After looking over the posts on Chrony’s my conclusion is that a good cost effective basic unit for me would be a new F1 for $60 from DNR Sports upgraded with Peterdragan mods (according to the Shooting Chrony website you can’t upgrade their $50 refurbs).
That being said, being sort of impatient, I wanted to check out my 26 year old Crosman 760 smoothbore pneumatic air gun that I use for plinking - NOW - like this weekend. What I wanted was approximate velocity figures for BB’s and the Beeman Silver Sting pellets at different numbers of pumps. I wanted to know approximately how much muzzle energy the old 760 was putting out. So I devised a variation of the setup described on the Seattle Airsoft website based on the chrony feedback in the postings.
Basically the Airsoft guys measured the interval between two sound events, in their case a paintball going through two sheets of paper spaced 10 feet apart using computer audio software to measure the interval, and calculating velocity from that. One small issue they encountered was that the paintball going through the paper slightly affected the velocity and hence the readings. I wanted to figure out a way to do it that didn’t affect the velocity. See
http://www.seattleairsoft.com/modules/news/article.php?storyid=15&page=0 In one of the chrony posts here in the GTA one member made the comment that he just used the “snap – thwack†method to tell which pellet worked best in his airguns – the pellet that went the quickest was the one that had the shortest interval between the shot being fired and the pellet hitting the target. Hey! Two sound events!
So I started digging through my accumulated computer accessory junk and came up with two microphones and a headphone “Y†connector, then downloaded the free Audacity 1.2.4b audio software for Windows from the web on to my laptop. See
http://audacity.sourceforge.net/ I had what I needed to set up an audio based chrony test. (If this is starting to sound too techie, don’t worry – check out the following pics for liberal use of duct tape)
I set up a piece of ½†thick mahogany board and placed a sandbag on a table 10 feet from the board – the front edge of the sandbag is exactly ten feet from the target board. I tried to keep the gun muzzle at the bag’s front edge to maintain a constant distance during the shots. I placed a microphone facing the bottom of the target board below my intended target area.
I placed one microphone on the table facing the sandbag directly below where I placed the barrel on the sandbag. I found it was necessary to place the microphones facing away from each other to keep spurious sound pulses to a minimum. I also placed an old (clean) sock over each mic. for the same reason, and also a folded cloth under each mic so they weren’t so “liveâ€. I found I also had to reduce the microphone recording level in Audacity to 0.1 to get decent shaped audio pulses.
After everything was set up all I had to do was start the software recording, fire each shot, stop the software, then look at the recorded event to get the info I needed and plug it into the spreadsheet.
I set the software in “plus†zoom mode for recording each event to get a close up look at it per Airsoft. From the close up zoom view I was able to identify the leading edge of the gun’s report created when the pellet left the barrel and the leading edge of the sound created when the pellet hit the board. The software enables you to mark each of these spots and drag between these spots to mark the interval. It then reads out the marked time interval to six places to the right of the decimal. I plugged the results into a spreadsheet I put together based on Charlie Da Tuna’s rifle tuning sheet (see attached), and came up with approximate velocity and muzzle energy figures for the 760 Crosman using BB’s and Beeman Silver Sting pellets at different numbers of pumps. Voila! Cheap and dirty chrony readings – good enough for gvt. work!
The readings seemed reasonably correct – remember I’m going for approximate here. According to this method my 26 year old Crosman 760 had the following results:
BB
10 pumps = 570fps / 3.7 ft/lbs
15 pumps = 626 fps / 4.4 ft/lbs (20.6 % gain in muzzle energy)
Beeman Silver Sting
15 pumps = 466 fps / 4.1 ft/lbs
20 pumps = 524 fps / 5.3 ft/lbs (26.7 % gain in muzzle energy)
25 pumps = 534 fps / 5.5 ft/lbs (3.9 % gain in muzzle energy)
What this told me, as a novice airgunner, was that the 760 was good for target shooting and plinking and could be used for very small pest animals and small pest birds at close range.
I also determined that the most power gain from this gun in stock condition was obtained at 20 pumps. Over 20 pumps the power gain was minimal for the effort expended with the gun in stock form.
And finally, maybe Charlie Da Tuna is right…. maybe I don’t have to buy a chrony after all.
(trying to attach pics for this - this is my first attempt adding pics so we'll see how it turns out)
