Has Anyone Chronographed a Benjamin 392 Using JSB Heavies ?

[wll2506]

Was wondering what velocities you were getting with a stock gun ? I know the average velocity with 14.3 gr pellets is about 640 fps. Was kind of hoping a stock 392 could get maybe 575 using JSB Heavies with 8 pumps ?

wll

[RBQChicken]

I think it would get at least 575 if it's getting 640 with the 14.3 grain.

I have a 392P (1993) which gets 657 to 660 with the Crosman Premier domes (14.3 grain), and gets 615 with the JSB 18.1 grain.

[wll2506]

I think it would get at least 575 if it's getting 640 with the 14.3 grain.

I have a 392P (1993) which gets 657 to 660 with the Crosman Premier domes (14.3 grain), and gets 615 with the JSB 18.1 grain.

I'm assuming that is with 8 pumps ?

wll

[RBQChicken]

Yes. Sorry I forgot to mention that.

[anti-squirrel]

Yes.  565 at 88* F in my 392PA.

I prefer Polymags, Gamo Redfire, and RWS Anything since they all shoot more accurately.  I'd rather have a little less oomph with more accuracy, though Sniper Mags also group better than JSB Exact Heavies.  Gamo Redfires in particular are favored by my 392, though I rarely use more than 5 pumps.

[wll2506]

Hey guys, thank you for all your info, looking forward to getting a good heavy pellet for this gun.

I know this gun is no real powerhouse by today's standards, but within its range limitations it is a meat getter as proven by it many, many, many years of use in the field. I'm checking FedEx every few minutes hoping the scope arrives early :- )

Well as of this 2-19-19 edit, it looks like Wed is the delivery date ;-  (

wll

[Hobbyman2007]

Hey guys, thank you for all your info, looking forward to getting a good heavy pellet for this gun.

I know this gun is no real powerhouse by today's standards, but within its range limitations it is a meat getter as proven by it many, many, many years of use in the field. I'm checking FedEx every few minutes hoping the scope arrives early :- )

wll

If you can find some BBT bullets in the bigger of the two sizes it might be worth getting some . I think this guy might have them https://www.ebay.com/sch/ricmor_hv3fazc3/m.html?_nkw=&_armrs=1&_ipg=&_from=

[wll2506]

Hey guys, thank you for all your info, looking forward to getting a good heavy pellet for this gun.

I know this gun is no real powerhouse by today's standards, but within its range limitations it is a meat getter as proven by it many, many, many years of use in the field. I'm checking FedEx every few minutes hoping the scope arrives early :- )

wll

If you can find some BBT bullets in the bigger of the two sizes it might be worth getting some . I think this guy might have them https://www.ebay.com/sch/ricmor_hv3fazc3/m.html?_nkw=&_armrs=1&_ipg=&_from=

Thank you for the info, but for me and my intended use a standard heavy pellet in the 18-22 grain area is more than enough. It will give me a good BC, and be able to be pushed by this gun fast enough to have a reasonably good trajectory in the intended  20-35 yards range.

wll

[wll2506]

Well with the few mods I'm doing, by the time I'm done I'm sure I can get over 535-545 fps with JSB Monsters ... this gun may need the extra volume in the valve and adjustable piston and be a 9-10 pump full power gun to get velocity with the Monster pellets .... I'm not sure ? As it stands right now with nine pumps there is no air left in the valve.
 
I will shoot it most of the time in the 6 pump area with JSB 18.1gr Jumbo Heavies I think for general starling, pigeon, rabbit, squirrel  use, and use the Monsters for heavy 30 yard smack on tougher, bigger critters.because of the extra downrange energy. The price of the Monsters with the discounts ($6.74 per tin of 200) and because of that and the good accuracy I have been getting with these,  I just purchased a supply from AGD..

I have some Nielson 23gr slugs that could be a specialty round as the BC is very, very high compared to the Monsters (.073 vs .039), testing will determine how I use them.

wll

[Robert 5mm]

You could calculate the new velocity using the FPE formula ( If you have a spreadsheet - very easy )
( This assumes all other things are equal - like efficiency of pellet, etc )

Known FPS1 = 640 = use VEL1 in formula
Known WT1 in gr = 14.3 = use WT1 in formula
Constant = 450240 for calculations

Calculated FPE = ( VEL1 x VEL1 x WT1 ) / 450240 = 640 x 640 x 14.3 / 450240 = 13.01 FPE

so reverse the calculation to get new FPS2 at new WT2

Calculated FPS2 = Square Root (( FPE x 450240 ) / WT2 )) = Square Root (( 13.01 x 450240 / 18.1 )) = 569 Feet per Second

[RBQChicken]

As you said, that's assuming the same efficiency, but it's a well-established fact that pneumatics are more efficient with heavier pellets.

[wll2506]

You could calculate the new velocity using the FPE formula ( If you have a spreadsheet - very easy )
( This assumes all other things are equal - like efficiency of pellet, etc )

Known FPS1 = 640 = use VEL1 in formula
Known WT1 in gr = 14.3 = use WT1 in formula
Constant = 450240 for calculations

Calculated FPE = ( VEL1 x VEL1 x WT1 ) / 450240 = 640 x 640 x 14.3 / 450240 = 13.01 FPE

so reverse the calculation to get new FPS2 at new WT2

Calculated FPS2 = Square Root (( FPE x 450240 ) / WT2 )) = Square Root (( 13.01 x 450240 / 18.1 )) = 569 Feet per Second

Thank you for the info for sure.

wll

[Yarddog]

William, I know that you've probably moved beyond your original question, but with eight pumps, my stock, our of the box 392 with JSB 18.13 pellets chronoed at 584.3 FPS at eight pumps.  That is my go-to pellet with this rifle.  It's the most accurate I've found in it so far.  Sorry I didn't see this thread earlier.

[35 shooter]

Wow, 584fps. With 18 gr, pellets is about 14 ft. Ft. lbs.?
Nice little hunting rig right there!

[Robert 5mm]

More Chrono Data - using 392 mod today
All 8 pumps - no air left over

Crosman PHP 14.3gr - 695 fps
Gamo TS22 21.75gr - 595 fps
H&N Rabbit II 25.62gr - 551 fps
Gamo Round Lead BB 15.43gr - 704 fps

[wll2506]

More Chrono Data - using 392 mod today
All 8 pumps - no air left over

Crosman PHP 14.3gr - 695 fps
Gamo TS22 21.75gr - 595 fps
H&N Rabbit II 25.62gr - 551 fps
Gamo Round Lead BB 15.43gr - 704 fps

I would love to get 550 with Monsters .. did you put in an adjustable piston ? and what mods did you do to the valve ?  did you cut off 2 or 3 threads on the valve and a bit off the check valve?

Oops, just saw your other post where you said what you did ... I took a picture of it and will do my best to copy you !!

wll

[wll2506]

Well with the few mods I'm doing, by the time I'm done I'm sure I can get over 535-545 fps with JSB Monsters ... this gun may need the extra volume in the valve and adjustable piston and be a 9-10 pump full power gun to get velocity with the Monster pellets .... I'm not sure ? As it stands right now with nine pumps there is no air left in the valve.
 
I will shoot it most of the time in the 6 pump area with JSB 18.1gr Jumbo Heavies I think for general starling, pigeon, rabbit, squirrel  use, and use the Monsters for heavy 30 yard smack on tougher, bigger critters.because of the extra downrange energy. The price of the Monsters with the discounts ($6.74 per tin of 200) and because of that and the good accuracy I have been getting with these,  I just purchased a supply from AGD..

I have some Nielson 23gr slugs that could be a specialty round as the BC is very, very high compared to the Monsters (.073 vs .039), testing will determine how I use them.

wll

Well my shipment of Monsters just came in, have enough for a few hunting seasons for sure ;-  )

wll

[RBQChicken]

Looking forward to a speed and accuracy report on them.

[Robert 5mm]

Will
We all have been hoping for some good results, any updates you have will be appreciated.
Thanks, Robert

[Creaker]

     FWIW....after many learning experiences over past few years, I was able to get to 650.5 avg. with 8 pumps.
That’s 17 ft/lbs measured on beta chrono. That’s with JSB 18 gr.  However compression o-ring developed a leak and have just replaced with Buna n-70 rings..Last were stock Crosman. Usual mods of 1/4 off inlet check valve
and remove 2 threads. I found headspace,volume and porting most critical. Took exhaust port to 5/32 in valve and barrel and relieved sharp bend internally..Using stock hammer and valve springs. Two other items though: Was older valve without Delrin exhaust valve so installed one from Mac1. Cut a 45 degree seat and lapped
in with bore paste on my brothers lathe.Kudos to Crosman on changing their exhaust valve. That added a little more volume and improved air flow. Also ground seat (very tedious) around valve exhaust port and superglued
O-ring in.Does have solid billet pump arm.
     I like shooting this gun so much I wanted to upgrade but not HAVE to pump it so much. Really love where it’s at now. Really performs at even 6 pumps. Hope this helps !