Cast bullets and choked barrels.

[EdinGa]

Spent some time with the Urban and the BBT 30gr fp. The only camera I have that does 120 fps is my night vision setup, so the video is black and white. I had some difficulty seeing the reticle in the sunlight so all shots aren't on target, but I was mainly looking for instability in the bullet flight. Velocity is in the 630-640 fps range so there is a lot of drop on the far target. I'm pretty confident in the accuracy out to 35 yards, but I don't think I'll be pushing it beyond that. The steel target is at 35 yards and the cardboard with the black dot is at 75.

[K.O.]

The Black and white really gives it an appropriate Halloween feel...

can really see the trajectory of the 75 yard shots also... am a little confused tho... I thought the Qb was set down at 20 fpe and  Qb barrels to my knowledge not choked... the tittle is cast and choked barrels... so I think maybe you meant your urban... or did you turn up the Qb..?

[EdinGa]

You are correct, it was the Urban. Not sure what I was thinking. I edited the post. Thanks!

[K.O.]

side rest? front  rest... sitting knee rest... hold looks a bit loose(movement) during/after shot cycle... possibly with the heavier slow round it needs a firm hold due to the extended lock time and more time for the harmonics to work/build...with the lighter faster pellets not so much an issue imo... I bet the shot cycle with the 18g JSB would show much less movement... heck some of the .22  rim and centerfire rifles I have shot like a looser hold and some do not... worth trying..?

[K.O.]

But yep lock time and follow thru...

Heck when hunting practice and improvised rest such as tree side rest I tend to push into the rest with a firm hold... just because I tend to do better that way... with shooting stick a slight down pull helps me... and that is without long lock time...

[rsterne]

FYI, even with a lightweight hammer the time from tripping the sear to the hammer hitting the valve in a PCP is at least twice as long as the dwell time of the pellet/bullet in the barrel.... Therefore, the percent difference in total lock time (sear release to pellet exit) varies only a small amount for pellets within the velocity range we normally use (700-1000 fps)…. The time it takes for your brain to tell your finger to pull the trigger is MUCH longer than the hammer travel time.... Between the time your brain sees the crosshairs on target and says "fire" and your finger pulls the trigger is about 210 mSec (for a young person with good reactions).… compared to maybe 5-10 mSec for the hammer travel, and about 2-4 mSec for the pellet travel in the barrel....

Bob

[K.O.]

Bob it is about the rifles comprehension of time not ours... and that the double heavy round may have more toqure load and more newton reaction 15g compare to 30g... same weight of air hitting twice the weight before break away...

point is different harmonics...  but stronger possible 1st order possible with forever and a day(~30-50% increase in forever) instead of forever...give more time for constructive and destructive resonances to build and be effected by small differences in that light hold..

so say the rifle feels it at the rate of ~5700 fps( speed of sound in 347 stainless) (MOE determined)... so that means in a 12" barrel  the vibrations have traveled its length and back...57 times in that centi second... 10 milsec...10^-2second...and the barrel is attached to something with its blending constructive/destructive resonances...

so lets add a few milliseconds or 3 before the round comes out and well a firmer hold might bring slightly moe accuracy... cause the resonances have had less time to build in different ways because of differences in hold...

now a heavier hold well it should act somewhat like a damper on all that...

least that me theory and I is a sticken tuh it..

[EdinGa]

It was a front rest, but the issue was the scope cam. The camera sits behind the scope so the entire rifle was out in front of me. I wasn't as concerned with accuracy as I was with seeing if the bullet was spiraling or losing stability. I'll try for groups this weekend.

[K.O.]

well It just looked to me like it was a light hold in the vid... would be cool to see what a tight hold would look like when recorded I think it would show...

But yep I do think it is possible that the urban may be a touch sensitive about hold with a heavier projectile... but hey it may prefer a lighter hold, sometimes these things are unpredictable...

[EdinGa]

It's not possible to get into a normal shooting position with the setup I'm using. I need to upgrade to a sideshot or something that will allow me to look through the scope.

[K.O.]

was still educational and interesting to me...especially watching the arc of the 75 yard shots...

[EdinGa]

I'm glad you enjoyed it. It was interesting to see the arc that Chairgun plotted. This is a great hobby.

[K.O.]

well in a few sessions watched more than a few times and it is pretty clear that there is not any spiraling in my view... but probably need a 1000$ camera to watch for wobble.... so can not really tell but as clean as the holes in the cardboard are I am pretty convinced it is stable out of the 1:19 barrel...do not know if a Go Pro 5  would be good enough 240fps at 720p...

A friend had a program that I think cloned each frame a number of times  to make it seem like more frames per second but I forget what it was called...

between  high speed vid cams an Doppler radar we are at a time we can learn quite a bit about shooting pellets and cast/swaged rounds at low fpe... the long range type challenge at shorter ranges... but yep could learn quite a bit about subsonic shooting all these different nose and waist shapes...and Boolits of course...

speaking of doppler... it would/will  be a help to chair gun...  the GA drag model for round nose pellets could be improved quite a bit... I think like Bob that it does not show the sub .5 mach rise in cd that most pellet have IMO... but also there could be a drag model for each pellet... now out of different barrels at different fpe(skirt blow out) level there will still be differences but would be more accurate than what we have now...

I do think the drag RA4 drag model for the BBT 30 is better it is based on 22 lr... BUT I just used the estimated B.C. that  N.O.E. has which I think may be a touch low... 

Chair gun does have a toolbox that includes ways to calc B.C. from velocity differences and poi differences...So you could download it your self its free...then you could get the B.C. out of your Barrel at the speeds you are sending them out at...

what I charted was just a rough estimate in the end... just to get an Idea of what t expect and judge about what hunting/target/plinking ranges could be expected...

But yep here is a link to download Chairgun  if you do not have it...

https://us.hawkeoptics.com/chairgun.html 

the next link is a search... top result is a pdf article on Doppler... "Accurate Measurements of Free Flight Drag Coefficients with Amateur Dopple Radar"   

https://www.google.com/search?q=drag+coeffient+from+doppler+radar&rlz=1C1GNAM_enUS688US688&oq=drag&aqs=chrome.3.69i57j69i65l2j69i59j69i61l2.7033j0j7&sourceid=chrome&ie=UTF-8

I  picked up the .25 pellet mold and was one of a group that proved you can cast pellets... Even with my experience at an aluminum foundry was a bit nervous about being able to get good skirts...

we are among those showing small bore air power works and enjoying the challenge... at a time were even more is being shared and learned...

the vid below is the AEAC channel testing the Fortitude but guess what he just got the Labradar so we can glean a little info... He is a member here so we may be able to talk him into some more in depth info...Maybe...

But yep it is a great time to be in this hobby... heck in the 1980's the best I could do was 18 fpe with green tin Benjamin .22 pellets out of my pumpers... had to learn how much wind I could handle, my drop and best zero the old fashioned way... lots or shooting... can/have to still do it that way but the tools do help speed that up... 

[K.O.]

well Ed hope you are still around I got a good deal on an Urban... I did some very relaxed checking out how it did with various pellet at 35 yards... it did well with quite a few and some of the groups should shrink with better form and effort from me... and then it is a matter of refining the sweet spots for each  pellet... did good with a good number of pellets so I am happy...it is not a pellet picky barrel...

Right now of particular interest to me are the Eun Jin 28g  got 4 in one large hole then one almost touching...pic in reply #7 in below link... Urban quick and dirty testing starts at reply #5

with  the BBT 30g the groups were .75" outside to outside(which is my hunting min acceptable accuracy) pic in reply #10 in linked thread 

https://www.gatewaytoairguns.org/GTA/index.php?topic=151471.0

So it was a full 3200 psi fill and rejects from my first cast of the BBT some month ago... was saving them to fire lap barrels if necessary cause they were poured with 4-5 % tin, .25% antimony, plumbers lead and roto 99.99... so they are about 10 -11 on the hardness scale...  Did not think they would do well because some had some rounding on the bases and or wrinkles... rejects... no lube...

I cast them just to see if the hardness would work in the very shallow rifling in the Disco barrel... like I said slow at about 550 fps...

my best guess stock out of the box The Urban is only spitting them at about 600 fps have not checked... it might be quiet enough for me to use Soft Chrono...

So between being able to get a bit more velocity and going to a 1.5 % tin softer round... and not sending rejects... and experimenting with lubes...I do think I just might get Moa or close at 40-45 ish yards... 

Point is Score for cast .22 out of the Urbans choked barrel in at least two instances.. will have to see how knurled rounds do soon... 

am linking to your other thread so how the BBT 30g behaved in your rifle is shown...

https://www.gatewaytoairguns.org/GTA/index.php?topic=149700.0


I have a used 24" Remmy 514 barrel it is only ~.001 larger in diameter than the Urban's barrel... I has decided it is for the urban...

So hopefully that will give velocity boost up to about 700-725 fps with the 30g BBT and ~600 with  the NOE .223-38-rn and 225-37-fn... If that goes well I may get it a bigger air tank and carve it a walnut stock...

So anyways your two thread helped me decide to go for the sale on the urban... if the tanks switch I will still have less into it than the $299 Buc... and yep the 30g BBT do good out of them