Tuning Interesting observation on FX Crown Mk2 and new, bigger dual port barrels

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ChRiSiS

ChRiSiS

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May 6, 2020
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CT, United States
I recently picked up a Crown Mk2 with, among other things, a .30 cal 700mm barrel. This barrel is the new style from FX which has the bigger port and a central bar in the barrel, very similar to the Ernest Rowe and Huma Dual Hole Transfer ports.

I also have another Crown MK2 with an older style 700mm barrel with the single hole transfer port. I found that when I installed the dual hole barrel in my original Crown, making no changes what so ever to the settings, that I lost approximately 20 FPS! That seemed so counter intuitive that I installed the older barrel in the newer Crown and found that it then gained about 12 FPS. So, in summary, the dual hole barrel was approximately 20 FPS slower in my original Crown (going from 948 FPS with the single hole to 928 FPS with the dual) and approx. 12 FPS slower on the newer Crown (going from 916 FPS dual to 928 FPS with the single). 

Note that I am using the exact same FX .30 44.8 grain pellets in each gun. The two Crowns obviously have different tunes (I actually haven’t adjusted the newer one in any way yet) hence the different speeds.

My question is, has anyone else seen the same effect, and can anyone suggest a good reason for this rather counter intuitive outcome. I guess I expected that with the improved airflow from the dual hole port on the barrel i would actually see higher speeds, not lower. By the way, the pellet probes are both standard FX Crown .30 probes.

Insights would be appreciated.

Chris
 
Its possible that the larger port area decreases air velocity, but will allow for more air volume(if you have more volume available). So it might be possible that with the new port configuration, when tuned correctly the FPS will go considerably higher than the old port configuration would allow... But a higher 'air volume' tune must be used then the velocity will also increase...

jmo
 
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ChRiSiS,

Is the liner inside the new barrel the same as the liner inside the old barrel?

all things being equal, I found that the 700mm Superior or Super Heavy liner fires pellets about 20-30 fps SLOWER than the older STX pellet A liner using either the old round port or the newer oval(bigger) port.
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Ghostranger69,

Great screen name by the way 😂

You make a good point. I was unaware of this difference. The older (single hole) .30 barrel could well be a standard STX liner. The newer (dual hole) barrel is, in fact. One with a Superior liner!

Chris
 
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Its possible that the larger port area decreases air velocity, but will allow for more air volume(if you have more volume available). So it might be possible that with the new port configuration, when tuned correctly the FPS will go considerably higher than the old port configuration would allow... But a higher 'air volume' tune must be used then the velocity will also increase...

jmo
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I wondered the same thing. I guess time will tell as I adjust whichever Crown I choose to use with the dual hole barrel. I will most likely test both barrels with 44.8 and 50.15 grain pellets and pick the one which shows better accuracy between 50-100 yards. I am not likely to shoot this very often with slugs, since these days I mostly shoot on my property and so am limited to 75 yards or less, most often 50. I also don’t really want any chance of a projectile going beyond my property and this likelihood increases significantly with slugs as opposed to pellets.
 
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Well... I have done some porting on mine, myself. I think the issue with the larger port cna be that the probe may not really line up with that larger port. So you end up with an overhanging edge creating turbulence. You'd always want to cut the probe to match the port.
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I am trying to think how I can determine this. I’ll take some measurements on the two and see if there is a possible mismatch.
 
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Pretty standard way of checking for port matching is to mark the probe with a sharpie where it goes into the barrel. Then remove the barrel and probe and insert the probe into the barrel up to the line you just marked on it. Makes things pretty plain.
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Well, I learn something new everyday. Thanks Biohazardman. It seems pretty straightforward once someone lays it out for you. I’ll try that tonight.

Chris
 
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Based on my observations, if one were to look at close up pics of the FX new transfer ports vs the Huma dual transfer ports, especially in the 30 cal, you will see that the Huma dual opening starts closer to the o ring on the entry side of the thimble, while the FX one looks about a strong 1/8” away from the o ring. 

That slight offset alone will cause that slight loss in speed. 



I think its its whoever is machining these. I’ve tried Ernest’s offerings when he first came out with the dual hole TP’s and they worked well. Maybe look at the one on your gun, and look closely at pics of the Huma TP’s to see the differences.
 
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Check this tread out. See post number 6, with pics of huma’s and FX’s transfer ports compared side by side, in a 25 cal-

https://www.airgunnation.com/topic/fx-crown-mk2-700mm-barrel-25-cal-70-8-fpe/
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Bigragu, based on these pictures, and on the fact that my barrels do not have an additional small hole on the other side, i am pretty sure that I have the Huma dual port installed on my barrels, rather than the FX one. I am going to check to see if the probes are aligned with the dual hole in the barrel.

Chris
 
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The Huma V2 dual transfer port is quite wide in the .25cal version. I haven't seen the 30cal in person.

20210327_222209.1618034408.jpg

 
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See... I can't think of any good reason the port should be that wide. Coming from a round .30" transfer port hole, dumping to a wide rectangle then back down to a round port again....? There has to be a loss of efficiency there. I imagine just going from the TP to the wide barrel port would cause some tumbling. It would be a different story if the TP was a smooth transition. Maybe someone should mill a custom TP to match the valve outlet and barrel inlet? 

My ports are setup with all smooth transitions. From TP to barrel port is smooth as butter. From barrel port to actual bore is as smooth as I could make it as well. But difficult porting inside of the barrel, though. Always leave a bit of a rough finish on ports. Actually aides laminar flow. (same concept as golf ball dimples) I have actually witnessed power loss putting a nice smooth polish on my ports. 
 
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Long_Gun_Dallas, I am starting to think that you are on the ball here. At least for “normal” levels of tune the older, round hole approach may actually be best. I also have a .22 barrel with the same dual hole to compare with the one which came on my Crown Continuum (both with Superior liners). I’d I see the same difference in velocity (i.e. the dual hole slower than the single round hole) then I think it will be case closed for me anyway. 
 
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See... I can't think of any good reason the port should be that wide. Coming from a round .30" transfer port hole, dumping to a wide rectangle then back down to a round port again....? There has to be a loss of efficiency there. I imagine just going from the TP to the wide barrel port would cause some tumbling. It would be a different story if the TP was a smooth transition. Maybe someone should mill a custom TP to match the valve outlet and barrel inlet? 

My ports are setup with all smooth transitions. From TP to barrel port is smooth as butter. From barrel port to actual bore is as smooth as I could make it as well. But difficult porting inside of the barrel, though. Always leave a bit of a rough finish on ports. Actually aides laminar flow. (same concept as golf ball dimples) I have actually witnessed power loss putting a nice smooth polish on my ports.
Click to expand...


I also bought the Huma pellet/slug probe to go with the new port

20210327_221001.1618070265.jpg

 
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