I'm going to put this question here, as finding out how fast your loads shoot is an integral part of handloading for many of us.
I have an old Alpha model Shooting Chrony and pull it out on occasion to check my loads. When it reads out an error I know that straight away because it will be a 1,000 fps higher than the load will call for. Case in point; 420 grain lead FNGC boolit with 26 grains of old Hercules 2400, a puff of dacron and CCI 200 primers out of the 45/70. 7 or so years ago those averaged 1,620 fps. It had been a while and I wanted to do this again along with other loads yesterday afternoon. Out of 10 shots 5 were true readings, where 5 were 2,052 up to 3,693. Certain loads will do this on occasion and I've learned to accept it as an error on the unit's part and go on. (Maybe the dacron flys past the sensors and caused this?) The 5 good readings averaged 1,670 fps, so I know these loads are in the same ballpark as they were some years back.
I began this session shooting the Lee 457-405 RFPB boolits over a charge of Unique. 6 out of 6 shots were good readings and they averaged 1,184 fps from the 26" Cowboy barrel. This Chrony can work splendidly when the stars align up.
Now, here is where I am a bit flumbuzzled: at 15' shooting 250 grain Hornady SP-SR bullets out of a 338 Win Mag, using 73.5 grains of 7828ssc, Federal 215 Magnum primers and Hornady Brass, the averages came out to 2,837 fps. This is NOT a max loading, that would be 74.5 or 75 grains depending on which manual I consult. The primers are flat on the surface but have a rounded edge, not smeary appearing that a too hot of a load gives. (Trust me, I've seen those before and stay away from extreme high pressures) The reloading manuals say this load should be in the 2,650 +/- range. {Hodgdon online loading data shows 75 grains of 7828 with the same 250 gr bullet is 2,628 FPS}
This is a proven load for accuracy out of this Ruger M77, as we've tested it 3 years ago when sighting in using the ladder load testing but with no chronograph available (chrony in Ohio, rifle was living in Colorado with its owner, I was visiting) Starting at 71 grains going up we settled on the 73.5 grains of IMR 7828ssc because it didn't give excessive pressure signs, whereas the 74 and 74.5 grain loads seemed to be more than we deemed necessary. Curiosity got the best of me yesterday, the rifle now resides in Ohio and I pulled out the Shooting Chrony. Va La, now I know that these bullets are spitting out impressively fast compared to what the manuals state they should. Hmm....
Another loading for the 338 Win Mag we never shot is 69.4 grains of IMR 4350 with the same Federal Magnum Primers but with Winchester Brass. 4 shots through the Chrony showed these averaged at 2,968 FPS. The Extreme Spread of 222 FPS. (yes, one was over 3,000 by 85 fps). MUCH Faster than the manual says this loading should be. {Hodgdon online Load Data says this same bullet with 69.5 grains IMR4350 clocks at an average of 2,634 FPS} The primer was very flat, just the slightest touch of a radius left on the edge, not smeary to the outside of the primer pocket mouth, but more pressure than I would want to continue to fire through this rifle. If this were reading error then they would be showing a substantially higher read out, from all the past experience I've had with this unit when it acts up. Still, this is quite fast for the 338 WM and 250 grain jacketed bullets.
In case you are wondering "How bad is the kick on these loads?" Well, with the Decelerator Pad and the Mercury Recoil Dampener Tube glued in the buttstock the 45/70 Marlin CB kicks considerably more noticeable.
Other than my 35 Whelen, which also seems to shoot faster than the books by 100 fps, other rifles are basically spot on when comparing the readings to the manual expectations.
What are your experiences with chronographs and handloads?