open sighted Diana N-tec 350 hits low with heavy grain pellets

[David110]

I tapped 3 pellets with a nail up the skirt hole to widen the head size a little for a tighter fit.
Still, all landed 5" below so it's not the snugness of the fit. Definitely barrel harmonics.
Would've been nice if the randomness of the throw was too high not too low.

[David110]

Here is the test with some makeshift weights I taped on. It straightened the shot half of the distance. Interesting!

[subscriber]

David,

What you are seeing makes me wonder how tight ball detent locks the breech on your 350.   I am  contemplating the idea that the barrel is able to pivot downward slightly during the firing cycle, due to the ball detent not being as positive as it should be:

It is clear from the bright witness marks that your breech closes all the way, metal to metal.  The question is if it stays there during the firing cycle...

If you pull the barrel down slowly, is there obvious movement at the breech before the detent "pops"?  While lubricating the ball might seem like the wrong thing to do, it may also remove friction in its channel, that just might be preventing the ball from travelling all the way out for the most ridgid lockup possible.

If you slam the breech shut a little too hard, it may be bouncing slightly open then staying that way because of ball friction in its channel.  Yes, a sticky detent should also be present when shooting lighter pellets.  However, the timing of the breech block bounce and the angle of the barrel at pellet exit may be different.

Should you lube the ball with oil or grease, be careful to keep that away from the breech seal area, as air heated by compression will tend to ignite any oil exposed to the hot air stream - further confusing the issue and possibly causing other trouble.

Experiment with closing the breech more softly.  Just hard enough to hear s light metal to metal sound. 

If you are the second owner of this rifle; also try closing the breech slightly more firmly.  This, while maintaining upward pressure on the barrel slightly longer than would seem required to close the breech.

[subscriber]

Here is the test with some makeshift weights I taped on. It straightened the shot half of the distance. Interesting!

That is a signal!

[David110]

The closing mechanism is very defined and tight like a swiss knife, it always stops at the exact point.
When it's cocked and broken, the barrel part is nice and tight, doesn't swing down with gravity when it's cocked.

[subscriber]

That is good to know, David

This suggests the barrel is flexing over half a degree, based on the scale construction image captured below.  Perhaps that is not unusual for such a potent springer.

All dimensions in inches and degrees.  504 "  =  14 yards

[David110]

Excellent, thanks for that.
I'll go ahead and order the tube and start with the easiest possible and most good looking fix.

[subscriber]

To avoid disappointment, call it an experiment.  You could end up with it shooting 5" high   

For CF tube to reduce barrel flex, the tube would need to be bonded to the barrel.  That seems aggressive to me.  A tight slip fit might make a smaller, but possibly useful difference.  You will only know if you try it.

[David110]

To avoid disappointment, call it an experiment.  You could end up with it shooting 5" high   

For CF tube to reduce barrel flex, the tube would need to be bonded to the barrel.  That seems aggressive to me.  A tight slip fit might make a smaller, but possibly useful difference.  You will only know if you try it.

Oh, I wish it would go 5" high, then I can tighten the sight instead of cranking it open.
I just finished shooting with different weights/metal objects attached, nothing changed, still closer than without weights and I did take them off to recheck and yes the shots went further down without any weights.

I will definitely try it without epoxy first, what about red Loctite is it permanent?
If the tube brought the shots a little up like the weights ill be happy.

[subscriber]

Some of the carbon fiber tube thread links I posted above discuss pros and cons of different adhesives. 

Loctite has thread lockers, gap fillers and adhesives.  I am not an expert, so some research is indicated.  Generally, some Locite products are considered more permanents, and require higher temperatures to release. 

https://www.henkel-adhesives.com/nz/en/insights/all-insights/blog/difference-between-threadlockers.html

https://www.henkel-adhesives.com/us/en/products/industrial-adhesives/retaining-compounds.html

Solvents, such as acetone will often dissolve Locitite.   That said, the "wetted" front where the solvent could contact the adhesive between barrel and sleeve is tiny.  So, you might need to submerge the whole barrel in a closed tank, for rather a long time.

So; because a CF tube will have such a large bond area to the barrel, I would consider the CF tube permanent.  Unless you don't mind scorching the tube with a torch.  You could then probably clean off the residue with acetone.  I could be wrong about this

If you contact the people who made the CF tubing tensioned barrel airguns in the above threads, I am sure they can give you advice, including about reversing the application.  If you feel odd about asking them via private message, then start a thread on the topic; or post it to this thread.  Either way, I have no problem asking people for input via PM, on threads that are not my own.  I accept that some will decline...

[subscriber]

I think if the muzzle weight you attached is more securely and rigidly fastened, it might have more effect.  Something more like two hose clamps, than a slightly loose Sellotape wrap.

[Yogi]

Before you go the the barrel tube route, try adding a muzzle weight/break to the barrel.  That in and of itself should help.
Something like this:
https://www.airrifleheadquarters.com/page/page/1205004.htm   

-Y

[subscriber]

The aluminum barrel weights Yogi suggested above are elegant, but rather limited for varying the weight, or moving weight around to change the barrel harmonics.  Also, you may have to file on the barrel to get them on:

Rather than filing on the barrel to remove the raised metal where the front sight mount is swagged into the barrel, how about the split collars below?  You can use more than one in a stack, if one is not enough.   There are also longer versions of these clamps.

The clamps are easily moved, or removed.  There is no grubscrew mark left on the barrel, should you move or remove the "weight":

Select the barrel diameter from the menu on the left:  https://www.mcmaster.com/bushings/shaft-collars-and-couplings/two-piece-shaft-collars/system-of-measurement~metric/

Assuming a 16 mm barrel diameter (?) https://www.mcmaster.com/6063K17/





https://www.mcmaster.com/9746T2/




You can get them cheaper from amazon:  https://www.amazon.com/s?k=Clamping+Two-Piece+Shaft+Collar&ref=nb_sb_noss_2

Which is the best source in NZ is for you to figure out.

[David110]

I like the idea of sleeving the barrel to make the barrel thicker to aid in the cocking grip. I ordered a 20mm OD/16mm ID. Do you think I should get split collars in a diameter that will fit over the sleeve, or put sleeving to the side and try the weights alone first?

[subscriber]

You may need to get two sizes of collars.  One with a 20 mm ID.  The other for a 16 mm ID.

Of course you could wind a short section of tape tightly around the barrel and just attach the 20 mm ID collar without the sleeve.

Trying different combinations of weight; weight and sleeve; and sleeve only should provide useful info.

My prediction is that until you bond the sleeve to the barrel, that the weight will provide the strongest signal.  Moving the weight an inch up or down from the muzzle should make a noticeable difference on target.  If it seems to make no difference, move it half an inch back.  You will find several vibration "nodes"  where the effect comes and goes.

[HectorMedina]

Thanks for all that great info and suggestions guys.

Hector, what I want to achieve is long-range shots which is why I'm excited about the heavy grains. I'd prefer to tweak the gun to be more pellet friendly if I can rather than have to order specific pellets online. I live in the rural deep south.

What do you think about sleeving the barrel with a 2mm thick carbon fiber tube?     20mm OD X 16mm ID.

The pellet goes in deep enough, I don't think it touches the chamber end at all when closed.
I try to push them as far as I can to avoid deformation
Here is the after picture.

Can you send me the link to the muzzle weight you recommend?

David;

The farthest south we reached was Queenstown, from there we flew back. You can read a little about our experiences in Aotearoa here:

https://www.ctcustomairguns.com/hectors-airgun-blog/veronika-and-hector-down-under

We had a lot of fun and will always remember fondly the Kiwis (the people, not the birds).  ;-)

IF you are interested in long range shots, then the use of Iron sights seems somewhat conflicting, unless you are planning in the future to use some sort of "Ladder" / Tang sight, like the Credmoors of old.



I have, and they are a ton of fun. IF that was the case, then the low shooting of your gun is really no problem, the bad accuracy is.

From your pictures, I would suggest you get some 5.52 and 5.53 head size Baracuda Match. Whoever is importing the H&N line should be able to get you a tin of each. And try those.The improvement of the shot consistency between the "nail tapped" and the "untapped" pellets tell me that you have a "largeish" bore and the 5.50 headed Baracudas are not achieving their full potential.

One thing about wrapping things around barrels is that they need to fit TIGHT. Otherwise, you are introducing one more degree of freedom into the system and that will create REALLY CHAOTIC vibrations.

Every single barrel I have CF sleeved has been permanently glued, tensioned barrels are not as interesting in airguns as they are in PB's because the reduction of the bore through the extremely high tensions used is not conducive to accuracy in airguns and we used choked barrels. So, I would not suggest you think of tensioned barrels.
CF sleeving is not complicated, but it does work better when there is NO SPACE between sleeve and barrel. You can use Loc Tite 680 to attach one thing to the other, though I use JB weld when the CF sleeve leaves little space to be filled, OR even casting resin (what is used to make fiberglass) when the space desired is large.





In GENERAL, for springers, I prefer to use the CF sleeve to create a Reflex device:



The same device offers harmonics tuning capabilities.

You do not need a lot of weight. Just 30 grams moved mm by mm will show in a target at 20 meters.

I would use Peter's suggestion of the split collars, and see where that takes you, again, mm by mm; plus different pellets.

Last thing you need to know is that ALL breakbarrels are sensitive to the way the bedding is achieved, and the bedding is, in part, dictated by the torque in the screws.

So, get a torque wrench and tighten the two forend screws to about 20 in-lbs.

Then tighten the rear stock screw to 30 in-lbs and "unscrew" that screw by half turns, you should readily see a change in the grouping till you reach the best point possible (DO NOT pay attention to the POI), then retighten and undo by 1/4 turns, then retighten and undo by 1/8 of a turn till you are satisfied that you found the optimum for your rifle.

Once you have done that, then worry about the POI.

;-)

Kia Ora Haere Ra!







HM

[Yogi]

I like the idea of sleeving the barrel to make the barrel thicker to aid in the cocking grip. I ordered a 20mm OD/16mm ID. Do you think I should get split collars in a diameter that will fit over the sleeve, or put sleeving to the side and try the weights alone first?

David 110,

My understanding is that the weave of the carbon fiber tube is extremely important to effect the barrel harmonics.  If the weave is not right you might make it worse.

-Y

[David110]

Hector,
Beautiful photos, it looks like you had an amazing time here and a successful competition. Isn't it a beautiful country, I feel like I live inside a calendar 😂 It's funny that I came from the flattest spot, Chicago. This is my 3rd year living here in Dunedin, not far from Queenstown.

All the toys look amazing but I had another look and no gun-related accessories can be brought into New Zealand without police permission and/or a gun license.

While I'm waiting for the CF tube and the clamps Peter suggested, I decided to make sure everything is working and clean from the inside out. I took out the piston seal very slowly and carefully so as not to damage it as the gun has yet to be deburred and I wanted to get the seal out in it's factory condition. It does seem to have a small nick on it.






I know it won't cause the drop issue we've been discussing, but will this have any effect on the timing of the pellet leaving the barrel with a  heavy grain?

[subscriber]

That seal appears "shaved" at the lip.  If it had been split, I think it would have much greater potential for leaking.  That said, any obvious damage to a new piston seal seems unnecessary and disappointing.

It is possible that the seal is leaking enough air that the piston crashes into the front of the compression cylinder harder than usual, on firing.  This crash could have more effect with heavier pellets because they take longer to leave the barrel.   

That could mean that you are seeing more barrel vibration and a slight loss of velocity due to the seal leakage.  With heavy pellets, those factors might very well account for the point of impact shift you were seeing.   I think it plausible that your original trouble will go away after you replace the seal...

[David110]

Yeah, I'll definitely be changing the seal and testing it with this same tin of barracudas before anything else, just to know.