Vertical stringing

[DeoreDX]

Beginner PCPr here. Testing some pellets out of my Benjamin Fortitude. I have it turned down to shoot a little more than 11FPE (800FPS with the CPHP 7.9'). My target is 15 yards from my shooting position. I'm shooting off a folding workbench with a sandbag in the front only using a clenched fist under the rear to control the elevation. 5 shot strings with some JBS and CHPH.



JSB Diablo Exact 8.44g



CPHP 7.9



JSB Straton 8.26g





I picked up some tins of RWS R-10 Match and found with either the 7.0gr or 8.2 I get vertical stringing. As the only pellet I get vertical stringing with I'm going to ignore the possible causes of the shooter being the issue or velocity fluctuations caused by the air delivery system being the issue. I get really consistent velocity on every pellet I've shot so far (didn't have the Chronograph on the RWS yet). Pellet diameter/weight could cause this right? Just going by the reputation but I would assume that wouldn't be an issue with the R10 Match. I don't normally shoot wad cutters. Am I pushing the wadcutters too fast or far? I would assume the poor ballistics of a wadcutter would make them more sensitive to environmental changes (i.e. wind) but would it show up at only 15 yards as a 30-40% increase in vertical stringing?

 

[Yo]

Your gun does that because they're not it's preferred pellet. Yo!

 

[Yo]

Those wadcutters were designed to specifically shoot at speeds ranging from 350-650fps for 10 meter competition however they may do well at higher speeds depending on if your particular gun likes them. Yo!

 

[nervoustrig]

That's an interesting pattern you're getting. Like you said, it would be good to go ahead and confirm the velocities are consistent but meanwhile it’s reasonable to assume they are based on the JSB groups and the fact it’s regulated and the distance is only 15 yards.

It's true that wadcutters in general don't perform their best at high-ish velocities (800fps+) but I have found enough exceptions over the years to keep me trying. For example, a recent QB HPA conversion I did was grouping the 9.3gr RWS Supermags under a dime at 30 yards with a muzzle velocity of 950fps. Bad, bad news for chipmunks and gray squirrels. Another setup I have with a LW barrel groups them slightly better at this distance at 750fps. That's a lot slower but still substantially higher than the 500 - 550fps range generally considered optimum for wadcutters.

With that said, yeah a lot of the ones I test do not group well enough at distances useful to me (min. 25yds) to make the cut, but what I usually get is a random dispersion, not one that is predominantly vertical. If anything, I see more horizontal stringing which I attribute to wind more than anything else. So no, I don't think the vertical stringing you're seeing is an expected characteristic of wadcutters when pushed too fast.

What would be interesting is if the behavior changes at a different state of tune. If it does, then that suggests harmonics are at play. It looks like the JSB 8.4gr is doing well for you (it's as close to a sure thing as there is in the world of lead pellets) so it would be understandable if you choose to leave it be but if you get curious to experiment, I'd back off the velocity by, say, 50fps or so and try again and see how they do. 

Or you could look carefully at the barrel attachment and support structure and try perturbing something there to see if the behavior changes for the better or the worse. The Fortitude is shrouded so that second option can get a little complicated to process all of the places where this touches that and then this other thing translates the attachment point to this other place. But...it's pretty easy to just pick something at random and try it and then if you don't like the result, back it out. For example, if there's a bushing that keeps the muzzle centered inside the shroud, it probably has play in how it fits. You could try wrapping the parts with tape until it's a slip fit on both the ID and OD and see how that affects things. Another possibility...some folks have found a big difference in accuracy between having the shroud threaded tightly or loosely onto its collar. 

The curious thing about harmonics is what is advantageous to one pellet may be detrimental to another, and that highlights the fact there's always some lingering uncertainty when we think we've "arrived" with a particular tune. Is its current state really the best it can be? Did I just condemn that last tin of pellets as garbage because it didn't do well this time around?

 

[Yo]

Sometimes believe it or not wadcutters shoot better when loaded backwards in some guns that don't shoot them worth of beans the correct way. Yo!

 

[DeoreDX]

We have 13mph winds here right now so probably not the best time to try to shoot a group. I'll throw on the chronograph and see what the numbers look like during my lunch hour.

 

[Yo]

13mph winds can wreak havoc shooting wadcutters. Domes are better suited to be ballistically advantageous in windy situations. Yo!