How does the overall case length of the bullet affect performance? I am loading .117 gr Sierra's with 52gr H4831 to a length of 3.321 (max 3.325). If I seat the bullet deeper or shallower, what am I gaining? Yes, I am still working on loads and am new to reloading.
I also have some .75 gr varmint HP's that I am told will flat out smoke a coyote. Should I keep the OAL the same? I am very confused here and I have read to my brain is mush. Any advice is appreciated.
What you gain by knowing the precise OAL of each bullet is consistent accuracy.
You have to experiment, via testing, with each type of bullet, what overall length your rifle likes. It's not likely to be the same for the heavier bullets as it is for the lighter bullets - even bullets of the same weight, but different make and type tend to like different OAL's for maximum accuracy.
Consistency is the name of the game when reloading for accuracy, which I feel is the most important reason for taking up this endeavor.
The way my mind reasoned it out; different OAL's for one load (117gn Sierra/52.0gn H4831) will create different pressures inside the chamber area of your rifle. This in turn creates different velocities and "barrel times" for those 117gn pills, which envokes different barrel harmonics. However slight these harmonics differ will create different points of impact for your bullets. Thus accuracy suffers.
I feel similar results could be obtained by keeping the OAL the exact same but varying the powder charge a grain or two on either side of the optimum charge, or accuracy node for your rifle.
Other things affect consistency also, such as temperature, humidity, altitude and barometric pressure. You can't expect your loads to print the same in December (30deg) as they did in June (98deg). Moderen Smokeless Powders are good, but not that good, and I don't burn one brand. I have approximately 20 different "rifle" powders and (half that many "pistol" powders) all create different "barrel times" in my rifle(s). This allows me to find the best powder for whatever weight bullet I decide to load for. Example: I wouldn't use Retumbo (although you probably could) to load the 75gn Sierra HP in the .25-06, you may not be able to get enough in the case.
Different bullet weights equate to different lengths. Some can be set at .010" off the lands and grooves and others will have to jump more. Good luck. JohnnyK.
I have loaded for and hunted with the 25-06 for a long time. It has been my finding that for some reason most of the rifles that I have used wanted the bullet to jump between 10 and 30 thousands off the lands with most closer to 30 thousands. Here are just some of my findings.
In 24" barrels 52 grs H-4831 with 117 Sierra Pro Hunter, the flat base bullet, seems to really shoot well over all. With 26" barrel 49 grs IMR 4350 and the 117 Sierra Pro Hunter seems to shoot better. There is not much different in the velocity either. My load that I have used for years in my 26" barrel consists of this. Rem cases, IMR 4350 49.0 grs, CCI 250 mag primer, 117 Sierra Pro Hunter, OAL 3.150 . I have killed well over 100 whitetail deer with this load from point blank to a little over 400 yards and 99% of them drop in their tracks.
Read the sticky at the top of the reloading section "how to get the best accuracy out of Berger VLD". If you have already tried some loads take one of your milder more accurate loads and try the Berger recommendations, then work up your powder increments. I have found this to be absolutely crucial for the VLD type bullets, but less so with other bullets. I have had really good luck with the sierra 117 sbt in my 25-06 and never even played with the seating depth with it. A bullet comparator is a much better way of measuring length as it measures from the ogive, which is what matters (the tip of the bullet will vary).
I have an older 700 BDL and it's favorite bullet has been the Hornady 117gr SPBT. It ended up that the best accuracy I got out of it was Hornady's recommended COL. With that COL it shoots sub 1/2 MOA. I didn't have to do anything special to the load at all other than switching to a magnum primer and started working up the load again with IMR 4350.