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PROJECT PCP MATHS & Q&A

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L

LewisP12

Member
Jan 28, 2017
89
0
Gauteng, South Africa
Hi All

I purchased a Jkhan .357 cal N400 as a project air rifle a few months back and I have come to the conclusion, that it is not suitable for my hunting and target shooting needs.

AIM
The following are my hunting requirements:
  • Feral and Rock Pigeons shot from 30 to 150 m
  • Guineafowl out to 200 m
  • Impala at max 50 m, ( app 110 lbs weight)
  • Warthogs at max 50 m (app 150 lbs weight)
The following are my target shooting requirement:
  • 1 “ groupings at 100 m
  • Shooting falling plates out to 300 m plus
Note: I require one bullet and power setting for each of the above mentioned.

PERFORMANCE OF PLATFORM
  • 130 fpe (JSB 81 gr @ 880 fps, 160 bar, 5 mm CTC at 50 m)
  • Max 150 fpe (JSB 81 gr @ 911 fps, 200 bar, 35 mm CTC at 50 mm)
  • Barrel length 600 mm
  • Transfer port diameter 6 mm
  • Air cylinder 400 cc @ 200 bar
MODIFICATION THAT COULD BE DONE
  • Machine transfer port to be Oval ( 6 mm wide & 9 mm long)
  • Upgrade air supply to 500 cc @ 300 bar bottle, will be willing to install two bottle if required (Total 1000 cc @ 300 bar)
  • Regulator set between 170 – 200 bar (modified paintball regulator)
  • Large plenum chamber I thought about 100 cc in volume
CONCERNS
Is it possible to mathematically calculate the following, I would be grateful for advice etc because I would like to try and get this right the first time:
  • The energy loss, if I change the barrel to a TJ .257 cal , 24 “, 14” twist
  • The energy gain (over the 24” . 257 cal), if I lengthened the barrel to 28” or even 32” still in the .257 cal
  • Amount of shots, using a 500 cc air supply and shooting from 300 bar down to 200 bar.
CONCLUSION
I thought of using the following bullets and speeds:
  • Hunting – 25742 HP mould 74 gr @ 950 fps
  • Target shooting – NOE FP 83.5 gr @ 900 fps
 
My guess is that you need several air guns for the variety you spoke of. I've never seen a single air gun with the capabilities needed for this.

Probably also want this to be seen by the folks in the hunting forum as well.

The same probably is true for powder burners. Anything that can take down a warthog would obliterate a rock pigeon. Anything that would leave more than feathers on a rock pigeon would bounce off a warthog. I do not know of any single gun capable of the adjustments required. Even FX guns where you can switch barrels up to a .30 caliber aren't enough for what you want.

Check the hunting forum, or perhaps contact Matt Dauber of YouTube Channel Air Arms Hunting South Africa (https://www.youtube.com/channel/UCthxJ7N1usTDbd4n_aSSO7g​) as he is much more familiar with South African wildlife than those of us in North America. He has a great deal of hunting experience there and could give advice on rifles appropriate for your terrain and hunting needs.

My only advice is to plan on multiple guns for the variety of needs listed. Too bad no one does something, I believe is called, a "Drill" where one gun contains multiple barrels and calibers. Not in the airgun world so far as I know.

As for the Maths, way beyond me. Good luck!
 
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As for switching barrels, I've heard grumblings that it really isn't practical. 

​Each time you switch barrels, you must sight the scope again, and adjust the power wheels accordingly. Some people are willing to do that, some aren't. Cost is another problem. Each caliber for an FX gun will run $400 to $500 US, almost the cost of a lower end gun.

Considering the cost of an Impact or a Crown, one could purchase a .25 Streamline or Wildcat (both FX Guns, but other brands could do the same) and keep the .357 for larger game. With the new pellets available from JSB for .22, even a .22 could be a good 100 yard pigeon gun. The new .22's are 25 and 34 grain pellets, so could allow smaller bore to be just as effective.

​All I can say is, good luck on the decisions. I hate choosing a new gun, but love it as soon as the decision is made and the gun is on its way.
 
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Hi All

Some feedback on what I have learnt, while off from work over the Christmas holidays.

I found several articles written by Bob Sterne of B.C. Canada regarding PCP builds (Goldmine of information). The deciding factor of calibre choice is determined by the relationship of sectional density, pressure and barrel length. By studying Bob graphs, I have to get my .357 Jkhan up to 250 fpe because when I fit a .257 barrel the energy level will drops to app. 130 fpe.

Bob also touches on calculating T.P. sizes to prevent supersonic air speed through the T.P. and the ratio size of valve throat to T.P. He also indicates that there is an app. gain of 10% increasing the barrel length from 24 to 30”. After reading Bob articles, I must say I am confident that I can covert my PCP to a (130-150 fpe) .257

At present I am only reaching 150 fpe at 200 bar with .357 JSB 81 gr, I found an article about a .357 Bulldog upped its power from 170 to 220 fpe when the transfer port was opened to .305. When a SS valve was installed it whacked out 302 fpe at 3100 psi. I have started modifying T.P and valve throat sizes.

Valve T.P. was 8 mm (Staying at 8 mm)
PCP T.P. was 6 mm (Boring out to 8 mm)
Barrel T.P was 6 mm (Boring out to 7.5 mm)

The .257 is known for shooting 74 gr HP bullets but I have also found out that it shoots 260-63-FN of N.O.E well, only requiring app 130 fps to drive this bullet at 950 fps. It also mentions the likes of Cedric and Doug Noble starting off with this bullet until they increased the output of their rifles.

For some reason I can’t find where in the States, I can purchase a raw 32 “ long ,257 TJ Enterprise barrel, I would very much appreciate if somebody could forward me some contact details of .257 barrel and bullet mould suppliers.

 
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