Thread: Barrel chop

Originally Posted by 6.5 CRD

A loss of 200fps for 4 inches doesnt add up with the .308, you should theoretically have lost around 100-120fps.

My suggestion would be that theres an error somewhere with either chronograph or loading. 44gr is a pretty light load to be doing 2820fps with a 150gr so may be as simple as a chronograph error, or those loads were hotter than 44gr.

End of the day its still going to do the job perfectly fine at the distances you indicate your going to hunt at, but you are right in thinking a loss of 200fps is abnormal. Velocity loss vs barrel length is not a linear thing, it is a curve that ramps up at the beginning of the barrel and then flattens off as the barrel length increases. Cartridge type / bore size / powder used all effect the shape of this curve and therefore how much velocity is lost when shortening can never be a set "x fps for y inches". Due to the shape of the curve, the overall length plays a big part too. For instance with a .308, going from 28 inches to 24 will lose you just under 100fps, however if you were to shorten a barrel that was 18" to 14", you would lose close to 200fps.

If he's using an optical chrono it could be anywhere + or - 100fps depends on lighting conditions....optical chrono's drive me nuts with their inconsistent velocity more than once I've gone out same load different days in different conditions (cloudy vs sunny) and got different velocities due to using an optical chrono, shot same poi just got different velocities due to lighting being brighter or darker. If you don't know someone with a Labradar or a magnetospeed your best option is to shoot at range say 100y an 200y then use drop to calculate your velocity.

Originally Posted by dannyb

If he's using an optical chrono it could be anywhere + or - 100fps depends on lighting conditions....optical chrono's drive me nuts with their inconsistent velocity more than once I've gone out same load different days in different conditions (cloudy vs sunny) and got different velocities due to using am optical chrono, shot same poi just got different velocities due to lighting being brighter or darker. If you don't know someone with a Labradar or a magnetospeed your best option is to shoot at range say 100y an 200y then use drop to calculate your velocity.

Yeah your not wrong, Optical chronos can be a pain in the ass.

Calculating velocity from shot drops can work but its not a very accurate way of doing things imo. Alot of variables & not only that, need to shoot at some serious distance to try filter out the noise. 200y just isnt enough distance to give you an accurate idea of velocity. Difference in drop with the 150gn Sierra at 2600fps vs 3000fps is less than 3/4 of a minute at 200y. Most peoples cant shoot a 3/4 minute group, let alone have a rifle capable of doing so consistently.

Originally Posted by 6.5 CRD

Yeah your not wrong, Optical chronos can be a pain in the ass.

Calculating velocity from shot drops can work but its not a very accurate way of doing things imo. Alot of variables & not only that, need to shoot at some serious distance to try filter out the noise. 200y just isnt enough distance to give you an accurate idea of velocity. Difference in drop with the 150gn Sierra at 2600fps vs 3000fps is less than 3/4 of a minute at 200y. Most peoples cant shoot a 3/4 minute group, let alone have a rifle capable of doing so consistently.

yup fair call on the distance thing but probably still more accurate than an optical , I don't use distance for my initial velocity verification it as I have a magnetospeed, obviously drop should always be confirmed by actually shooting at range.