A few thoughts on reloading accurate rifle ammunition...
A few questions have come in on the subject of building accurate rifle ammunition, and I thought I'd share the answers here. From a reader..... "Why do you ream out primer pockets? Is that better than just cleaning them?" There is a big difference between cleaning a primer pocket, and actually changing the dimensions with a reamer. The idea behind cleaning the pocket is to get the carbon out and make a clean hole to push the new primer in..... and that's all well and good. 'Reaming' a primer pocket, or better yet 'Squaring and truing' the primer pocket, those are done for other reasons. Just simply cleaning the pocket up with reamer might be done to eliminate a military crimp, or take out a ding in the brass that's interfering with seating the primer.
'Truing' the pocket, and squaring it up with a Lyman Primer Pocket Uniformer.... that is the gold standard of primer pocket preparation. While removing brass from the case is always something that should be approached carefully, taking away a little in the primer pocket itself to make sure it's perfect does have advantages. A 'square' primer pocket has the bottom and sides straightened with a precision reamer so the bottom of the pocket is exactly 90 degrees from the sides. That allows the primer to sit square in the pocket. The operation also assures the bottom of the pocket is flat, once again letting the primer sit square, and not cockeyed.
Another advantage, truing the primer pockets assures they are all the same depth across the entire batch being worked.... something that is NOT assured on even virgin factory brass. A different depth pocket from one shell to the next means a slightly different initiation of the powder charge burn.... and that means accuracy variations. Most of the time, just cleaning the primer pocket occasionally is fine for everyday shooting. When the hand loader is trying to shrink groups sizes already measuring in small numbers... the extra step of truing and squaring the pocket is worth while. Also, it's a once and done deal... as they generally stay true once made so. Another question from a reader: "What is the point behind flash hole deburring?" Good question.... glad you asked! You see, most ammunition manufacturers do not drill the flash hole in a case, as that is an expensive and time consuming operation. They simply punch the hole in the soft brass while forming the case. This is faster, easier, cheaper, and works just fine.
The thing is, the act of punching the flash hole in the case almost always leaves tiny brass flashings around the hole inside the case. While normally never an issue, these flashings do serve to alter the way the fire from the primer enters the case, and can reshape the flame front impacting the powder charge, changing ignition characteristics. Now, again, most shooters will never notice the minute change this makes in the exterior ballistics of the round, and cases with untouched punched flash holes will still turn in sub-moa groups all day. That said..... when searching for ammunition perfection, the time it takes to deburr the flash holes is not misspent. A few minutes with a Flash Hole Uniformer are all it takes, and you only ever do it once to a case. And a last question: "Is case length really critical? Do I need to trim my shells every time I load them?"
Case length IS critical from a safety standpoint, as a case which is too long may intrude into the throat of the barrel, and might be squeezed into the bullet, unable to release it properly on firing. This can raise pressures dramatically, and cause an unsafe condition. That means cases need to be trimmed when they reach maximum length, and most rifle cases 'grow' in length with each firing as brass flows forward. The higher pressure the load, the more cases tend to stretch.
All this is important, but there is another factor when we are discussing hyper-accurate rifle ammunition. The amount of bullet seated within the neck, and the tension that neck maintains on the bullet.... can dramatically change the way the powder charge fires. More tension, and the powder charge is contained slightly longer in the case as it begins ignition, jumping chamber pressure and altering the entire pressure curve of the firing event. A shorter neck, or less tension, and the bullet is released more easily, thus lowering chamber pressures and powder burn rates. I believe neck length and tension issues are one of the larger factors in variations in pressure curves, and thus velocity variations. Velocity variations invariably lead to poorer accuracy than might be desired. So.... yes, case length needs to be checked with every loading, max length cases need to be trimmed, and ideally every case in the batch will be trimmed to exactly the same length.
Regarding neck tension, and making every bullets journey start as evenly as possible.... many folks take more time polishing the inside of the case neck than they do the outside of the case. The thought behind this is a carfefully cleaned inside to the neck will make bullet release more consistent. I tend to agree, and on cases I truly care about will clean the inside neck with a bore brush chucked in a low speed power drill. Only five seconds per case, but that is enough. Remember friends.... when we are thinking about putting together rifle ammunition with thoughts towards stellar accuracy, the Name Of The Game Is The SAME. Every round needs to be as identical to the others as possible!