Addressing the OP, get one of these if funds are sparse:
I have had one for several years and although it only measures in MPS (meters per second), I still recommend it as it is reliable, works WITHOUT ANY external light source (in other words, you can use it in pitch black dark/no light). I have never had a failure to get a reading. Compare that to the reviews you can read about Caldwell, etc.
Or you can spend a lot more and be frustrated by about as much $$$ as you spend.
Good luck!
P.S.
I don't recommend the "stand alone" of this same unit that does NOT attach to the barrel as it is harder to use (easier to shoot/destroy it), although it does give you tenths of a meter measurements. The muzzle mounted version is nearly fool proof. Works every time and is accurate, up to plus or minus about .5 to 1 meters per second. Probably more accurate than that, but I like to err on the extreme side. That way I am sometimes pleasantly surprised instead of vice versa.
For MOST folks, like me and probably you, the muzzle mounted version, even though it only measures in MPS, is enough to tune most airguns and you should download
ChairGun Pro for fpe and trajectory calculations. Although no longer supported, it is free, it works well and does more than you will likely ever really need. Don't over think it and you will save a LOT of money!
Good luck!
Oops...
Take a good look at the picture I posted and find the best price you can. They are ALL the SAME, regardless of "brand". Same goes for Chinese hand pumps. Find the best price from a vendor with good customer reviews... LOL! Even then, I always cross my fingers... I mean it is Chinese Communist junk. Yes... it is, but when will anyone in the USA make comparable products at a competitive price? And where are the other products competing for your money actually made? Probably China, Indonesia, Japan, etc, etc, etc, etc, etc. (heavy sigh)
