hinge. This contact has always been there, but the gun always shot true before I replaced the seal at the entrance to the barrel. Could I have damaged the hinge pin? The seal had a raised lip as compared to the one I removed, it was compressed considerably. As a matter of investigation I cut most of the raised lip off and improved the situation some. My fear is that the seal did not compress and it caused the hinge pin to bend. Has anyone seen this before? Now, with the seal trimmed down I can shoot the normal tight groups, with the seal in it's supplied condition I could not get a consistant group at all. (shooting crossman premiers) Also, with the scope removed and the regular open sights it shot perfectly, further pointing towards a problem with the hinge as the rear sight is mounted on the barrel. Any help would be greatly appreciated.
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Sure does sound like the trick. However, since I was planning on using the gun next weekend, would replacing the factory plastic parts get me through? I like Paul's method of repair, but I need to make it functional by this Friday. Also, any advise on removing the hinge screw? It is extremely tight. I have only tried with hand force so far, I did heat the threaded end of the screw with a small butane torch figuring to soften any loc-tite, but no luck. I do have an impact driver, but i'm not sure about using it on this. Do I just need to put more umph to it or am I asking for trouble? I would rather shoot without a scope this weekend than not at all. Long planned trip for camping/fishing/plinking with friends.
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The breech block to action screw on the Gamo's are very tight. In most case's you will need an impact screwdriver and possibly a couple of spare tips. Sometimes the lockup is mis aligned and this will cause a problem. If you have any movement in barrel (down movement) after it is cocked then you very well may have a bad lock up. Gene
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hey gene, here's a ? for ya.
if your lockup is weak, what do you do about it? is there any way to fix it, or is it just toast?
i don't have this problem with any of my guns, just curious
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heres a ? for ya mike, is it shooting consistantly, or erratic?
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Dave a weak lockup will cause the barrel to droop when fired. Not only droop but lose the air seal at the breech seal. It depends on which kind of lockup you have Dave. Chsel or Ball. In some cases you are able to add an addition spring behind the barrel chisel or the ball. Each case is different and there could be other causes as well that may need tending. Gene
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what makes a ball detent stronger(or is this just a myth?)?
my 350 has ball, but my pat has chisel...both work fine, but i thought chisel was thought to be not as good.
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I personally do not think either one is better than the other. It really depends on if both were set right at the factory. I had one incodent where a Gamo Shadow 1000 had a mis alighned chisel which would never lock up completly. But that was the only one I have ever seen and I do own several Gamo springers which use the chisel lock. I also have a few springers that us the ball lock.
Gene
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ok, thanks, gene...good to know.
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It was very eratic until I cut down the seal to the proportions of the former seal, then the groupings were very tight, as it was new. It was not locking the barrel to the action consistantly as I was measuring with a .0015" feeler guage, when fully engaged it will not fit. However, even with the consistant groupings they are consistantly way off.
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Thanks Gene. It doesn't seem to have any movement that I can detect. Would you check for movement with a dial indicator? Or are we talking significant enough to feel?
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Dave if you think the barrel has a bad lockup then take off front sights. Lay a straight edge accross barrel breach and action block. With it locked up see if you can see a gap when you pul down lightly on the barrel. With a light pull nothing should move. Hope that helps.
Gene
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Inspired by Paul's method of bolstering the breach block and encouraged by Gene's advice on the hinge screw I attacked the project. I was able to remove the screw with the impact driver and some 24oz encouragement. The plastic washers measured equal still, but I manufactured some out of 5/16 washers. I upped the thickness to .067" for a nice firm fit. I did notice that the hinge pin screw does not fit into the breach block very tightly, but I do not have a way to fix that right now, so we'll see how it does. I re-assembled the barrel to action, it has a nice fit and solid "click" when locking barrel to action. Also, it appears to provide consistant fit on each cycle. I was thinking that I will get out the feeler guage and measure the breach block to action clearances to double check, but it looks alot better than before. Later tonight I'll see how it shoots, provided the measurements are OK.
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k, let us know how accuracy is
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Still no good. After replacing the plastic bushings the barrel appears to be still misaligned to the right, favoring the same side that was rubbing the fork on the action, though there is .010" clearance on the left there is still .017" on the right. I am beginning to wonder if my gun was dropped at some point, as it was o.k. at one point (note: the shims are .067" each side. The only reason I can figure for the breach block to look off that way is for the alignment pin to be misbored, or the fork bent. Question, what should the distance between the forks be at the pin location? Maybe knowing that would help determine what is going on. Also, in looking carefully at the breach block and action, I noticed that the face of the breach block did not appear square when looking down from the top, further there is a witness mark on the action to confirm the appearance. It is as if, when looking at the breach block seal surface the upper left corner was raised. I took a small file and worked this over to the point that it is nearly square across when the barrel is engaged. I can still fit a .oo15" feeler guage on the right side , but the center and left are tighter than they were. I took some time and knurled the right hand side of the pin to a slight resistance fit, figuring it to encourage the barrel to come back to the left also. Haven't shot it since the filing and pin refit, but we'll see.
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hmmm...can you show a pic of where you say its rubbing?
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Not able to do pics. If you were holding the barrel to the left, breach block to the right looking at the left side of the breach block that would be facing you, there is witness markings to the right of the hinge pin hole. This would be consistant with the barrel being pointed to the right as compared to the action.
With all I have done so far there has been no change in performance.
I have been rethinking the cause. I know it happened all at once. I don't know of the gun getting dropped, it is unlikely, but always a possibility. IF: the forks of the action were not square with the action, the pin would be misaligned thus causing the barrel to be off. I'm not sure about this, but what else could cause this?
I took the thrust washers out and re-installed the barrel to have a look at what it is like with only the pin in place, low and behold the breach block is closer to the right fork at the pin and closer to the left fork at the breach. (looking from above). AAARGH!!!! Just getting a little frustrated. Anyone been here before?
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...hmmm...i've never had this problem before...did you try what gene said about lockup spring?
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As far as I can tell the lockups are working properly, there is great spring tension on both of them and the barrel does not pull down at all when engaged to the action.
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HMM sounds like its time for a new breech/barrel
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I think you are right. I have resigned myself to the fact that it will take at least some machine work to correct this gun. Lucky for me my Dad is highly skilled and equipped in that area. Only problem is that he is out of town and I am no machinist. However, I can Yankee engineer pretty well and I figure that for now, it is shooting consistantly tight groups (in the wrong place) so if I can not bring the shot to the scope I will bring the scope to the shot. If I fail, worst case it is a 10.00 set of mounts sacrificed. Oh well, good news is that I recieved my GRT trigger and it is installed! Can't wait to try that out.
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Sorry about the no pictures. I am at a loss at this point. I don't sincerely think that dropping this gun set it off. I got to a point of frustration last night and did something rash. I set the breach block on a piece of wood, the front of the action on another piece about 2" from the forks and let it have it with a 48oz soft face sledge. After hitting it harder than I thought I should it hasn't changed a thing about the gun. My sleep deprived thought was that if dropping it did it than I would un-drop it. I have fixed much more complex things like that in a moment of inspiration, persperation and frustration. That's just too much 'tion.
So, after that I modified a set of scope sights, mounted up and shot some rounds, what I noticed is that the breach would have a gap after the shot, even when it had mated well before the shot. Consequently, I could put two shots through the same hole and the third would be 1.5" off at 15'. It took getting close to zero-ing the scope to really understand that the concern is barrel movement. So, I played some more with the pin, carefully knurling and fitting the best I could with a center punch and file, very crude but I have no machine shop at my house. I have also found that by appearance the breach block is only spaced squarely in the forks when using a .067" shim on the left side of the breach block and the OEM .063" on the right. Speaks of something tweaked. My thought is to have my Dad set the action in the milling machine and do some indicating to see if the forks are the source. That will have to wait 'till he gets back from a trip. I also took out the lockup, inspected and added about .090" behind it to snug it up, haven't shot it yet to see if it helped.
Meanwhile, the gun is packed up for the trip this weekend, shoots fine with open sights, and who knows, I may mess with the scope and stuff some in a more relaxed environment, maybe a few cold frosties will bring some clarity to the situation. Anyhow, I will get the opportunity to shoot the likes if a Gamo Big Cat (or three), a BAM that a friend just got (tuned and lubed), a winchester something or other and put a few hundred rounds through my PT80. Thanks for the interest and ideas. Oh, by the way, the GT III trigger is a dream! Fine piece of work and a cinch to install. If nothing else I will have the finest trigger action of all the Gamo's in camp! I'll have to yank it out if I decide to sacrifice the gun in the fire.
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well don't do that! i'm sure one way or another, we'll get your gun working right
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It would take more than a little frustration for me to do that. At the very least I have an open sight gun to use.