Hi Harry,
Sorry but I've not tuned it to shoot sub 20 but I am sure its possible. However, the new Compact version might be better suited to those lower tunes since it has a completely different barrel and faster twist rate for lighter pellets. This Performance barrel is specifically engineered to shoot Monster Re-Designed 25.4's, using a slower 1:32 twist, at much higher power levels and velocities. Therefore, I have only used the 25.4 MRD pellets in mine.
I've been tweaking this rifle for weeks and having a blast shooting it and have learned a lot about its nuances. I didn't include any hammer settings because I am using a 3D printed butt hook plate so my measurements would be quite different and much longer than with the stock butt plate. If you use my bar settings and velocities and tune your HS to get these velocities then you will accomplish a similar tune.
I went through some virtual spin drift calculations using Streloc Pro, made the spreadsheet below and found what the pellet's sweet spot should be. Then I focused on tuning within those speed ranges which are 800-920. Although a person from JSB said the range they found to be the most accurate for MRD's is 880-970 using their barrels. They had no data with a slower twist rate of the LR barrel.
This range in GREEN (800-920) shows the least amount of spin drift (3.6 mils) using a GA drag model and .050BC with a 15 mph 90* cross wind factors. 940-960 was 3.7 and 980-1000 3.8. Above 1000 the pellet's calculated spin drift increased dramatically and linearly up to 4.3 mils of drift. Right now I am shooting at 965 but will continue to work my way down to 900-ish all the while by taking copious notes. The 965 tune is shooting the best this rifle ever has so I am very pleased with it as is but being a former Mechanical Engineer I will continue chasing perfection
I experimented with tunes up to 1050 and I had a lot of fliers and severe leading in my barrel even after just 20 round shot strings. The rifle was not friendly to shoot at these higher velocities and seemed to jump around with each shot. I tried all kinds of different lubes and waxes on the pellets and nothing reduced the leading until I dropped my pellet speed below 980. Then White Lightening Bicycle chain wax did the trick at completely reducing the leading in the bore, fliers were almost non existent and group diameters shrank.
It wasn't until I started using the HS to harmonically fine tune the reg setting that I started to notice significant accuracy improvements and more efficient air consumption. Before, I was just randomly tweaking the reg then the HS then the reg and then the HS mostly chasing my tale hoping to find a lucky combination that showed up as accuracy on paper. It wasn't until I started to harmonically tune that I saw huge accuracy improvements.
For those of you that don't harmonically tune (like me) try it because it works. Set the reg to any PSI you want, increase the HS 1/4 turn at a time until the velocity peaks or starts dropping. Lets say the max peak velocity is 1000 FPS. The harmonic tune should be somewhere close to 3% under the max velocity of 1000 or 970 FPS. To get there start by backing off the HS 1/4 turn at a time until you reach 970 FPS. That is close to where the reg and HS are in harmony. You guys who have done this for years I am preaching to the choir but for a relatively newer shooter like me this was an enlightening A-HA moment.
Once you get to 970 then start shooting for accuracy and take notes. REMOVE the chrono if its an FX and hanging on the end of your barrel!!!! Just the weight of the FX chrono will affect POI, barrel harmonics and accuracy. Are the pellets going where you expect them to at this velocity (970)? Is the gun friendly to shoot? Is it efficient on air consumption? If the accuracy isn't where you expect it then fine tune the HS either way by 1/8 turn and see if the accuracy improves or suffers. If the accuracy simply isn't there then repeat the process using a lower or higher reg setting keeping in mind the data set you calculated in StreLoc Pro or similar shooting APP. Its not a fast process so be prepared to take accurate notes that you can refer back to and hurl lots of lead down range.
I want to publicly thank loneranger, Cworkman74, Centercut, MikeVV and zx10wall because they were all very helpful in sharing rifle set up, tuning advice and shooting tips which has made a huge improvement in my tuning process as well as my shooting techniques. Thanks guys!