Post by: grundy53 on April 03, 2011, 02:39:32 PM
Post by: grundy53 on April 03, 2011, 02:41:25 PM
Post by: sakoshooter on April 03, 2011, 07:22:23 PM
Post by: GregMcFadden on April 03, 2011, 07:25:50 PM
Unless I am A. Loading for multiple rifles with the same round, or B. loading to fit in a magazine that is constrained, I do not pay any attention to the Cartridge length specification. What I pay attention to is the maximum length (for me, measured from base to ogive) that will fit in the rifle with the bullet just touching the lands and whether that round will feed from the magazine of said rifle.
Every cartridge combination(bullet, powder, primer, brass) and every rifle has a particular jump that will give you the smallest group size. I've had some that liked to be rammed into the lands for best accuracy. some wanted to be seated back 0.030 thou. some folks have reported jumps as high as .125-.150 with some bullets in some rifles. It is something that you must experiment with while watching for signs of over pressure (watch the primers in particular). Personally, I always start my load development as close to the lands as possible, preferrably just touching them. With that I generally start below the recommended starting load in the books, as touching the lands can run up the peak pressure. Once I get a round that works well there, I seat the round farther back and see what/if there is an effect.
Post by: grundy53 on April 03, 2011, 08:41:05 PM
Post by: sakoshooter on April 04, 2011, 12:32:00 PM
Post by: GregMcFadden on April 04, 2011, 04:12:29 PM
One more thing to note, just in case you are wondering if it is worth the trouble to change seating depth. My pre-64 M70 featherweight 30-06. Seated to the lands, 5 shot group size is .9 inches at 100yds and 2515 fps. I proceeded to seat back .028 inches from the lands. same speed on the chrony but the group size dropped to .5 inches. I stopped there and deemed it good enough.
Post by: carpsniperg2 on April 04, 2011, 04:17:16 PM
If you're working on seating close to the lands for accuracy, COAL is mainly determined by what will cycle thru your rifle's magazine. Max lengths are guidelines. Rifles vary, magazines vary.
x2
Post by: Bob33 on April 04, 2011, 04:30:59 PM
GregMcFadden gave a very good explaination. Pay attention to his mention of increased chamber pressures from 'touching' the lands.Hornady expains it very well here: http://www.hornady.com/ballistics-resource/internal (http://www.hornady.com/ballistics-resource/internal)
To illustrate the effects of variations in bullet travel before the bullet enters the rifling, we'll compare a standard load with adjustments made only in the bullet's seating depth.
In a "normal" load with the bullet seated to allow about one 32nd of an inch gap (A) between the bullet and the initial contact with the rifling, pressure builds very smoothly and steadily even as the bullet takes the rifling. Pressure remains safe throughout the powder burning period (B), and the velocity obtained - 3500 fps - is "normal" for this load in this rifle.
Seating the bullet deeper to allow more travel before it takes the rifling, as in these next two illustrations, permits the bullet to get a good running start (C). Powder gases quickly have more room in which to expand without resistance, and their pressure thus never reaches the "normal" level. Nor does the velocity; with the same powder charge it only comes to 3400 fps (D).
When the bullet is seated to touch the rifling, as in the accompanying illustrations, it does not move when the pressure is low (E); and not having a good run at the rifling as did the other bullets, it takes greatly increased pressure to force it into the rifling. As the rapidly expanding gases now find less room than they should have at this time in their burning, the pressure rise under these conditions is both rapid and excessive (F). Velocity is high at 3650 fps - but at the expense of rather dangerous pressure. Many rifles deliver their best groups when bullets are seated just touching the rifling. Seating bullets thus can be done quite safely if the reloader will reduce his charge by a few grains. The lighter load will still produce the "normal" velocity without excessive pressure.
Post by: grundy53 on April 04, 2011, 10:06:50 PM
Post by: KNOPHISH on April 05, 2011, 06:02:41 AM
Post by: GregMcFadden on April 05, 2011, 08:07:08 AM