Tuning the Crosman 1720T

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Nov 21, 2017
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OREGON, United States
Hi Guys!



There isn't enough time in the day to play! Yesterday I posted about a personal project regarding the Jkhan Noblesse Bullpup. It is a work in progress! However it is a my own personal gun so I will come back to it on another rainy day. Today it has been somewhat depressing. The rain started and the so much work needs to be done around the house! Too bad! It's time to work on airguns!



What I have here is a Crosman 1720T .177 caliber pistol. So far I have done a full shot
1537145827_20952429655b9efbe3e2b862.64705779_20180916_171134.jpg
string on it before I tune it. I filled the gun to 3,000 psi or around 200 bar. I am shooting the JSB 8.44 grain pellet. The initial shot string showed that over 40 shots I got an average of 688 fps, with a standard deviation of 16.98 . Minimum velocity is 650 fps and Maximum velocity is 711 fps. Spread was calculated at 61 according to my chronograph. I am using a Caldwell Ballistic Precision Chronograph.

While shooting the gun I noticed it was expelling some air into my face. This I will have to investigate...this means to me that some air is being wasted and not going where it needs to - behind the pellet! Also this gun is not regulated and the customer would like one to be added. My choice of regulator is HuMa. The customer also expressed his desire to keep the pressure gauge on the gun. I like this because it allows us to see the if the reg is working and when it falls below reg pressure. Some customers may opt-out to leave the gauge on the gun because it will provide more volume of air versus having the gauge adapter in place (which takes up room in the air tube). In the next few days I will report the results of the tune and regulator install...
1537145682_20007519845b9efb52a77594.56128138_20180916_171141.jpg

 
Full Shot String Data:

Airgun: Crosman 1720T

Caliber: .177 (JSB 8.44 grain)

BC: .019

Distance to Chronograph: 10 feet

Fill pressure: 3,000 psi

FPS of 40 shots:

663, 673, 671, 680, 690, 687, 692, 685, 696, 691, 702, 700, 703, 703, 696, 698, 705, 709, 691, 711, 703, 706, 700, 706, 698, 711, 707, 695, 696, 688, 695, 682, 677, 679, 664, 671, 663, 665, 652, 650.

Average velocity: 688, StdDev: 16.98, Min: 650, Max: 711, Spread: 61, True MV: 730.96
 
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Hello spysir,



I wonder (I'm almost sure its been written on some forum...somewhere) how the bstaley oring thingy works with the reg. Ive tried the bstaley mod and found it to work surprisingly well without a regulator! I will try and see if this works better or worse with a reg as I go along... The tp port on this guns is .067" or a #51 drill bit. I'm almost positive I will use a of 3.5mm tp port or even a bit more when I add the regulator. In this project, I would like to free flow the entire system and let the regulator do most of the work. Sometimes I find a different scenario, depending on the customer's overall requirements.
 
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My buddy wanted his 1720T set up to stay just under the 12fpe field target limit so I did a bstaley O-ring buffer and ended up with 34 shots averaging 11.4fpe inside of a 4% extreme spread, or 24 shots inside of a 2% extreme spread.



Full writeup here if you are interested.

https://www.gatewaytoairguns.org/GTA/index.php?topic=86564



Be advised that this type of tune can be a fairly time consuming trial-and-error affair compared to using a regulator.



It is most effective when down tuning from a higher power level to give a longer shot string.



With regard to combining it with a regulator, generally there isn't a good reason to do that. With a regulator installed, all you have to do is adjust your hammer strike (hammer spring preload) until the velocity is at about 97% of its maximum. By doing that, efficiency will be quite good and it will hold a tight velocity spread even if the regulator exhibits a modest amount of creep. If the resulting power level turns out higher than you wanted, you could then add an O-ring buffer to bring it down but the usual way would be to either reduce the regulator's set point or restrict the transfer port. The former is somewhat preferable in terms of shot count.
 
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Hello nervoustrig,



Nice read! And yes you are correct, there really isn't a reason to use the bstaley mod (except for maybe trying reduce some hammer bounce) its a cheap and easy way to get better performance without a regulator. i just thought it would be fun to see the results. Usually I adjust regulator pressure, transfer port size, and hammer spring tension, and sometimes the valve stem spring when I do these tunes. I have tried the bstaley mod and its a good get around....but nothing like a regulator in my experience. Maybe I'll try it with the flat washers just for kicks and giggles! Customer wants 10/ftlb power level.
 
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