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

[HectorMedina]

We seem to continually forget the pressures that are reached inside a piston/compression chamber arrangement.

People! we're creating peak microsecond pressures in excess of 4,000 PSI's. Even across a millisecond time average, pressures exceed 800 PSI.
That nick in the lip gets flattened out completely during those times.

Parachute seals seal the LEAST under LOW pressures, that is why the "Blow with all your lungs into the barrel" test is so telling, it is the worst possible condition for the seals to do their job.

Yes it is disappointing to see a rifle assembled hastily and without the care WE put into our airguns, but production workers are what they are and as long as the rifle meets the specs, it leaves the plant.

In this particular instance, we want the rifle to perform with a pellet that is not quite within the "normal" realm for spring-piston guns. So, it's not like this should have popped during registration firing (power level documentation for legal purposes), as in that case, the most efficient pellet for the powerplant is used so as to keep to legal limits.

By all means, de-burr and polish the slots, it never hurts, but more important than that, get a strip of measuring tape, anneal one end and create a lip, then you can insert it between piston and mechanisms tube and cover the slots while you are pushing everything in.
Once the seal is past the last round hole where the cocking arm engages with the piston, you can remove the strip and finish the assembly. This will protect the seal as it is going in.

Lastly, you CAN "regulate" the barrel so that it shoots to where you want it to shoot. IF you are really set into what you are doing, that is the best option. BUT, I will insist: YOU NEED TO TRY DIFFERENT HEAD SIZES. You need to find accuracy before you want to attain precision.

I can almost grasp your liking of NZ. As a "flatlander" the Southern Alps must be simply amazing. Hope your stay will be happy and healthy and that you find a bright future there.
You'll never stop being a "Chicago boy", but that does not mean you cannot become a proud New Zealander as well.
NZ is a country that reminds me strongly of the USA in the 60's. And there is nothing wrong with that.

;-)

Keep well and shoot straight!




HM

[subscriber]

Thanks Hector for pronouncing that seal OK.

That said, you know that I can't resist a good argument:

While that seal may conform to the cylinder just fine at 4000 PSI; when the piston is half-way through its stroke, the pressure is around 15 PSI (maybe a little more due to adiabatic heating).  If the seal might leak at "low pressure", how much air volume might leak by the seal, before the air pressure reaches a few hundred PSI?

I think an easy way to answer the above would be by shooting benchmark pellets.  If they they reach velocities, with FPE that is within 5% of nominal of those with a "virgin" seal, then my argument is purely theoretical.   Else, it may be part of the problem seen with heavy pellets.  Even a stronger indicator, would be to compare FPE with heavy pellets; when using a virgin seal compared to this particular damaged one.

[mpbby]

David, I think you are complicating things. By now, I would forget about harmonics, and restart from the very basics.

No matter where are the POI, at 15 yards, you need much better groups.

Adjust the trigger to be light, find the holding and resting to achieve much tighter groups. To think about long distance demands that you are being excelent at short distance.

At 15 yards, with the HN FTT 5.53, the groups should be ok.  Or, you may try the JSB, 15.89 and 18.1 gr.  If you can't achieve very good groups, the barrel is not the culprit. Considering holding, resting and squeezing the trigger, the shooter is doing something wrong.

Just to let you know, for my D 350, the HN FTT 5.53 is the more consistent one. Trying heavier pellets, the Baracuda 5.53 was too tight; the 5.51 fitted ok, but I couldn't have good groups.  The JSB 18.1 5.52 is being much better.

If you have access to a chrony, to check the consistency of fps is very important.   

[Chouchin66]

David, I've been reading this thread & it seems possible that while harmonics may be at play, it may also be shot timing( position of barrel @ time pellet leaves the barrel.The heavier ' Cuda, taking longer to " pop the leade, in relation recoil cycle.
For a good long range pellet( as others have said, try a JSB/ AADF 15.9 grain. Try these out to 30 - 35 yards. At that point , yardage - wise you can seperate the wheat / chaff for a "distance" pellet. Also , pay no heed to head size labeling( rarely accurate) go by a die & batch #
Happy shooting.
See if you can get a JSB sample pack from Gunworks Ltd. NZ.

[David110]

Hector, I ordered a new seal, but after reading your comment I decided to give this seal a chance while I wait for the new one to arrive.

I also went and got a tin of barracuda match which has a bigger head size (5.51) fits nice and tight, same results ( 5-4" below aim point).

I did deburr everything and used your sliding method of reinstalling the seal. It even cocks smoother now.

Next, I'll try the CF sleeving with JB Weld, then the last resort will be the split collars.

Christopher, thanks for the Gunworks suggestion, I'll try that.

[HectorMedina]

Thanks Hector for pronouncing that seal OK.

That said, you know that I can't resist a good argument:

While that seal may conform to the cylinder just fine at 4000 PSI; when the piston is half-way through its stroke, the pressure is around 15 PSI (maybe a little more due to adiabatic heating).  If the seal might leak at "low pressure", how much air volume might leak by the seal, before the air pressure reaches a few hundred PSI?

I think an easy way to answer the above would be by shooting benchmark pellets.  If they they reach velocities, with FPE that is within 5% of nominal of those with a "virgin" seal, then my argument is purely theoretical.   Else, it may be part of the problem seen with heavy pellets.  Even a stronger indicator, would be to compare FPE with heavy pellets; when using a virgin seal compared to this particular damaged one.

Peter;

What you suggest is more or less what is done in the "Fire for register" process.
A gun needs to yield specced power or it does not leave the plant. A seriously cut seal will NOT yield specced power.

Even a trained person can rarely exceed 15 PSI's on lung power alone (more like 10 or 11 PSI's), so, AGAIN, blowing into the barrel with all your lung power is the WORST test you can inflict on a parachute seal.  If it seals with your lungs, it will seal within the first 1/4" of piston travel.

I agree with Marcos, and have been insisting that David takes care of the group size first.

I cannot agree with Chouchin because, as heavier pellets DO leave the barrel at a different point in the recoil cycle, they do leave the barrel when the muzzle is pointing "more upwards" than a iighter pellet, and at 14-15 yards, this is one of the typical quandaries. At longer ranges, yes, things get reversed. Unless David's bench technique is different from the rest of us and he lodges the heel of the stock in the crook of the shoulder, I cannot see a reason why the recoil would make the barrel point "more downwards" than under normal circumstances.
Though, in airguns, everything is possible . . .

Keep well and shoot straight!






HM

[David110]

Hector,
What I usually do to check the seal is half cocking while blocking the air chamber port and feeling the pressure release. It does feel well sealed with that test.

Remembering that the original problem was the barracudas specifically shooting 5 whopping inches low. Also remembering my test of holding the gun horizontally "gangster style", the pellets in that test shot 5" to the right (shooting right-handed).

1, it's not the weight or power plant lacking, since it did not go down with gravity in the gangster test.

2, it's not shooter error. I'm shooting handheld standing or sitting (half kneeling) so groups will naturally be looser. Yes, I'm not the best shooter but you can see similar-sized grouping with all pellets whether 5" low like the barracudas or on aim like the rest. Perhaps I'll get a chair and table out to show you all cleaner grouping 😂

[HectorMedina]

Hector,
What I usually do to check the seal is half cocking while blocking the air chamber port and feeling the pressure release. It does feel well sealed with that test.

Remembering that the original problem was the barracudas specifically shooting 5 whopping inches low. Also remembering my test of holding the gun horizontally "gangster style", the pellets in that test shot 5" to the right (shooting right-handed).

1, it's not the weight or power plant lacking, since it did not go down with gravity in the gangster test.

2, it's not shooter error. I'm shooting handheld standing or sitting (half kneeling) so groups will naturally be looser. Yes, I'm not the best shooter but you can see similar-sized grouping with all pellets whether 5" low like the barracudas or on aim like the rest. Perhaps I'll get a chair and table out to show you all cleaner grouping 😂

Now that you have explained further, I THINK I understand now.
I like the way you think, quite "unorthodoxically", LOL!
Given your "Gangsta" test, I would look into the breech seal again and make sure that I don't have a shim under the ORing.
The logic is that a bigger gap between barrel and breech face might give the high pressure spike a chance to "open" the breech a little.
The more time it can do so, the more difference there will be between a faster exiting pellet and a slower exiting pellet.
That "opening" would mean a downwards pointing barrel when held in the normal position and a right hand pointing barrel when held in the "gangsta" position.

It is also a possibility, in the world of airguns that take +45#  of peak cocking force, that the barrel got bent downwards, and while the effect is not that noticeable with FTT's or JSB's in the medium weight range, it IS noticeable in the Baracudas.
One of the things that few people notice is that in 0.177" and 0.20" the difference between the FTT and the Baracudas is MUCH smaller than in the case of the 0.22"; for example: in the 0.177" the difference between 8.64 and 10.6 is less than 25%, but between 14.6 and 21.1, it is almost 50%.
When discussing with JSB their lineup, many years ago (2009), the decision was to create a 14.3 gr. 0.22" to allow those users of Premiers to have an alternative, and then the next step up was the 18 grs. offering as the "heavy" pellet. Now you have the Monsters and the Beasts, but those are definitely PCP territory. Same could be said for the 0.22" Baracudas.
Another thing to note is that, as far as long range shooting, the best pellets for that are the 16 grs. JSB's (Exacts Jumbo). They are in the sweet spot between power/velocity & stability.
Gunworks seems to have them at the best price, though AirRifle Co also seems to have them.
GunCity seems to stock only the RS's (too light for long range), and the Predators.
https://www.gunworks.co.nz/shop/item/22cal-air-rifle-pellets-jsb


The OTHER thing that might be affecting the POI in such a hard kicking airgun is your trigger hand hold. If you place your hand high on the pistol grip and with a tight hold, it COULD make the gun shoot low with slower pellets.
Try using the benchrest trigger pinch and see what happens.
Normally, for accuracy's sake, all airgunners learn to have a very loose holds. In 90% of the cases, this works. Not always. But it is worth testing.

In the end, David, if your gun is yielding comparable groups between quality pellets (and that is something that was not evident from the first set of pictures), then you can easily "regulate" the barrel to shoot where you want it.
Barrels are "regulated" at the factory with some reasonably standard pellet, and the Baracudas in the 0.22" cal. are not one of these.

Thanks for the additional information!






HM

[David110]

Hector,
It's been raining here for days so I haven't had much opportunity for shooting. I checked all the things you mentioned and didn't find any shims or any evidence of problems with the barrel. It's still only the barracudas shooting south.

I'm now just waiting for the sleeve to arrive to see if that helps the barrel harmonics. I'm worried that the 16mm ID tube won't go past the front sight dovetail, which is 16.30mm at some points. If I do manage to get it on, will sleeving the barrel have a negative effect on the other kinds of pellets? I can't think of any way to test before making it a permanent change.

I read a long time ago somewhere that some magnums don't like a completely loose artillery hold. I tried your advice and let the gun float on my palm and the results were amazing. Still, barracudas shoot way south. Two people tried and the barracudas still shoot south. Should I just give up on the barracudas, get the 18gr JSB and move on with my life? 😂 It just guts me that it's a powerful magnum and didn't like the heavy pellets. It could be an H&N brand thing like you said.

The bottom is a 9 shot group using cheap gamo pro magnum penetration 15.42gr and a very loose hold.

[HectorMedina]

Hector,
It's been raining here for days so I haven't had much opportunity for shooting. I checked all the things you mentioned and didn't find any shims or any evidence of problems with the barrel. It's still only the barracudas shooting south.

I'm now just waiting for the sleeve to arrive to see if that helps the barrel harmonics. I'm worried that the 16mm ID tube won't go past the front sight dovetail, which is 16.30mm at some points. If I do manage to get it on, will sleeving the barrel have a negative effect on the other kinds of pellets? I can't think of any way to test before making it a permanent change.

I read a long time ago somewhere that some magnums don't like a completely loose artillery hold. I tried your advice and let the gun float on my palm and the results were amazing. Still, barracudas shoot way south. Two people tried and the barracudas still shoot south. Should I just give up on the barracudas, get the 18gr JSB and move on with my life? 😂 It just guts me that it's a powerful magnum and didn't like the heavy pellets. It could be an H&N brand thing like you said.

The bottom is a 9 shot group using cheap gamo pro magnum penetration 15.42gr and a very loose hold.

Yes, it DOES RAIN n Aotearoa! LOL.

As you have seen the loose hold does improve the groups, now if you really want to make your gun shoot to POA, then regulate the barrel! It's not a PB that holds tens of thousands of PSI away from your face. Just regulate the barrel and shoot whatever pellet you want that shoots well.
Really, there is nothing magic about an airgun barrel's orientation.

Good luck and keep us posted!





HM

[David110]

When you say "regulate the barrel" do you mean the sleeving and weights?

[HectorMedina]

When you say "regulate the barrel" do you mean the sleeving and weights?

Nope!

I mean "bending" the barrel in an extremely controlled manner at the proper place to obtain the most stable POI at the right spot where you want it because, IIUC, all this comes from the usage of the iron sights that have a limited travel. ¿no?

Though you COULD try a simple "riser plate" underneath the OEM iron sights . . .  ;-)

¿no?







HM

[WolfyW]

Ya those holds are a funny thing. My new spring D350 didnt like fingers wrapped around the stock. But didn't like an open palm either. At a loss, I did the open palm again but applied my thumb and index finger only. Magic ! And repeatable. Go figure.

[subscriber]

What about shortening the front sight post?  That removable front sight element seems like a cheap part to risk, and very little effort to shorten; or replace. 

You only need to lower the front post by 1 mm to get those heavy pellets to target center at 14 yards.  You could take off 1.5 to 2 mm to re-enable rear sight travel. 

You might also consider a lower front sight base that fits the barrel dovetail.  Perhaps one from Weighrauch.

A spacer plate on the rear sight would achieve the same thing, but is more work to make than a shorter front sight.  Depending on the rear sight screw lengths, you might be able to just put in 1 mm thick washers.  2 mm longer screws should not be too hard to find.  Then use 2 mm thick washers or two 1 mm washers...

[David110]

If you see my second post on the 1st page I posted a picture of the whole rifle, you can see the iron sight sitting on a bunch of washers. It does help but cosmetically it looks and feels like a bandaid. I ordered longer hex 10.9 grade steel screws to get a deeper bite and I'm trying to find an appropriate aluminium plate as suggested.

Shaving the front sight post will mess with the half-circle view and will need more than half of the needle to be trimmed. I had thought of that but decided against it for this reason.

CF sleeve and weights are still on the menu, just waiting for them to arrive from China. I'm interested to see what effect they have.

[subscriber]

David,

So, is the low grouping even with the washers installed; or was that an early experiment?

The barrel weight and sleeve will probably shift the group.  They may also make groups tighter or wider.  Mass damper tuning is usually used to tighten groups, rather than primarily to shift the central point of impact.

Anyway, trying a few option can only provide more information.

Bending the barrel sounds horrific, but there are a few threads on this forum about how to do it.  The amount of bending would be about 1 mm down between front and rear sights.  This assumes they are 19 inches apart, based on the 19.5" barrel length spec here:  https://www.pyramydair.com/product/diana-350-n-tec-magnum-premium-air-rifle?m=4171

If the distance between front and rear sighting planes is actually 18.75", then the degree of shift or bend will be proportionally less.

Barrel bending would focus the bend near the breechblock, because it will shift the front sight the most there.  Also, it is a natural point for the barrel to yield, due to the stiffness of the breech block.  Of course, this depends on the technique used.  The threads below show a few variations.


Posts about barrel bending - a bunch of selected links below the quoted post:

My new-2-me vintage Beeman C1 is shooting low....5" low at 10M with no hold-over.
It has a new Kruger 3-9X32 AO scope, I thought maybe it was defective so I swapped it with a Beeman 4x32 that has been fine on other rifles. Same deal.
Soooo, I swapped back the Kruger and put a .003 shim sandwiched with another layer of friction tape in the bottom of the rear ring. Now 4" and out of adjustment. 
  Standing back, looking at the barrel, I thought it had the dreaded Barrel Droop.
I took a straight edge and laid it on the barrel, turned it over and flipped it end for end... Yep, barrel is bent. 
 (In the pic I have a ruler to show the distance, but I used a true straight edge.)
 Problem is, the bend is close to the pivot block, about 1-3/4" away.
If it were the whole length of the barrel I would just support each end on wood blocks and smack it with my deadblow BFH in the middle.
  But being this close to the pivot block I will need to make a support and press some how. I know barrels can get bent "up" here from pulling the trigger when open and slamming shut, but mine droops down. I suspect from years (decades) of breaking it open. The barrel length on this C1 carbine is only 10-3/8" but it has a very FIRM lock-up, so you really have to palm smack it to breaking open.
 Also, the cocking linkage is pinned to the block, not screwed or bolted so while I can remove the stock, I would like to do it with the action assembled.
 - What have you used in such a case?
 - How do you know how far to go?
 - I assume baby steps, test firing each time but am worried about stressing the steel barrel making it weaker to bend again, or am I being paranoid?
 - I am sure there will be some spring back so thinking it would need to go "over-center" in the jig?
 I have an assortment of work benches, wood blocks, vices, C-clamps, Beam clamps even a Pipe clamp,  so anything home made I can probably fab up.
 I have "searched" the topic and nced did something like I am thinking except mine would need to be off set, not centered. (thanks Ed)https://www.gatewaytoairguns.org/GTA/index.php?topic=68011.msg155778015#msg155778015

 If you made it this far, thanks for your consideration and let me hear your methods other than "putting it in the fork of a tree and pulling".   
LOL!

I've been bending my HW airgun barrels for years so the poi is within 1" or so at 30 yards after optically centering the scopes and fine adjustments are done with the turrets. Here is the construction lumber, steel eye bolt, Delrin shoe, flat washer and nut contraption I've been using for a few years now...........

I always place the end of the "barrel tweaker" against the barrel pivot block so no bending force is transferred to the pressed in barrel/pivot block joint. Also, if the barrel is bent too far it's easy to bend the barrel in the opposite direction since the "pressure point" is always at the same place along the length of the barrel. Here is the bending sequence I used with the new HW95 barrel I bought on sale from that Canadian retailer when they stopped selling hW parts a few years ago.........

There are times when it's a bit more tedious to "put the poi where wanted" but with patience it has always worked. Matter of fact, a bend in an airgun barrel that moves the poi 4" at only 18 yards can't be seen with the naked eye!

Barrel bending threads:
https://www.gatewaytoairguns.org/GTA/index.php?topic=49873.0
https://www.gatewaytoairguns.org/GTA/index.php?topic=168557.msg155892495#msg155892495
https://www.gatewaytoairguns.org/GTA/index.php?topic=95539.0
https://www.gatewaytoairguns.org/GTA/index.php?topic=156328.msg155728487#msg155728487
https://www.gatewaytoairguns.org/GTA/index.php?topic=170803.msg155942963#msg155942963

[David110]

I bent it today but was too late to fully test it.

The elevation is tightened all the way down with the normal 15ish grain pellets, 'cudas are still 5" below so it will need about 12 clicks to compensate.

I'll shoot more tomorrow and post the results.

[subscriber]

Jolly good, Dave

It would be interesting to see a picture of the barrel out of your bending fixture.  Thanks

[David110]

Jolly good, Dave

It would be interesting to see a picture of the barrel out of your bending fixture.  Thanks

Doesn't really look bent 

[subscriber]

Thanks David,

Not looking bent is exactly what you want.  It needs to be subtle; not obvious.  And besides, the adjustment you want is just slightly more than one millimetre...