Less dead air space in the compression tube (from sanding the seal and or using a better seal if available) will net you more power, better accuracy, less risk of scope damage and be quieter. This may vary with different guns but on avg the std B18 Crosman likes it. But dry fire it once and it may break your scope right there, which is why I mention in the guide
to be careful. So I'd leave it at the 15 thou and continue to look for the real issue, then later you can go further if you want. That # will vary like so many things so you really can only push the limit with you and your gun via trial and error. Same with the transfer port; I'd call it good for now but if you wanted you could try an smaller one to see what happens.
Meaning I believe a smaller one would work better but only one way to find out. How much better would be a guess at best but I do not believe that is your problem.
Check out that link in the guide for Chairgun, which will tell you how fast speed drops based on BC.
The phone app no doubt goes by sound so it would need the exact target distance and air temp to work. Then it would give you an avg speed, not muzzle, which is why I mention Chairgun so you can plug in higher muzzle #'s to get the avg your phone is giving you to est muzzle. Pellets slow down very fast and it also depends on speed and the BC of the pellet, so kinda a pita... So a closer target would give you a better idea of muzzle velocity, but I'd imagine also higher risk of error? I spoze if could also give false readings if the shot echos off something, but I'm just assuming all the above since I've seen the app.
Btw I'd save that old nitro since (based on what you said) it's likely about as strong as a regular B18 Crosman spring but my guess is the NP2 spring will hold air longer since the shaft is smoother. So it might come in handy if you ever buy a B18, especially to convert a coil version, or simply want a lower powered/smoother shooting NP2. Considering the much cheaper price of the NP2 spring (assuming the lower powered one is cheap too) it might be worth it for someone to try it in the regular B18, either a coil conversion or to replace a dead oem nitro. I haven't tried putting one in a B18 but seems like it'll fit. I'd be interested too since I've lost several B18 nitro's and would love a cheaper and better replacement.
The regular Crosman guns can do 650 w/ 18gr if tuned, I have one doing 690 @ ~1000' elevation, so maybe 700 at 0'. Point is an NP2 should do at least 700, but maybe more like 750. That's one reason I mentioned using the lighter spring b/c maybe it'll do 650 and be sweeter all-around? Back burner but keep it in mind...
The 18gr are no doubt better on avg at longer ranges but really the 14.3 are excellent too. All depends on what you want, and I like to get speed up if I can. Like if an 18 passes thru a squirrel then a faster 14 would do more damage. Just a thought. Have you tired the the Crow Magnum pellets yet? If anything you're using is passing thru I'd give 'em a try, even if not I'd still try 'em.
Sanding the melted seal is fine as long you verify the OD is good, which you mentioned you did. Mine was barely oversize... Bizarre how they're like that, which had to have been done on purpose and the only reason I can think of is an attempt to seal up the cuts. One big neg imo was mine were off to one side due to the weight of the piston, so the piston was kissing the receiver on one side.
I don't know if I mentioned it but when I sand a seal questionably thin to bump compression I also sand the face of the dovetail smooth so it'll tell me when it hits. Most people are unwilling to take their gun apart again to check that but since you have been in there over and over I'm guessing that's no problem. It's the cork effect the pellet provides which reduces impact, so a lighter pellet, weaker skirt, deep seating etc will hit harder. Pretty easy to understand, as is what higher compression will do, changes in Xfer port volume, leakage, piston speed/weight, spring strength/rate, ambient pressure, breech to pellet stiction/friction etc. Some alternate ways to est velocity are ballistic putty or possibly duct seal. Putty comes with a chart to est speed, duct seal would likely only be a guess or to compare to a known gun. Then there's the sound barrier which is easier in a 177 but the idea is to lighten pellets 'till you get the SS crack. Of course you need a precision scale to weight down to 1/10th or less grains. Reloader type people have one, but I also scored a digital one on ebay for ~$6 (jewelers scale).
The reason I prefer ftlbs over velocity is most people say xxxfps but rarely say what pellet weight, or they may not even know so then I have no idea where there gun is. Like recently a guy posted some exceptional #'s but apparently it was b/c the pellets and the wt on the box didn't match and they were .5gr or so lighter. Generally the cheaper the pellets the less likely the weight is correct, as is the spread within the tin, tin to tin and lot to lot. So worth it to by good pellets...
Horsepower is just torque X speed so I prefer both when possible
I'm like you, backyard gearhead, offroader, all-around tinkerer. Not sure how much torque my Chevota makes but at least 450, maybe 500 if I'm lucky? Then I rev it so get HP too
I tried lower rpm motors at first but it did NOT work out for me...
With pellets ideal is supposed to ~890-900fps, which is b/c pellets BC gets worse after that on up to SS where it's really bad, so what's a guy to do. I still greatly enjoy 1000+fps, especially at close range with hollow points. I also like to keep my std guns in 177 and mags in 22 so trajectory is similar.
As for noise you can't do a lot, but I'd imagine the 18gr pellets are pretty good. You could try Barracuda which are sweet, but too slow imo. I think 18 is as far as "I" would push it since that's awfully slow, but whatever works for you and if you're like me experimenting is half the fun. So pellet wt plus all those things mentioned earlier that affect power will affect noise too. Basically the less efficient the gun is the more mechanical noise it'll make. The lighter spring would help so one more reason it maybe try it once you get it dialed in.
If you have a syn stock then I'd fill in the hollow areas with nice rubbery open cell foam, both under the receiver and butt.
If you could make the foam so that it seals the cocking slot pretty tight but the linkage can slice thru I think that would be sweet.
The oem shroud assy no doubt reduces sound over no shroud so I'd leave it on.