rnbra-shooter
CGN Ultra frequent flyer
- Location
- New Brunswick
That "Riflemans's Journal" article that BearClan linked to is a good one (BTW BearClan you spelled his name correctly; also, FWIW his first name is spelled "German" but it is pronounced "Herman" - he's a really good shooter and a helluva nice guy, he used to come to our Canadian national matches). Among the many good ideas in that article is the "bring a .22" to confirm your setup.
trix_17, those 168s weren't actually going 3300-3500. Your Chrony was reading that, so this would have been caused by one or more factors:
- muzzle blast (though unlikely at 10')
- lighting (too much; too little; bad angle, i.e. very low angle causing glints off of the bullet). Try using skyscreens; if it is a clear day, try to avoid having sunlight shine directly on the sensors (if you can place the sensors in shade but looking up at a bright sky, that works well)
- Chrony not parallel to the line of fire (it's not critical, a few degrees misalignment won't make a meaningful difference, but a big enough misalignment will indicated faster-than-true speeds. 10 degrees off-axis will indicate 1.5% faster than true (so instead of 3000fps it will indicate 3045fps). 15 degrees off-axis will indicate 3.4% faster than true (so 3000fps will indicate 3100fps).
- Chrony not fully unfolded as Andy mentioned (the way that my tripod connects, can make this happen unless you are paying attention)
Of these, when I get "I'm not really sure about this" readings from my Chrony, it usually is a lighting problem
(re 168s at 3300-3500.... I once shot what I intended to be a fireforming load, which fired 155 Sierras at 3150fps from a 30" barreled .308. All of the brass was ruined (loosened primer pockets). A 168 from a .308W at 3300+fps won't show mild indications of overpressure, it will show severe or catastrophic pressure signs.)
trix_17, those 168s weren't actually going 3300-3500. Your Chrony was reading that, so this would have been caused by one or more factors:
- muzzle blast (though unlikely at 10')
- lighting (too much; too little; bad angle, i.e. very low angle causing glints off of the bullet). Try using skyscreens; if it is a clear day, try to avoid having sunlight shine directly on the sensors (if you can place the sensors in shade but looking up at a bright sky, that works well)
- Chrony not parallel to the line of fire (it's not critical, a few degrees misalignment won't make a meaningful difference, but a big enough misalignment will indicated faster-than-true speeds. 10 degrees off-axis will indicate 1.5% faster than true (so instead of 3000fps it will indicate 3045fps). 15 degrees off-axis will indicate 3.4% faster than true (so 3000fps will indicate 3100fps).
- Chrony not fully unfolded as Andy mentioned (the way that my tripod connects, can make this happen unless you are paying attention)
Of these, when I get "I'm not really sure about this" readings from my Chrony, it usually is a lighting problem
(re 168s at 3300-3500.... I once shot what I intended to be a fireforming load, which fired 155 Sierras at 3150fps from a 30" barreled .308. All of the brass was ruined (loosened primer pockets). A 168 from a .308W at 3300+fps won't show mild indications of overpressure, it will show severe or catastrophic pressure signs.)
Stuart
