Originally Posted by yzm19
does it matter if you are + or - like .03 on your loads? i am going to load some .308 and was wondering if i need to be "spot on"??
If you're referring to SAAMI length, then no. SAAMI listed COL's are a guide only for the shortest chamber, and do not have to be followed to the letter.
An easy way to determine the COL for YOUR rifle is to cut a slot in the neck of a dummy round, seat the bullet long, and chamber in the rifle. This will show you where the origin of the rifling is with THAT bullet. This measurement will change with different bullets.
I always set my handloads either for the maximum magazine length, or for single shot target rifles, to the length that the rifle likes.
This may be shorter than SAAMI or longer, depending on the above restrictions/usage.
Each bullet profile has a different ogive length, which dictates how far into the lands it will seat and the COL will need to be adjusted for this, normally a length just shy of the lands is preferred, but this cannot always be had. So, if a magazine rifle, seat 3 bullets that are .010"-.015" short of magazine length, then 3 at .020", and so on at .005" increments, until you find the 'sweet spot' with THAT bullet, this may/will change with each bullet profile you use.
Hope this helps you out.