What Makes Sense for PCPs?

[rsterne]

Even a 5 foot rifle won't have a barrel much more than 3 feet.... If you want to build a rifle with a 4 foot barrel, then you can double all the FPE levels I based this post on originally....

Bob

[RHytonen]

Even a 5 foot rifle won't have a barrel much more than 3 feet.... If you want to build a rifle with a 4 foot barrel, then you can double all the FPE levels I based this post on originally....

Bob

And with enough plenum to half- fill them, right?
Tubing is cheap... Now where do I get the fattest commercially made valve?
(No lathe...)

Rod

[PakProtector]

Even a 5 foot rifle won't have a barrel much more than 3 feet.... If you want to build a rifle with a 4 foot barrel, then you can double all the FPE levels I based this post on originally....

Bob

And with enough plenum to half- fill them, right?
Tubing is cheap... Now where do I get the fattest commercially made valve?
(No lathe...)

Rod

'Fattest'? There is no single one. There are effectively two ways, one balanced to reduce the opening strike, and the other w/o balance. The latter suffers a quadratically increasing energy to kick open as its flow potential increases, the other suffers internal friction form the seals on the balancing parts. I suspect the balance point lies somewhere around the .357 bore area port area...
cheers,
Douglas

[rsterne]

Plenum volume still needs to be at least 1cc per FPE you want for a regulated gun.... much more than that unregulated unless you only want one shot....

Bob

[rsterne]

I thought it might be time to revisit this thread with the idea of coming up with a range of maximum bullet weights that make sense for different PCPs.... In particular, I'm thinking about relatively common guns like the MRod, Condor, even the DAQ, where you are dealing with a 3000 psi fill and a barrel of about 24", and unregulated.... In practical terms, this means that the last shot of a usable high power shot string occurs at about 2400 psi.... In other words, looking at how heavy the bullet can be using 3000 psi isn't really practical for these guns, unless you want just one maxed out shot.... So, what kind of maximum bullet weight are we looking at using 2500 psi and a 24" barrel, to achieve 950 fps?.... Here is what I get for the various calibers.... The Sectional Density works out to 0.140 for all of these bullets....

.177 cal - 31 gr.
.217 cal - 46 gr.
.224 cal - 49 gr.
.250 cal - 61 gr.
.257 cal - 65 gr.
.284 cal - 79 gr.
.300 cal - 88 gr.
.308 cal - 93 gr.
.357 cal - 125 gr.
.408 cal - 163 gr.
.457 cal - 205 gr.
.508 cal - 253 gr.
.580 cal - 330 gr.
.720 cal - 509 gr.

This doesn't mean you can't use heavier bullets than the above.... just that you are unlikely to achieve 950 fps unless you tune for just 1-2 shots, using a decreasing style shot string.... If you increase the pressure or the barrel length, you can use a heavier bullet, of course.... but if you want to hit about 950 with a usable shot string and a 3000 psi fill these are about as heavy as you can go.... To achieve this performance, of course, will require a very high-performance PCP with large ports and enough hammer strike to make use of the potential pressure and barrel length available.... in other words a lofty, but achievable goal....

Bob

[PakProtector]

That matches well with my experience Bob( limited as it is ). I have slightly more barrel ( 27" ), but lower weight in .224 ( 46 gr ), and have hit nearly 1000 fps around 25-2600 psi. More hammer is clearly needed. The 3k6 psi bottles leave a reasonable bit of headroom in terms of velocity curve pressure; kick the valve open well at higher pressure, but don't hit it so hard you get the Korean cliff velocity profile, and you're off to the races.

Doing it accurately means managing the barrel harmonics...and perhaps living with a smaller than usual velocity spread. Can't help wondering if the pressure-variable hammer hit excites the system differently enough to make it even more troublesome...LOL Which ov course is perhaps one reason the pellet energy stuff is easier to tune well.
cheers,
Douglas

[rsterne]

Douglas, between the extra 3" of barrel and 100 psi, plus 3 gr. lighter weight, you could easily hit 1000 fps.... as all three of those fudge the numbers to greater FPE.... Nothing is written in stone, this is just intended to be a guideline anyway.... Increase the hammer strike to take advantage of the 3600 psi you have available and all bets are off.... but you will find that at 2500 psi the velocity won't increase, just use more air....

Bob

[RHytonen]

Every time I do this (read your Geek Posts) I end up talking to myself and wishing I had taken mechanical engineering rather than electronics. Then I would not have to think so hard

+1 on that... once I realized every calculation had been done by someone (and studied Heisenberg;) I gave it up, having acquired the gut feeling by doing them in school and designing systems IRL (where the math is at best a rough guide and never works out perfectly on imperfect conditions and materials.) And then the bean counters, time misers, and cost cutters attack....

Rod

[RHytonen]

.... So, what kind of maximum bullet weight are we looking at using 2500 psi and a 24" barrel, to achieve 950 fps?.... Here is what I get for the various calibers.... The Sectional Density works out to 0.140 for all of these bullets....
This doesn't mean you can't use heavier bullets than the above.... just that you are unlikely to achieve 950 fps unless you tune for just 1-2 shots, using a decreasing style shot string.... If you increase the pressure or the barrel length, you can use a heavier bullet, of course.... but if you want to hit about 950 with a usable shot string and a 3000 psi fill these are about as heavy as you can go.... To achieve this performance, of course, will require a very high-performance PCP with large ports and enough hammer strike to make use of the potential pressure and barrel length available.... in other words a lofty, but achievable goal....

Bob

If we're using "bullets" rather than diabolo pellets, why the subsonic MV limitation at all?
-without getting into a discussion of time to fill the barrel, or valve port speed limitations-
which seem to me a little too academic given the myriad added parameters and unpredictabilities
(like machining precision, friction and wear) when approaching that level of analysis.

Still fascinated by that Xs/FDPCP GenII concept of multiple barrel ports...
Rod

[PIPERMAN]

If we're using "bullets" rather than diabolo pellets, why the subsonic MV limitation at all?

Airgun barrels have riflings that make the pellet rotate but not as fast as a "powder" barrel.
Diabolos stabilize due to their skirt,some barrels make them rotate slightly (smooth twist) and still have exc. accuracy.
Using bullets on a barrel designed for pellets? Then better go subsonic or go hypersonic all the way from gun to target,
so that the bullet won't have to deal with it's own sonic blast.
My 2 cents

[PakProtector]

If we're using "bullets" rather than diabolo pellets, why the subsonic MV limitation at all?
-without getting into a discussion of time to fill the barrel, or valve port speed limitations-
which seem to me a little too academic given the myriad added parameters and unpredictabilities
(like machining precision, friction and wear) when approaching that level of analysis.

Still fascinated by that Xs/FDPCP GenII concept of multiple barrel ports...
Rod

Because it becomes increasingly wastfull of stored air as you pass 950 FPS( not to mention projectile drag sky-rockets around that point). The push available is diminishing then too...the porting starts suffering from the sonic choke, and the gas will only expand so quickly...stored energy has to be put into a faster and faster column of air in the barrel....

On a relatively powerful .224 boolit shooter, I have managed a 1174 fps shot with a 28 gr E-J pellet...I got the 'crack' indicating SS travel. Then went back to more sensible testing speeds.
cheers,
Douglas

[rsterne]

The same reasons exist for staying subsonic with bullets as with pellets, with one exception, diabolo pellets have even greater instability than bullets when transitioning from supersonic to below.... However, the drag on bullets also skyrockets as you pass about Mach 0.8-0.85 (900-950 fps).... and the efficiency of the air accelerating the pellet inside the barrel takes a dump as well, unless the ports are bore-sized.... Even with bore-sized ports and high pressures there are sonic limitations to be considered.... Even though they aren't hard limits, who need to use ten times the amount of air to try and get to 1200 fps?.... If you start looking at wind drift, you will find that it reaches a minimum at somewhere around 950 fps even with bullets, because as you drive them faster the drag, and hence the drift, increases rapidly....

Bob

[PakProtector]

Given those numbers, I have a 50 gr, .224 boolit mold on the way( 22596 ). With more barrel, and slightly more pressure I think it will go fast enough. That particular Marauder is slightly short on hammer energy; it is going to get the hammer energy stretched as far as I can go...slide the trigger backwards a bit, lengthen the cocking lug slot in the air tube and breech and modify the stock's inletting. I think an extry .125" hammer travel is possible this way. If I can shoot a 940-960-940 string from 3k starting pressure I think I am going to be OK with it...   ( yeah, right...as long as it is accurate )
cheers,
Douglas

[TimmyMac1]

Great discussion. Relatively low pressure is fairly hard to make speed with and our breech pressure averages are in the hundreds not thousands like firearms. We definitely have to work within a narrow envelope before inefficientcy spikes and accuracy plummets. I stay below Mach .8 but for sure don't toy with .82 as it has been a huge disappointment for decades. For 4 years now I've played at 815 FPS in .22 as I have a 20 FPE performance max to stay under in both FT and HV Bench Rest. I think it is a better solution in conditions and my results speak for themselves. I've rarely shot better. I wish I could go 865-875 but there are no accurate .22 pellets that weigh that little. I use RS JSB 13.43 grain at Mach .72. Since resistance goes up @ the square of the speed going faster has its consequences and there are some real advantages to going even slower like .22 Pistols at Mach .52-55. The slower you start out the less you lose because you are dealing with higher BC and lower resistance. Wind drift and trajectory aside you just don't lose as much.
I've always thought going fast was highly over rated.
Supersonic is out of the practical envelope for Airguns when the discussion is about accuracy at distance.
The best work so far has been done sub-transonic and I believe that will always be the case.

TimmyMac1

[Bill G]

Seems that the reality is that unless you can have an avg barrel psi of 2000+psi, supersonic isn't going to happen on a shot string.  IMO, it's not worth chasing.  More efficient projectiles(better BC) at 950 fps is the best route.  Bobs study on the connection with SD vs PSI is priceless.  This information gives a wealth of info for those who want to establish a shot string with an acceptable ES% deviation.  It also hints to the possibilities of what to shoot to get a declining fps string..  This is a real decision tool in a nut shell.  Each of us have personal likes as far performance.  None of them are wrong, they're just prefernces.  This will allow us to asses a platform for a window of performance and apply the best weight projectile for that preformance.  Or..... Let us know we need a new platform

Bill G 

[rsterne]

I just had a look through the data on some of my wilder PCPs, and the lowest pressure I saw supersonic with was 2200 psi, but to be truthful I never tested down to 2000 with the lightest pellets.... However, I can assure you that the efficiency takes a dump past Mach 1, and is declining much before that.... Add to that the fact that no projectiles have a "drag discontinuity Mach number" (the speed at which the drag increases drastically) higher than Mach 0.9 (and most are 0.8-0.85, some as low as 0.7) and I really see no point in trying to push to Mach 1, let alone beyond, with an airgun.... Here is a graph of the drag of the standard G1 projectile....



Note that the Drag Coefficient is below 0.25 up to Mach 0.8 (900 fps), double that at Mach 1, and peaking at 0.65 at Mach 1.3.... The wind drift will follow a similar curve.... doubling at Mach 1 compared to at Mach 0.8....

Bob

[PakProtector]

Bobs study on the connection with SD vs PSI is priceless. 
Bill G

That and many others reside in a 'Best of Bob' favourites folder...
cheers,
Douglas

[Bill G]

I'm gonna have to start one of those. It would be easier to buy his Book

Having fun Bob

Bill

[dv8eod]

Check this out, Bob-

http://bulletin.accurateshooter.com/2009/04/us-army-team-tests-radical-new-dimpled-bullet/

I actually envisioned this a long time ago but didn't know how to act upon it. Now someone else is collecting all the money. Ain't that always how it goes.

[QVTom]

I'm pretty close to Bobs projected number for a 357.  24" barrel, 3400 psi, 138 gns, barrel port 100%, valve area at poppet 110% and now getting an average of 967 fps.  Still on a regulated tether while trying to sort out  some valve/hammer issues.