I bought a green laminate HW77k from Krale about 18 months ago. After shooting it for about 8 months and getting really fed up with poor accuracy I tore into it and found all kinds of problems-melted piston seal, broken spring, really tight first inch of the barrel. Well I resolved all of those issues and got it shooting about 760fps with 8.64gr H&N FTT. That was with a OEM replacement spring from Pyramyd Air and one of Maccaris blue "bullet seals." That settled into 750fps over the course of a couple thousand pellets and held at that speed. I think that is just a hair less than 11fpe. Since that corrective action about a year ago, the gun has been quite accurate but very hold sensitive. Fast forward to this evening. It has always bugged me that it was a couple fpe weaker than I thought it should be, as I ordered a FAC gun (non-12fpe). I'm not a big number chaser but I do sometimes use this gun to shoot field target and thought a couple more fpe would net me a flatter shooting gun. I finally decided to open it back up and put a new spring in and see how the piston seal looked. Pyramyd Air spring was fine, maybe 1/4inch shorter than the new Vortek spring I put in this evening. The piston seal was also still in great shape but while I had it open, I decided to size it since it was still fitting really tight and I wondered if the too-tight piston seal was maybe robbing me of those couple extra fpe. So, I changed two variables: I put in the new Vortek spring, and re-sized the same blue Maccari piston seal that I'd put in a year ago. Got it all put back together and put a few through the chronograph. I just knew I was gonna be in the 850-860fps range.......nope. End result of all of that-760fps. Which I strongly expect to settle into about 750fps over the course of the next couple thousand shots. So, I changed multiple variables and got right back where I started-about 11fpe.That lead me to my title question: Does anyone know how HW limits their 12fpe guns to non-FAC levels? I'm wondering if it is via reduced transfer port diameter. That would explain why I cant get past 11fpe. It would also explain why the gun is hold sensitive as I would think it "oversprung" with a transfer port that simply cant handle all the air coming at it. Or, does every HW97/77 come off the assembly line with the same sized transfer port and the power reduction is obtained through other means?I just checked my order confirmation email from Vortek (to make sure I'd ordered a OEM power spring) and this is what I just put in the gun:HW 97 OEM Replacement SpringProduct Code: V10-810125-31Price: $18.90 Nicely fits the HW/Beeman HW97/77 and Clone airgun guides for OEM+ power.
Could those last two fpe be simply explained by my altitude?
Another perhaps relevant details is that other springers I own are weaker than advertised too. My HW50s only gets about 8.5fpe. Two different fwb300's that I own (one 300s and one 300) only get a little over 6fpe with fresh Arctic springs and new seals.
If that's it, I'm losing somewhere around 12% of velocity to altitude. With the other guns having the same problem though it kinda seems obvious. Cuz I haven't done any piston resizing on those other three I mentioned being underpowered. Thanks guys. I was hoping it was something I was doing wrong and could fix, other than "move to a lower elevation."
Chronograph set up indoors with non-flourescant lighting. Gives the same readings outdoors under the sun though. Muzzle about 8 inches from the first sensor. Hector, I've shot a couple of the Diana recoiless guns and the only one that didn't feel like a bomb going off in my hands was quite likely the very gun you're referring to as Mark let me shoot his a couple times last summer. I attribute that major difference to your magic.Without intending to hurt feelings, I don't feel like the Diana guns are finished to the same level as HW springers. But, if it's a way around altitude, it might be an acceptable trade off. I kinda like the idea of trying a weaker spring to reduce some jumpiness. If the physics of my 77k at altitude caps the energy limit at 11fpe, I might as well use the most tame spring possible to get there (not my idea-someone suggested it in a response).