I could use a little guidance tuning my regulated 1720T

[cwcarrera]

I have a new 1720T and a Huma regulator on the way. I want to get to 12 ft-lb for Field Target with either 8.4's or 10.3's. I am looking for baseline settings for regulator pressure, transfer port size, hammer spring preload, and hammer stroke. I would love to hear from anyone that has experience with a regulated 1720T.

Thanks! Chip

 

[nervoustrig]

Cue up Motorhead. I know he’s done it.

The one I did for my buddy was a ~12fpe bstaley tune that yielded 34 shots inside of a 4% ES. On a small tube like this, a regulator provides a similar shot count (due to the regulator body and plenum overhead) but with the benefit of a tighter ES.

 

[cwcarrera]

Thanks nervous, I searched before asking and found your thread about the bstaley tune you did back in April of 2015. I was hoping you would respond and that maybe by now you had done one with a regulator... I guess not. I will wait and see if Scott responds to this thread and if not I will give him a call and see if he has any info to help me.

Thanks, Chip

 

[nervoustrig]

Hi Chip, I do like regulated PCPs so although I haven't regulated a 1720T, I can touch on a few things that apply universally. 

To maximize shot count, open up the porting so it can flow plenty of air. For .177 pellets, I tend to use the "75% of the caliber" guidance. So something in the vicinity of 0.130" - 0.140" for the valve exhaust, transfer port, and barrel port. The valve exhaust and barrel port may already be big enough, I'm just mentioning them as possible choke points that you'll want to check. The metal transfer port can be challenging to open up because there isn't much to hold onto. If the drill bit snags, it's going for a ride. You can use a grinding bit if that happens...there are no points awarded for being pretty.

If memory serves, there aren't any other notable choke points in a 1720T, save for possibly thinning the nose of the bolt probe.

Secondly, if you can make or buy a lightweight hammer, that will be a big help. It will further improve efficiency by reducing dwell and hammer bounce. That's true for full length rifles but even more so for a short barreled gun like the 1720T.

Adding a SSG would be nice as well but if I had to pick between it and the lightweight hammer, I'd take the hammer.

My guess is a regulator setpoint of 1400psi will get you where you want to be. 1600psi tops. With a 3000psi fill, that's a good amount of overhead for a solid shot count. 

 

[Brazos]

Get a lightweight hammer. That will be more important than the reg. If I had to choose between the reg and a lightweight hammer it would be the hammer. Same goes for Marauder rifles.

 

[Motorhead]

Lol ...



Done a few of them. Generally reg pressure ends up @ 1200/1300 psi. Transfer path valve throat to barrel @ .120" Lighter poppet spring and a Lighter hammer ideally, or oem hammer and a SSG device with a different / softer hammer spring.



My personal conversion was a 1701 with an added 1720 / p-rod air tube making it 1720 spec in air capacity. Keeping the 10" barrel it easily made 11 1/2 ft/lbs giving me @42 shots on the reg.

Actually won 2 state pistol championships with it before selling and building something different.

 

[cwcarrera]

Nervous and Scott, thank you both for the input! I will open up the transfer path to .120" and see where that takes me, if I need to go bigger from there I can take another crack at it. Is there any reason I shouldn't just do some lightening on the stock hammer? I have a nice drill press and an X/Y vice so I can make holes in the hammer. The stock hammer weighs 58.6 grams, how much weight am I looking to loose, 25%? 50%? With the lightened hammer will I use the stock hammer spring which I calculate to be 8.5 lbs? I am calculating the poppet spring at 25 lbs, what should I be changing that to, and any advice on where to buy that spring?

Thanks again to you both for all of the help and willingness to share your knowledge! Chip



Edited, left out the decimal point in the hammer weight. 58.6 is the correct weight.

 

[Motorhead]

IIRC the MDS hammer I ended up with only weighted @ 16-17 grams using the OEM screw in steel striker. Spring used was one from my stash of spring from my Paint Ball tuning days. Was larger in diameter and wire than oem however.



because other internal valve mods were done to gain increased breathing feeding valve, plenum increases etc ... and the parts You will ultimately use are different than mine which will yield different results. Just get into it and learn cause & effect as you do it. To be a tuner is to pay attention to the small things, make sense of them and adjust accordingly.



Scott S

 

[cwcarrera]

Thanks Scott, I think I have a good idea what I need to do. If I get stuck I will be back with more questions.

Chip

 

[cwcarrera]

Scott and Nervous,

I'm pretty happy with where I ended up on my tune. This is the basic final configuration I got to after much trial and error. I installed the Huma regulator set at 1300 psi. Opened up the valve exhaust and transfer port to .120" and the barrel port was already .130 from the factory. I was able to lighten the factory hammer to 42 grams with some judicious drilling from the rear of the hammer. Installed a lighter poppet spring that changed the static preload on the valve from 9.5 lbs to 4.5 lbs. I tried a significantly stiffer hammer spring but was not happy with how hard it made the gun to cock so I shortened the factory hammer spring by .250" (which did make the spring constant a little stiffer) and I am happy with the way that is working. The shot cycle 'sounds' good to the ear, nice and crisp. With 10.3's I am right on 11.5 ft-lbs but with the 8.4's I can only get to about 10.8 ft-lbs. I think I would either need to open up the flow path a little more or bump the reg pressure up to maybe 1400 to get the 8.4's shooting up at 12 ft-lbs. It doesn't really matter because it shoots the 10.3's better than the 8.4's and looking at the trajectory of both out to 35 yards there just isn't enough to gain by shooting the 8.4's.

String data;

AV = 710

ES = 6.59

SD = 1.41

38 shots from 3K to 1300 PSI.



I have some .750 Delrin in hand so I may make up a much lighter hammer at some point in the future to give that a try. But for right now I think I am just going to shoot it and have some fun!

Thanks to you both for all of the help!

Chip