Really bad groups from .177 Discovery.

[fingerace]

Hey guys...

I am having a problem with the Benjamin Discovery. My groups at 18 yards can be up to 1.5 inches.

I mounted the scope on it yesterday. I chose a very light simmons scope for it and mounted it using 2 piece b-square adjustable mounts.

The rifle remains very light. - I think that's part of my problem. I can shoot sub half inch groups with the CFX at the same range all day long. The CFX is a lot heavier, with a heavy scope and a heavy one-piece base.

When I am using the discovery, my pulse makes the whole rifle move.

I have tried breathing slowly and steadily. It does help but the groups are still crazy.

I have to note that I have checked for scope and mount tightness. All the screws are tight, nothing is moving or vibrating.

The pellets I am using are crosman 10.5 ultra magnum premiers (that's what it says on the tin). I don't think they are exactly the same as the premier heavys. I have tried H&N field target trophy pellets as well - same disapointing results.

I am resting the rifle on a sandbag I made, filled with sandy soil. That's the way I shoot with the CFX.

I tried holding it tight and I ve tried holding it loosly. The loose hold has less movement.

Over at the pyramydair blog a reader suggested that I must adjust the barrel band so that the contact between the muzzle and the HPA tank is uniform and even.

Is there anything else I can try or check? Any suggestions will begreatly  appreciated.

Thanks a lot

[Progun]

Welcome aboard to the GTA Andreas. You'll get a lot of feedback on this question so please be patient. Have you chronyed your shots to see if you are going supersonic? If you are that could be part of the problem as diablo pellets tend to destabilize at supersonic speed. If that is the case step up to heavier pellets to get your velocity below supersonic speed . Also the chrony will tell you if you are getting wide velocity spreads which would affect POI.  How many shots have you made with the gun?A lot of barrels need a few hundred shots to season before accuracy improves.Let's go the other direction. Could your barrel need cleaning?If you clean it you'll likely have to season it again before accuracy improves.With inch and a half groups you don't have a catastrophic problem so it could be you need to relax more and get your heart rate down before taking the shot. just a few ideas here and others will surely chime in with suggestions.You might also check the crossman forum to pick up ideas. Good luck.

[ribbonstone]

Test the rifle without your influnece as much as you can.  It's not a recoiling design, so you can fully support it at the bench. Basically, get it "dug in" solid front and back so it's difficult to jiggle it around even when you try (just don't lay anything across the barrel).  Certainly not how you'll shoot it for real, but if you can demonstrate the accuracy of the GUN, can look for inaccuracy in the technique used when you shoot it away from the bench.

If it's still shooting poorly, then look for mechanical concerns.
1. Can you wiggle the barrel back and forth at the muzzle with just finger  tip pressure?  Some guns seem to have this issue.  One TEMP.EXPERIMENT is to shim between the barrel and gas tube with a bit of cardboard or plastic tape....just enough to take out play, not enough to really bind the barrel.  If groups settle down, then you might concider relocating the barrel band towards the muzzle.
2. Try a wider variety of pellets.
3. Check the muzzle crown (should be a whole lot of advice...some of it even good...if you do a search).
4. Most triggers suck. They aren't hard to work on (again, there are on-line pictorals...the 2260's trigger is much the same).


Chances are nothing is wrong...the gun is just light, the trigger pull heavy, and as you are a springer-shooter as well,you aren't use to the idea that you can really strongly support the rifle when testing.

Also, when testing just the ability of the rifle, don't sweat the small stuff.  If your groups are 2" to the left and 3" low...who cares so long as they are on paper?  IF the groups are good, can adjust the scope to center them....but way waste time when there are other pellets to test?

[melloroadman]

My .177 is very accurate at 25 yards with CHP 10.5 FULL FILL CHRONYING AT 890 - 880  but when I take it to 50 yards the high fill pressure gives me 3 inch groups but once the FPS lowers to 850 it shoots 1 inch groups which tells me the pellets can not take the speed and a heaver pellet or lower psi is required so as mentioned earlier use a chrony if possible or try a heaver pellet or lower pressure and see if that helps. Also the trigger pull might be throwing you off. With the heavy pull of the stock trigger make sure you are using only the tip of your finger. I have a post here that tells you how to do a lower link trigger mod if you are looking to improve the trigger and it also shows you the chrony numbers that I have been shooting . I hope this all will help you sort it out . Marvin

[fingerace]

Ok some more info for you guys so you can help me out...

I shoot two pellets through the rifle. The 8.64gr H&N field target trophys (which are the ones I use in the CFX) and crosman 10.5gr ultra magnum pellets. I ve recorded the velocity for the light pellets and it is below 950fps. so no supersonic flights here.

After changing the possition of the barrel band these are the groups I got:

http://i46.photobucket.com/albums/f146/fingerace/IMG_0091.jpg
http://i46.photobucket.com/albums/f146/fingerace/IMG_0094.jpg (same picture - no glare)

I moved the barrel band about one inch from the muzzle.

Maybe it needs a few 100s shots through it as you said. Just today with all the barrel band possitioning tests etc I must 've shot about 100-130.

yeah, the trigger is heavy and without a first stage but I believe I can pull it consistantly. However, I will perform the succesful trigger mod  melloroadman posted as soon as possible.

[melloroadman]

Please remember you are at the door step of the sonic wave which means the pellet is already starting to make its own wave motions and shifting the pellet a round . If you calm it down the issue of the barrel band will resolve it self more than likely . Also when you do the trigger mod KEEP YOUR FINGER OFF THE TRIGGER UNTIL YOU ARE READY TO SHOOT  the first time or two . The difference is like night and day . Marvin

[fingerace]

Marvin, the 950 fps are for the H&N pellets. The 10.5 gr crosman should go a lot slower but I didn't measure their speed yet. Funny thing is that H&N group better than the Crosman pellets.

I just can't understand what's wrong here. I am using the same technique as I am using on my springer. HOw come I get sub half inch groups all day with the CFX and get larger groups with a PCP such as the discovery. Doesn't make sence.

I am not discarting the chance that I may have a bad technique for the discovery, it's just weird.

That's why I am suspecting that there is something wrong with it.

How many shots did you fire before accuracy reached top performance?

[Splash]

Take the front site off and check the crown, I think some of the earlier ones went out with a bad crown. Also try a lower fill pressure maybe 1500-1700, that may help. Mine has had a tiny bit of work and works best starting at 1300 down to 900.

[melloroadman]

Slow down and quite worrying . Note that each of us has said lower the pressure or up the pellet weight . And the trigger in its self could be a issue for you if you are not able to adapt to a heavy pull. Try and note at what shoot count the gun groups best at . You need to find the sweet spot  . Most of the rifles in stock form seem to do best at 1700 - 1800 start to about 1000 . The chrony is your best friend right now if you have one to help you find the sweet spot. And even if the barrel is beyond repair which I drought its about a $17 replacement item . Marvin

[ribbonstone]

Am shooting a .22, so can't give pellet advice.  Can run 14.3gr. pellets at 870fps, but the best area for speed, consistency and shot count seems to be when adjusted for 830fps (added a "power adjuster" to the end cap to vary the force of the hammer strike to the valve).

Mine had a problem with the breech end.  the metal trasnfer port tube was a hair too long...which made the action not lay down onto the gas tube (it kind of 'tetter-tottered", balance on that slighty too high transfer port).  Tested it with the issue band..shot well.  Added a second band and it still shoots well, but now it's harder to knock the barrel out of adjustment (using the issue band, it shot just as well....that 2nd band is there becasue I tend to hunt in heavy cover and have been known to smack a tree limb with a barrel now and again).

Did the trigger mod., but used smaller diameter screws (4-40's). Housing  already had small holes in the right places, and a 4-40 tap cut full threads, so I just used the small screws for the two-screw mod.

There is a trigger TEST you can try.  IF you remove the safety, the tail end of the rat-trap spring that powers the sear moves farther in...which lessens the force on the sear...which lessens the force needed at the trigger.  HAVE TO TEST that there is enough spring force to set the sear when the gun is cocked.  If there is, and the sear sets and holds, can test some shots with a trigger pull that's about 1/2 to 1/3 as heavy as issued.  IT IS NOT A SUBSTITUTE for a trigger job, but you can go out and test right now without having to do any work.

[ribbonstone]

..go out and clean the barrel.  Even if you cleaned it well when you first got it, clean it again.

"When I am using the discovery, my pulse makes the whole rifle move."

All the barrel cleaning, site/scope adjustments, trigger mods etc. aren't going to help that.  Seriously, do you have a high blood pressure problem?  I hand pump my Discovery and within a couple of  minutes I am ready to shoot, and I am old and out of shape.

[fingerace]

I think I have eliminated that problem. At 18 yards with 24x magnification there is barely any movement of the crosshairs.

However, when I pull the trigger I can't keep perfectly still. In my opinion that little ammount of movement doesn't account for the 1.5 inch groups at 18 yards. I am not an experienced airgun shooter, but IMO it doesn't account for all that inaccuracy.

Maybe it seems small to me and it does affect my groups so much. You guys know better...

I'm glad to hear that.  I don't know if Tom Gaylord's post today is a reply to you or not, but it sounds like he is addressing exactly the same problem you have.  If you haven't read it I would encourage you to do so.

[fingerace]

Yes, that blog is a direct reply to my problem.

I have found a tin of JSB exact 10.2gr pellets as well. Now I can shoot and test the rifle.