Seating depth question

Silentstalker · Oct 13, 2012

So I am fairly new to reloading and have built up a couple decnt loads using just powder increments. I know from reading this website and a few articles that adjusting the overall length can help improve accuracy too. So my question is this.

Is the COAL the maximum length I can go to or can they be longer than that if they fit in the chamber and the magazine?

Also, do I want to typically go longer than this length to begin with or could the sweet spot be shorter?

Anyone want to go through the best process in layman terms for a noob? I a ppreciate any help I can get!

Thanks,

Chad

Wincheringen · Oct 13, 2012

The published COAL is what SAAMI defines the maximum. Your chamber may accept a longer COAL but most of the time you are limited by the magazine length.

DrMike · Oct 13, 2012

Chad,

Your COAL is part of the customizing aspect of hand loading. It is adjusting the length of the cartridge for your rifle to achieve the best possible accuracy. If, when you measure your cartridge, you cannot come near the lands with a particular bullet, either because you are restricted by magazine length or because of freebore, you have no choice but to adjust for accuracy by varying powder charge or using a propellant with a slightly different burn rate. However, if your cartridge and bullet choice allow you to load to the lands (that is, to actually touch the lands), you may be able to adjust the cartridge overall length to adjust for the best possible barrel harmonics.

Most barrels will have a couple of "nodes" (points where the harmonics are most conducive to an accurate load), and adjusting the COAL will many times permit you to find those nodes. There is no "hard and fast" rule for finding those nodes. However, as a general rule, cup and core bullets (and bonded core bullets) tend to shoot best when loaded near the lands) It is not a good idea to actually load them to touch the lands, except in very special instances, such as benchrest shooting. The reason for this caution is that in the field, a bullet touching the lands may be pulled out when unloading a cartridge, dumping the propellant into the chamber and creating quite a bit of trouble for the hunter. Again, as a general rule, monolithic bullets tend to give the best performance with a longer jump to the lands. The frustrating (I'd say intriguing) part about this is that there are exceptions to every rule, and that is assuredly true for many rifles. Nevertheless, in an unknown case, these rules are a good starting place.

Many hand loaders will test the COAL to seat the ogive of the bullet within 0.010 inches of the lands, if the magazine permits. Again, for many hand loaders, monolithic bullets are initially tested at 0.100 inches from the lands.

When I am working up a load, I watching for consistency in the velocity. A good load will give me a standard deviation of less than one percent. It is not unusual that my loads will give me single digit standard deviations. Even though the accuracy I am seeking may not be present with a load, if I can adjust the COAL, I will likely focus on the load with the lowest standard deviation as measured on my chronograph. The implication is that I will find a node that gives the accuracy for the load. My practise is to adjust the test cartridges in 0.030 or 0.040 inch increments until I witness a significant improvement in accuracy. This will then permit me to adjust in smaller increments to fine tune the load.

Again, for me, my starting points are 0.010 inches off the lands for a cup and core bullet (Ballistic Tip, Custom Competition or Partition) or a bonded core bullet (AccuBond). My starting point for a monolithic bullet (E-Tip) is 0.100 inches off the lands. In the case of a monolithic bullet, I will need to adjust toward the lands or away from the lands. I have had instances where a monolithic bullet required a much tighter jump to the lands (even as tight as 0.030 inches); and I have had a few rare instances where a cup and core bullet required as much as 0.250 inches of jump.

Ridgerunner665 · Oct 13, 2012

We could maybe give more definitive answers if we knew what cartridge and what rifle...might save you a few rounds...most production rifles are pretty darn close to the "standard" for that maker.

Example...Rem 700 magazines will usually feed 308's loaded to 2.82" OAL (SAAMI says 2.80")...that will still be waaaay of the lands, but its all the mag will accept.

Winchester M70's...you can feed from the mag and jam into the rifling.

Silentstalker · Oct 13, 2012

Thnaks guy's. I really appreciate the detailed answers. To give you some detail:

Ruger M77 in 7mm Remington Mag.
H4831sc with both Nosler Accubonds 160 gr. and Hornady SST 162 gr.

Rem model 700 in 22-250.
50 gr. Vmax using varget.

Savage 7mm-08 (this rifle shoots great groups currently).
140 gr. Nosler Ballistic tip using varget.

Any starting points for those in particular?

Ridgerunner665 · Oct 13, 2012

I'm no help with any of those rounds...but I'm sure somebody will be.

DrMike · Oct 13, 2012

On either your Ruger or your Remington, the magazine is likely to be your limiting factor on cartridge length. The same probably holds true on your Savage. However, each of them can be made to shoot very well though you won't be able to greatly extend seating depth beyond SAAMI standards.

TXbaldhunter · Oct 14, 2012

This is very good advise and it explains it very clearly.It works with not only Berger bullets,but all bullets.

Tech note from Berger on seating depth
The text below is from the Berger website on how they recommend finding the correct seating depth for their bullets

---------------------------------------------------------
The following has been verified by numerous shooters in many rifles using bullets of different calibers and weights. It is consistent for all VLD bullets. What has been discovered is that VLD bullets shoot best when loaded to a COAL that puts the bullet in a “sweet spot”. This sweet spot is a band .030 to .040 wide and is located anywhere between jamming the bullets into the lands and .150 jump off the lands.

Note: When discussing jam and jump I am referring to the distance from the area of the bearing surface that engages the rifling and the rifling itself. There are many products that allow you to measure these critical dimensions. Some are better than others. I won’t be going into the methods of measuring jam and jump. If you are not familiar with this aspect of reloading it is critically important that you understand this concept before you attempt this test.

Many reloaders feel (and I tend to agree) that meaningful COAL adjustments are .002 to .005. Every once in a while I might adjust the COAL by .010 but this seems like I am moving the bullet the length of a football field. The only way a shooter will be able to benefit from this situation is to let go of this opinion that more than .010 change is too much (me included).

Trying to find the COAL that puts you in the sweet spot by moving .002 to .010 will take so long the barrel may be worn out by the time you sort it out if you don’t give up first. Since the sweet spot is .030 to .040 wide we recommend that you conduct the following test to find your rifles VLD sweet spot.

Load 24 rounds at the following COAL if you are a target competition shooter who does not worry about jamming a bullet:
1. .010 into (touching) the lands (jam) 6 rounds
2. .040 off the lands (jump) 6 rounds
3. .080 off the lands (jump) 6 rounds
4. .120 off the lands (jump) 6 rounds

Load 24 rounds at the following COAL if you are a hunter (pulling a bullet out of the case with your rifling while in the field can be a hunt ending event which must be avoided) or a competition shooter who worries about pulling a bullet during a match:
1. .010 off the lands (jump) 6 rounds
2. .050 off the lands (jump) 6 rounds
3. .090 off the lands (jump) 6 rounds
4. .130 off the lands (jump) 6 rounds

Shoot 2 (separate) 3 shot groups in fair conditions to see how they group. The remarkable reality of this test is that one of these 4 COALs will outperform the other three by a considerable margin. Once you know which one of these 4 COAL shoots best then you can tweak the COAL +/- .002 or .005. Taking the time to set this test up will pay off when you find that your rifle is capable of shooting the VLD bullets very well (even at 100 yards).