OCW Questions.

[farmer7]

Hello all,

I have always used Varget for my .223/.308 loads but over here it's nigh on impossible to get currently so I have moved over to N140.

I did an OCW development for the .308 and have a great load now but the N140 from what I've read is quite a bit more temperature sensitive.

My question is this: Is my OCW load purely based on the charge weight or will muzzle velocity drop in the winter months affect it? Do I want to tweak my load to match my summer velocity?

I know "Optimum Charge Weight" itself should answer my question but wondered about the velocity drop affecting it!

Any insight or thoughts folks?

Thank you.

P.S. The suppressor I use is an ATEC Maxim with four baffles but have ordered an extra two baffles which will add just under two inches in length and a wee bit of extra weight, maybe 1 1/2oz.

Will I have to start over with my OCW? From what research I've done some say yes some say no. The no argument backed up by the fact that FGMM shoots well out of just about anything supressed or not and its based on the charge but wondered about altered harmonics affecting the OCW , Confused!

Hope it all makes sense!

Thanks again.

 

[steve-o]

Ok OCW method will get you in the accuracy node where you have the smallest spread in velocity and being off a few 10ths of a grain really won't matter. There is a burn rate chart for different velocities that will show you how much to expect velocity change due to temperature changes from 0 degrees F to 100 degrees F.
Basically you pick the chart that closest matches your velocity at standard pressure and temperature of 70 degrees. based off that velocity you could have as much as 200 fps change over the total temperature range. it's just a curve, if you charted your loads at each 10 degree increment it will follow the curve, maybe a few fps faster or slower but in general follows the curve, now that is for ammo temp as well.
if you want further or better explanation of temp curve other than some army or marine field manual there is a guy who goes into detail on how to develop your ballistics table for different atmospheric pressures, ammo temp, spin drift, coriolis effect, wind, lead...very informative.
go to youtube and look at TiborasaurusRex sniper 101 series, about 70 videos ranging from 8 to 30+ minutes.
he can really explain it better than me. He also has those burn rate charts and ballistic tables downloadable for you free, just plug in jbm data, it's excel spread sheet.