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.

From 24"to 18" barrel, my 308 165gr dropped about 300ft/s=50ft/s=1". 18inch drop at 300yds from 100ydszero.

Cold bore 2 shot group 300yds,18in drop in nor-west win left to right.No crony,drop is what it is to work out velocity.

Greetings again @yogi,
From my records 44 grains of AR2208 with the 150 grain Hornady in a Winchester case chronographed 2,560 fps in a 20" barrel. This would work out to 2,610 fps with your 45 grain load. Your load is 2 grains under max which is more limited by case capacity than pressure and you may struggle to get the full charge in your case but if you can 47 grains of AR2208 should give you about 2,710 fps or perhaps a little more. It seems that once the pressure drops below a certain point velocity drops at a faster rate. Your loads are only giving around 45,000 CUP or a little more where a full pressure load would develop between 50,000 and 52,000 CUP. Your Sierra Game King projectiles should be a good match for the Nosler BT used in the Hodgdon/ ADI data. I assume that the 159 grain is a typo. If you wanted to go to a different powder I would recommend AR2206H 45 grains of that should give 2,720 fps in your rifle and it will fit in the case better. 45.5 grains is book max. Work up using your chronograph though and expect around 80 fps loss below 24" data as you approach max loads. Or just go hunting with what you have, your choice.
Regards Grandpamac.
PS. My Chronograph is an Oehler 35P with a 4 foot screen spacing and has proved reliable for me.

@yogi. Ar2206h is a faster burn rate powder than 2208 and so a bit bettet suited to yr shorter bbl than 2208. The mv gain is likely to only about 120fps though. 2206h takes less space than 2208 so at around 45- 45.5 grains there should be no need for compression. Cheers mate.

Thanks alot for the input. Certainly food for thought

A couple of typos on the load.
150gn sierra not 159
46 grains 2208