Horizontal stringing in non-regulated PCP

[toddbrat]

Just like the title says, I'm curious about something I've been noticing on my Urban. I can get a nice long, fairly flat shot string, at least 50 at 14.5FPE. The spread is well under 4% so the air should be pretty consistent. What I'm seeing is horizontal stringing, nothing in the vertical, as the pressure changes. I do have the barrel free floated so the pressure difference in the tube shouldn't matter. Also, I see this on calm days at 50 yards, granted their could be a lot of small wind changes between me and the target but wind flags have stayed flat.

The stringing I see is usually to the right and can be as much as 1-1 1/2" and seems to only be in a given pressure range then moves back to the left to be back on target. During this range, the velocity is within 2% or better. I'm hoping to do some more "testing" this weekend at different yardages and see if it repeats.

Not sure if anyone has seen this before or if it is common but has me scratching my head.

[bandg]

Not familiar with the urban but a very interesting condition you noted.  Barrel harmonics springs to mind initially but seems you would need a higher velocity/pressure change to see that.   Some type of positional change in receiver at a given pressure?  Will be interesting to see if you find a solution.

[mann]

What hand are you and how heavy is trigger pull you could be pulling it that way. Just a thought

[Motorhead]

Loosen up the screws holding upper receiver to the air tube ... re-tighten paying attention to doing so evenly.
It may or may not help

[toddbrat]

Thanks Motorhead, I've seen that really impact my .22lr and thought of it here but haven't tried it yet. These are the things that go thru my mind when I don't get to shoot for a few days 

I'm right handed, generally shoot with a thumb up position rather than thumb over, and while I don't have a trigger gauge, I've got this one adjusted as much as I can without it being a hair trigger or going off when struck. I'd say it is a pound, give or take a couple of ounces. When I've been shooting something else and then pick up the Urban, it always surprises me the first shot, hard stop then bang, well more of a pfft, but you get the idea.

The valves in these do a really great job of self-regulating, my only other thought is as the pressure drops, it stands to reason the lift and dwell will be changing, curious if that would be enough to disrupt the air flow somewhere down the "chute"--that being the transfer port, barrel, and baffles-- before the pellet leaves the barrel. I may paint some pellets white and see if i can spot any spiraling. It would be great if there were someway to measure the amount and flow of air as it leaves the tube. I'd be interested to see how much turbulance there is in any given PCP rifle.

[Ribbonstone]

IF it's no a sneaky wind catching you, then it's likely some type of barrel stress going on.

Would follow the advice about bolting doen the upper to the lower evenly. 

Would certainly keep the barrel band loose whil I did the above, letting it move wherever it wants to while you mate the top and bottom halves.

Even if it looks a little cock-eyed to you, if it ends up in a stress free assembly,then try it.

Tighten the barrel band to finger tight...try it again.

Tighten the barrel band to normal tight...try it again.

Wouldn'thurt to be sure the brake/LDC is as firmly attached to the barrel as it should be...any shifting/wiggling there is sure to casue problems.

Keep on limiting the shots to the 4% are shown by your chronograph....but perhaps try 2 or 3% even though it makes for fewer shots.

[K.O.]

Just like the title says, I'm curious about something I've been noticing on my Urban. I can get a nice long, fairly flat shot string, at least 50 at 14.5FPE. The spread is well under 4% so the air should be pretty consistent. What I'm seeing is horizontal stringing, nothing in the vertical, as the pressure changes. I do have the barrel free floated so the pressure difference in the tube shouldn't matter. Also, I see this on calm days at 50 yards, granted their could be a lot of small wind changes between me and the target but wind flags have stayed flat.

The stringing I see is usually to the right and can be as much as 1-1 1/2" and seems to only be in a given pressure range then moves back to the left to be back on target. During this range, the velocity is within 2% or better. I'm hoping to do some more "testing" this weekend at different yardages and see if it repeats.

Not sure if anyone has seen this before or if it is common but has me scratching my head.

Hmm is highest variation at peak fps..? if so I might suspect some slight misalignment of the LDC blowing the pellet to one side...

But my Buc also has a sweet spot for  some pellets in the most noticeable in the 10g range but the difference is much smaller... on the order of .25" but it is also a Heavier rifle with a 50cc larger tank... and also no ldc/shroud...

Are you using a light or firm hold... it is a light rifle and with the small tank the overall harmonics of the rifle thru the fill range may be accentuated... would see how it behaves both ways...

First thing I would check is by pulling the LDC and checking its behavior...

just more food for thought to add to those ideas stated by others...

[AlanMcD]

Is it a predictable consistent movement from one location to another through a shot string?  I had that once on a .22 Marauder when I was tuning for power, and it was diagonal in nature.  I chased it every which way I could, and the only thing that got rid of it in that gun was regulation - that convinced me of what it really was: barrel harmonics . . . .

Everyone focuses on shot string speed and spread, but the reality is that two shots going the exact same speed on opposite sides of the curve (for example, the 5th and 35th shot of a 40 shot string) will have a very different shot CYCLE due to the conditions in the gun (pressure, dwell, lift, total air flow) and this can make for a different response from a harmonics standpoint.

I would try tuning to a different speed and see what happens - it could shift the resonance enough to maybe change things.

[bandg]

You might try a barrel weight to see if it eliminates or at least changes the stringing.  Might be a simpler way to narrow the cause down to possible harmonics.

[K.O.]

Is it a predictable consistent movement from one location to another through a shot string?  I had that once on a .22 Marauder when I was tuning for power, and it was diagonal in nature.  I chased it every which way I could, and the only thing that got rid of it in that gun was regulation - that convinced me of what it really was: barrel harmonics . . . .

Everyone focuses on shot string speed and spread, but the reality is that two shots going the exact same speed on opposite sides of the curve (for example, the 5th and 35th shot of a 40 shot string) will have a very different shot CYCLE due to the conditions in the gun (pressure, dwell, lift, total air flow) and this can make for a different response from a harmonics standpoint.

I would try tuning to a different speed and see what happens - it could shift the resonance enough to maybe change things.

I am usually a proponent of watching for barrel harmonics but in this case it is a about 15"  5/8" (.625) diameter thick walled .22 hammer forged barrel...

The Mrod .22 is a about 19"  7/16" (.4375) diameter thin walled barrel that is much more prone to harmonics... That is why if  I want a .22 Mrod I would start with a .25 Mrod and sleeve a Maximus barrel to .5"...

[Marc In Iowa]

I've been reading up on single shot loading vs. magazine fed. This isn't so easy to do with an Urban. What you can do, for diagnosis, just put a single pellet in the mag. Insert, fire, remove, 1 pellet, fire, repeat.

Do 10 single loads and see if it makes any difference. I had jamming issues with my first Urban and I always wondered, even though it might feed, what sort of marking might be going on with the pellet.

Easy enough to try for 10 shots, might give you another issue to think over, or not. 🙂

What range so you notice this stringing?

I've definitely had stringing issues and never tracked it down. You're not alone.

[toddbrat]

Thanks guys, lots of good info here. My Urban is not stock, I removed the pickle on the end and made a full shroud for it. The internals are ugly but seem to be working, I'll try to get some pics up later, and that could be part of my problem. It is only attached to the barrel via an insert made of PVC that is "clamped" at the breech end via duct tape which also adds thickness to keep the shroud in place. At the muzzle end, I left enough of the plastic to act as the spacer along with the plastic extension the original baffle stack was held against. The baffles themselves came from my Maximus BNM kit. It does function and there is a reduction in the report, but it also leads me to believe that could be part of my issues. I made an LDC for the Buccaneer and will probably do the same for the Urban to at least see if that will help any.

Also, I've removed the outer O-ring from the magazine, that helped tremendously with feeding issues. There is no friction at all when loading now. I see this at 50 yards, if the weather cooperates this weekend I'm going to try at different yardages and may end up cranking the power back up a bit. I've got it set for 14.5fpe, that's fairly slow for 50 yards. The least little puff of wind downrange can skew the path and the results.

 

Everyone focuses on shot string speed and spread, but the reality is that two shots going the exact same speed on opposite sides of the curve (for example, the 5th and 35th shot of a 40 shot string) will have a very different shot CYCLE due to the conditions in the gun (pressure, dwell, lift, total air flow) and this can make for a different response from a harmonics standpoint.

This is exactly what I'm thinking as it does seem to be pretty predictable, something in the cycle is different, be it barrel harmonics or the amount of vibration from the hammer spring or whatever. I'm going to go about it a bit more scientificly and record conditions/results from each string. If that's the case, it is easy to correct for if it is repeatable as all I do is punch paper in the backyard, but it is one of those things that will drive me crazy until the "ah-ha!" moment 

[K.O.]

 I feel there are almost always harmonics...  in .22 lr  trying to get that last mm of accuracy it is bull barrels and harmonic tuners...

 but that is at 100 fpe and we are talking mm... not an inch and a half... that much stringing to me points to something else... the Buc/Yote/Urban at the fpe level they are at would maybe have .5" movement at 50 yards at most in my opinion and most much less...it is a decently solid platform... with a nice stiff hammer forged barrel...

That is why I would look elsewhere this time... he said free floated the barrel I interpreted that to mean he removed the barrel band... on my Buc the band pulled down on the barrel and it did cause issues all thru the fill/string... My groups instantly went from about ~.8"-1" to ~.5"-.7" at 50 yards... then noticed the sweet spot for the 10.3 CPUM was to start with a 2400psi fill and that gained me another .25 or so... Still do not know if it is harmonics or just that there is a velocity difference that the CPUM do not like... the 10.3 AA behave similar but are just slightly less accurate... and they seem to like just a touch less starting psi...

Point is it is the Magnitude of movement was .3"-.5" ish not 1"-1.5"...  with the way the shroud is attached well that makes me suspect it much more now...

[K.O.]

The reason I brought up hold is that it is possible that the harmonics of the rifle as a whole can be different thru the fill range... with the lock time of these subsonic rounds and a light rifle a firmer hold may help...

here is a link to an urban sending the BBT 30g out at 640 ish fps... a slow heavy round that would imo accentuate those harmonics and have extended lock time...

https://www.gatewaytoairguns.org/GTA/index.php?topic=150114.msg1534120#new

[bandg]

As noted above, magazine?  I've posted this before-I have a Taurus .17 HMR revolver that has one chamber that will not group.  The indexing cut on the outer body of the cylinder is larger for that chamber than for the others and allows more rotation when cocked on that chamber.  It shoots extremely well if I avoid that chamber. 

[Ribbonstone]

Just to be sure I understand.

It didn't act this way before (with the factory LDC ), but acts this way after putting on a shroud...right?

So..yeah...it could be the shroud/clipping,could be the shround and barrel acting in independant vibration patterns, or could be that the breech attahcment it not strong enough to support  free floating the weight.

[bandg]

Missed that part.  Put it back to stock configuration.

[Gertrude]

since nobody has mentioned it yet, what Pellets are you using and seeing this horizontal stringing ?
 Also, have you tried shooting different types, brands, weights of Pellets producing the same results ?

[Ribbonstone]

You are right...even if they were the "same" pellets by the brand.tin, the current production (2018) may not match the ones you were using (2017).  Understand that pellet forming dies wear iout (acutally...they wear "big") and they have to be replaced now and again, si I kind of expect a little varation between "then" and "now"...but sometimes they just change the whole she-bang without bothering to relable the tins.


 But I kind of doubt that (an ASSuption on my part) as you've kep pretty exact records along the way.

Were it my7 rifle, I'd start with the SIMPLEST possible fix:

Clean the bore and start testing over.

[K.O.]

Hey Ribby is it possible that comment was meant for Jason's thread about Crosman pellets?