Seating depth and accuracy

[BJung]

I'd like to share with you my experience and then thoughts about seating depth and charge weight.

Yesterday, I shot a string of test loads within the span of the load table and given OAL for my T99 Arisaka. The SAMMI specs for the maximum case length makes it such that a seated bullet cannot touch the lands.

My previous tests used these specs. The results were modest. I ran out of a powder and had to start over so fine-tuned finished results were not achieved. The idea that I could get the bullet ogive always bothered me. To have a reference point and have the bullet at a length longer than the magazine was always something I thought possible.

And so, I assembled better brass by purchasing Norma 30-06 brass and resizing/fireforming them to just under "chamber length" than SAMMI specs so a seated bullet could touch the lands. They did! And, the case wasn't jammed into the rifling so the bullet would release when the powder ignited and pushed the bullet forward.

My previous experience related to this now extended cartridge was testing .38 special wadcutter loads. One set used 2.8gr BE and was seated at the crimp groove. The second set was discovered to be as accurate as the first using 3.1gr BE but seated to the first grease groove. The groups were about the same but the extended bullet had a very slight lead. I was shooting these through my S&W Model 66.

My results are as follows. 7.7-174 loads using load tables totally missed an 18'x24" target at 100 yards. From previous tests with R15, R19, IMR 4350, I'd hit the paper. I am using AA4350. I moved my Point of Aim. Still missed. The primers are coming out. I looked this up online and the primer issue was due to too low of a pressure. I moved my target to 50 yards. I shot really low and this was the midrange load. Only with the maximum load did the bullet hit POA. And, the bullets printed 1" apart and just above the POA by maybe 2-3". So, like .38 wadcutter experiment, the performance was similar.

Another experiment was 7.7-150 Barnes seated to magazine length. I shot this load and of them, one load shoes potential to printed 1-1/2"

So, my question to you readers is this. I can increase the load to bring up the pressure. Could I find a more accurate load extended close to lands? Or, seat the bullet to fit the magazine because OAL doesn't matter but the charge only and then I might fine tune that accuracy load with slight seating depth changes.

[ammo.crafter]

What components are you using? Primers? What powders have you tried?

Sights?

How is the barrel? How is the stock? Cutting down brass to spec?

There are so many variables involved.


Sight in at 25 yards and see how it shoots.

There are many, many reloading sights you can research and I'm certain there are Arisaka sights as well.

[tangolima]

Do you intend to load the rounds in the magazine and shoot the rifle as a repeater? If you do, then you will have no choice but to stay below saami coal spec.

Chasing the lands is controversial. I stopped doing it a while back. Instead I use other methods to turn for better groups. Varying coal (below saami spec) is one, barrel tuner being the other. They all work. Whether they are as good as chasing the lands I would never know, as I don't chase lands anymore.

As poi versus poa, you can always adjust the sight or optics. The markings on the sight slider are just numbers. It is cool to make rounds shoot exactly on the marks, but it doesn't have to be. The war is over. The rifle is no longer an implement of warfare. It has become a tool. The owner should be able to use it whatever way he see fit.

I still have one T99. Bought from a friend whose father-in-law took it back from the Philippines where he fought. After divorce it was left in his ex's garage rusting way. His beloved son sold the bayonet to get money for video games. He final recovered it as part of his share of properties after a prolonged legal entanglement. I helped him clean up gun. Not exactly a gun guy, he sold the rifle to me. It wasn't a last-ditch but it doesn't have the bells and whistles as the early productions. No mum. Shoots normal. Better than 2" R50 10-shot at 100yd.

-TL

Sent from my SM-N960U using Tapatalk

[BJung]

I separate my loads as follows:
7.7-174 HPBT with resized Norma 30-06 cases. OAL for maximum accuracy
7.7-175 RB Hornady with LC brass. OAL under magazine length for follow ups.
7.7-150 SP Hornady Norma 30-06 brass. OAL to magazine length for follow ups
7.7-150 TTSX with 7.7x58 cases. OAL beyond magazine length if necessary and under
77-185 LRN Lee with resized LC 30-06. OAL to magazine length.

[Unclenick]

BJung,

Terminology can get under foot sometimes, and I be want to be sure I am following yours correctly. Chasing the lands is when you set up a load with the bullet a certain distance off the lands, and then change it's seating depth toward as throat erosion gradually moves the point at which the bullet touches it forward. It makes intuitive sense that if you've found some magic amount of bullet jump to the lands that you would monitor the throat and do this. The problem with it is that an eroded throat doesn't have the same profile as the original throat, so while chasing the lands may keep your jump constant for one part of the bullet ogive, it may not be keeping it constant for another part, and thus, according to Eric Cortina, chasing doesn't actually work very well to maintain accuracy.

A second reason this is falling into disfavor is that it has turned out a lot of guys trying to find the best bullet jump distance we're not actually succeeding in doing more than fooling themselves into embracing dimensions that were made to appear to be best by random luck. In other words, they were fooling themselves by using test shot sample sizes too small to reveal whether they were seeing something real or not. Often, they were not.

A third factor is that many top shots have gradually come to realize the actual best amount of bullet jump is much bigger than they previously supposed. Scott Satterly is currently using up to an eighth of an inch of bullet jump and winning. As an example from someone who was a top benchrest competitor, in the 1995 Precision Shooting Reloading Guide, the late Dan Hackett described being frustrated by a 40X he had chambered in 220 Swift. He could get it to group five shots into about half an inch on average, with his best groups being about three-eighths of an inch but also a few bad ones printing three-quarters of an inch. He seated his bullets for 0.020" of jump back then, as that was one of the "magic" accuracy jump numbers that were popular to believe in at the time. Then one day, while loading, he switched to a 50-grain Nosler Ballistic Tip from a bullet he didn't mention. He did say the Nosler BTs were 0.015" longer from the bearing surface shoulder to the seater's ogive contact point, so he adjusted his micrometer to back it out by 0.015" to maintain his 0.020" jump. However, he accidentally turned it the wrong way, thus adding 0.015" to the bullet's extra 0.015" and now seating bullets with a jump of 0.050". He had 20 rounds loaded before he noticed the error. He said he considered pulling the bullets and reseating them but decided just to shoot them in practice and be more careful which way he turned the seating adjustment next time. At the range, he was astonished when his "practice" loads produced two quarter-inch groups and two true bugholes in "the ones" (groups that are 0.1##).

Anyway, the bottom line is not to be over sure of what you think you "know" without adequate testing. The SAAMI max COL is just to guarantee magazine fit and feed. It has nothing to do with what optimizes accuracy as the wide variation in available bullet shapes makes attempting to established a fixed ideal number for all of them on par with getting believable Ouija board readings.

[BJung]

Do you have an opinion or answer to bullet seating depth and nodes.

From what I've read and observed, given the OAL in a load manual and the range one would load, there might be more than one node. Starting with a seating depth with the ogive touching the lands and then increasing seating depth, would a group tighten, open up, and then tighten as the bullet seating depth increases. All this within an observable pressure when the primer starts to crater or flatten, meaning the pressure is too much.

[tangolima]

There are multiple "nodes", which is the point Cortina was trying to make. One doesn't need to chase the lands to have similar results.

Increasing seating depth doesn't immediately increase chamber pressure. The peak pressure, and MV, would remain roughly constant, till bullet jump becomes excessive.

-TL

Sent from my SM-N960U using Tapatalk

[BJung]

Hi All. After your responses, I started catching up on my bullet casting and then got sick, so I haven't been at my reloading bench for a while. My wife is blocking the way.

Since Tangolima's last text, I scored on powder from a club member that is scaling down on his reloading supplies. Among the powders is AA2460 and AA2700. I am using AA4350 and my nephew is experimenting with AA4064 and AA2520. I started with R15 and R19 and they are the best but hardest/most expensive to get now. Then there is IMR 4350. So, far, it's okay. A reloader online going as RunFiveRun reported that AA2700 POA with the military sights comes closest to POI. My sights are fixed.

I've learned that the load data is based on a specific seating depth. I never paid attention to this and never noticed because with the ogive close to the lands and using the commercial load data, I always found a node. Another friend who looked for the highest pressure and then backed down to the highest node didn't make me think about seating depth. Only until I experimented with 38 wadcutter seating depths did seating depth caught my attention.

Now with the extended case and bullet in my Arisaka, I have excessively lower pressure and plan to carefully increase my charge with the same powder. Yet, the case capacity has increased so I will calculate the internal volume of this case to a 30-06? If so, then AA2700 which is WW760. WW760 was designed for the 30-06.  

[RC20]

I find it hard to believe you are spending time on an Arisaka chasing accuracy.

If you like that caliber area, a heavier barrel 30-06 would be the thing to buy and spend your time with. 308 would be the same. Not because a heavy barrel is more accurate, its just more stable. Remington made some factory or you can go the Savage/Rem route and get a custom barrel. Shilen makes good ones and others. I am fond of Precision Barrel as they have been very lenient in multiple bullets and powders. Far from the only two.

I am shooting 7.5 Swiss. I have a gun I built up that is accurate. I have a K31 that is not. 1.5 inch at best and more 2 inch. That is a worth while gun to try at least.

I have a 1911 Swiss that is more accurate than the K31. It will do 1 inch and a bit under at times. Day in day out its more 1.25 at best. They are both fun to shoot. They both have decent to good accuracy reports so worth playing with. Military guns are not going to be target guns.

You are not going to see any consistent results in an Ariska no matter what powder or what seating depth.

No disagreement if it tickles your fancy to work with it, but do it for fun not serious target shooting.

[Unclenick]

Originally Posted by BJung

Do you have an opinion or answer to bullet seating depth and nodes.

There's been at least one recent article in which the author doesn't believe they exist. He thinks they are all a matter of shooting groups with sample sizes too small that deceive the shooter into thinking he's got something he doesn't. I don't agree. Since barrel tuners work, it is at least the case that getting barrel bending or vibrating motions synchronize correctly with the bullet exit from the muzzle will tighten groups,

There are several ways of going about changing the timing of your bullet exits. The tuner is one example. Changing powder charge or powder type are two more examples. And changing seating depth will affect it, too. Dr. Lloyd Brownell theorized that this is because changing seating depth changes the amount of gas that bypasses the bullet and heads down the turbe ahead of it. Bypass causes a slight stall in the rise of pressure in the chamber while the bullet starts moving forward. This running start affects the rate of expansion in the powder-burning space behind the bullet, and that changes the peak pressure. This is how seating depth affects the timing of the bullet's exit from the barrel.