Accuracy=seating depth or tenths of powder

[ColoYooper]

I always verify the group performance at (10) different seating depths from 0.010 into the lands to 0.050 off the lands, using the Ladder test at (10) different charges that vary by 0.2 grains, with (3) possible bullet weights, from Sierra, A-Max, and Lapua, using RWS, Lapua and Norma brass, using both Federal magnum and non-magnum match primers, and setting 3 different neck tensions. Unfortunately just as I get 1/4 the way through my testing I always have to restart the above tests as my barrel is shot out.

 

[barefooter56]

Excuse this question if I should already know, but what cartridge were you using when experiencing the throat erosion rates you describe? Trying to put some context to those throat erosion rates in comparison to the cartridges and powder charges I use for LRH. Thnks

.308. Not an overbore cartridge I know and a cartridge that is not known to be hard on throats.

 

[barefooter56]

Cant remember if the load was 45.7 or 47.5 grains. Sold the rifle and included the Loading log book with it.

 

[Pdvdh]

I always verify the group performance at (10) different seating depths from 0.010 into the lands to 0.050 off the lands, using the Ladder test at (10) different charges that vary by 0.2 grains, with (3) possible bullet weights, from Sierra, A-Max, and Lapua, using RWS, Lapua and Norma brass, using both Federal magnum and non-magnum match primers, and setting 3 different neck tensions. Unfortunately just as I get 1/4 the way through my testing I always have to restart the above tests as my barrel is shot out.

Good one! Too funny...

I understand. I spend less time refining load development now than ever. I try to develop a load accurate enough for my use with my bullet of choice, and then try to ensure the ES/SD is reasonably low. Proof it at distance and go hunting.

 

[Pdvdh]

.308. Not an overbore cartridge I know and a cartridge that is not known to be hard on throats.

Thanks. Maybe Bart will sound in again and shed more light in a throat erosion rate discussion.

 

[Mikecr]

I know if your best seated CBTO was determined to be off the lands(OTL), throat erosion won't make a bit of difference. Best seated CBTO remains.
I have no idea about relying on pressure provided by seating into lands(ITL), and affects of throat erosion there, as I've yet to find that as best seating.

 

[Pdvdh]

I know if your best seated CBTO was determined to be off the lands(OTL), throat erosion won't make a bit of difference. Best seated CBTO remains.
I have no idea about relying on pressure provided by seating into lands(ITL), and affects of throat erosion there, as I've yet to find that as best seating.

CBTO = cartridge base to ogive dimension/length?

 

[Mikecr]

Yes on the CBTO.

 

[Bart B]

I've tracked throat erosion on several .308 Win barrels over the years. They all eroded about .001" for every 45 or so rounds. Barrel life for acceptable accuracy was about 3000 rounds at which time the throat had moved forward at bullet contact diameter about .070" to .080" This is with IMR's 4895, 4320, 4350 and Varget. 20% of all ammo was reduced charges for short range use, the rest were max loads but at or close to SAAMI spec.

Four 30-338 Win Mag barrels lasting about 1200 rounds of acceptable barrel life had their throats advance that same .070" to .080" for that round count, That calculates to about .001" for every 17 shots. Powder was all IMR 4350 with max loads.

One .264 Win Mag match barrel lasted 640 rounds before going from a 3/4 MOA rifle at 600 yards to almost a 3 MOA rifle over 5 shots. Its throat had advanced .090" over that round count which translates to about 7 rounds per .001" of throat erosion. H870 powder with max loads was used.

Rifling leade angles on all were about 1.5 degrees. Lesser angles will have fewer rounds per thousandth; greater angles will have more. Powders with a higher heat index will erode the bore faster. I think the metal erodes away at about the same rate regardless of the angle for a given powder.

For a comparison, the US military 30-06 and 7.62 NATO throat erosion gauges taper .010" over their 1.000" gauging length with a 2.9 degree gauging surface taper. Diameters of .3100" at the large end and .3000" at the small end. For every thousand rounds of barrel life, the gauge goes 1/10th inch further in that when new. At around 10,000 rounds of barrel life, the gauge would read '10' and be about an inch further in than when the barrel was new. Match grade arsenal barrels were typically replaced when the gauge read '5' after 5,000 or so rounds, regular service ones at about 10,000 rounds when the gauge read '10.'

My accuracy limit has been when whatever the barrel had when new had increased to 50% larger test groups. New 30 caiber Hart, Obermeyer and Kreiger barrels shot Sierra's and Lapua's inside 3" to near 4" at 600 and about 6" to 7" at 1000. That .264 Win Mag wasn't quite that good with the best bullet available, Norma's 139-gr. FMJBT nickel plated one; no really good 26 caliber match bullets were available in the late '60's.

Seating Sierra's into the lands typically shot them the most accurate with max loads. Seating bullets for magazine loading needed to be shorter and shot with almost the same accuracy; but whatever it was stayed the same for the life of the barrel with both reduced and max loads.

 

[Edd]

One .264 Win Mag match barrel lasted 640 rounds before going from a 3/4 MOA rifle at 600 yards to almost a 3 MOA rifle over 5 shots. Its throat had advanced .090" over that round count which translates to about 7 rounds per .001" of throat erosion. H870 powder with max loads was used.

Is the .090" the only thing that caused the change in accuracy or was there other changes in the condition of the throat that contributed?

 

[Pdvdh]

I know if your best seated CBTO was determined to be off the lands(OTL), throat erosion won't make a bit of difference. Best seated CBTO remains.
I have no idea about relying on pressure provided by seating into lands(ITL), and affects of throat erosion there, as I've yet to find that as best seating.

Well I was hoping someone else would take the bait, but since I see no takers, do you have a theory on the causitive factor(s) for bullet seating depth influencing accuracy, independent of the ogive to lands distance? Your conclusion surprises me because I know that distance to the lands can be used to vary MV just as a slight change in powder charge can vary MV. So I presumed that a significant change in bullet ogive distance off the lands would affect accuracy as the throat lengthens with increasing fired rounds, and OTL distance increases.

Do you suspect the distance the bullet is seated into the case is a determinant factor in prime accuracy, somehow creating a specific (ideal) burn rate and pressure curve?

Somethine else? Magic?

 

[Mikecr]

Seating is magic to me for sure.
From what I've seen, and consistently, seating is the largest single reloading adjustment to accuracy (far more than powder).

Interesting(to me) testing results led me to 8thou OTL for best initial CBTO with a 6.5wssm Imp/139Laps. I could shape grouping with +/- 3thou, but beyond that window grouping flat out opens from 1/4-3/8 to 3/8-1/2moa at various distances.
This is interesting to me because it's the sharpest seating window I've run across.

This is not a problem, my seating with qualified ogives/inline dies is perfect and every round is always verified. I remained at this setting until seeing step change degraded performance at ~1750 rounds logged. My basis here is inline with Bart's descriptions and defining of barrel life.

At that point I re-ran my version of Berger's seating testing, and as I've found with other guns, best CBTO still held as initially determined(only now it was ~35thou OTL). The actual land relationship was way harder to lock in on by that point, but it don't matter while I'm OTL. Only best CBTO matters. The 'window' now seemed wider but did me no good as the barrel was not solid at 3/8 or better anymore, no matter what I tried. That's just what going past accurate barrel life means -when loading OTL. See, there is a price for everything. I didn't have to chase lands, but even a barrel setback won't bail me out in the end.

This gun(6.5wssm Imp) is purely a testing system. It's 16.5lb, BAT/Shethane, Border barrels, meeting Williamsport/IBS 1kyd rules. I had predicted ~1800 rounds barrel life, and had 2 barrels finished in a set. So after learning what I could about a barrel's dying, I spun it's sister barrel on, and amazingly every attribute & dimension matched perfectly. Didn't change a thing(still initial CBTO, formed cases & load) and grouping returned to 1/4moa. The brass fired in the new barrel didn't change a bit, MV didn't change. I could have swapped those barrels in the middle of a 10sht string with no detriment to the results. I have 1280 rounds on the 2nd barrel now.

I've challenged myself to understand exactly how & what seating does, and I do not know. For best OTL I'm thinking results are more to do with bullet release timing affect to peak pressure.
For best ITL, I think it's about going to another peak pressure level (a pressure node), and underbore cartridges benefit predictably here. But most hunting cartridges are not competitive underbores, so full seating testing with hunting cartridges likely leads to OTL as best. That's what I find anyway.

Also, anyone who's calibrated something(anything) knows you adjust coarse first, fine last.
Here, barring primer/striking issues, and annealing issues, it's seating first, followed by powder.
You can adjust powder to single kernels,, it's by far the finest of adjustments.
The only time it makes sense to adjust powder first is when seating is a predetermined factor.
While that's known in some conditions, most of the time(especially with hunting capacity cartridges) nothing is known until testing proves it.

What causes a barrel to go past accuracy peak, forever? Well, the ultimate killer is carbon constriction(not erosion). Carbon impinging into the metal surface profile. That's a barrel's cancer.

 

[Bart B]

Is the .090" the only thing that caused the change in accuracy or was there other changes in the condition of the throat that contributed?

I think the increased rough surface of the lands and grooves contributed to the accuracy loss. The slight change in pressure curve shape may also be a factor. Here's why.

Sierra Bullets' ballistic tech's testing their bullets for accuracy in rail guns record each 10-shot test group for size. Over the life of the barrels, the average group size increases at a fairly constant rate. When the average group size is 50% more than when new, the barrel's replaced. For example, the .308 Win chambered test barrels shooting bullets up through 180 grains last about 3000 rounds, then get replaced. They start out in the 1/4 MOA or so range with match bullets and reach the 3/8 MOA average point that many rounds later.

As far as I know, Sierra keeps the same OAL on all their test rounds fired in virtual SAAMI spec chambers. They don't prep cases and full length size them in Redding dies now (I think they used RCBS or Lyman ones years ago). The tech grabs 10 bullets as they come out of the pointing die (at about 90 per minute) forming the ogive on the cored jacket, then seat them in charged and primed cases and shoot them in their rail gun. They do this several times during a production run. If a run of bullets is extremely accurate, they keep some of those as "standards" to qualify test barrels; some standards shot in the 1/10th MOA range and they used to sell them at bigger rifle matches in plain brown boxes of 1000 with the sizing lube (lanolin) still on them and not polished bright and shiny like retail ones in green boxes.

Some time ago, I think someone from Black Hills posted a thread in this forum about a test of one lot their .308 Win match ammo. They fired hundreds of 10-shot test groups at 100 yards from the same rifle. Groups started out averaging 1/4 MOA or better but slowly opened up to about 3/8 at about 3000 rounds, then 1/2 at 5000 and at 10,000 rounds up to almost 1 MOA or something like that. I've searched a couple of times to find it but nothing uncovered.

One interesting thing about cartridge OAL in 7.62 NATO M1 and M14/M1A rifles used in matches. Some lots of M118 and M852 match ammo had enough muzzle velocity spread that a 1 MOA vertical shot stringing happened at long ranges. Some people brought their Lyman 310 nutcracker to the match then seated those bullets .010" deeper breaking the asphaltum seal between bullet and neck. They shot much more accurate with that much more jump to the lands as well as much less release force needed to push them out of the case neck. That modified ammo, as well as good lots of it, shot about the same accuracy level for the 3000 to 4000 round lives of the barrels. The bullet jump distance had increased about .080" or more in the chrome moly arsenal match barrel's I wore out and gauged for throat erosion.

 

[Pdvdh]

Seating is magic to me for sure.
From what I've seen, and consistently, seating is the largest single reloading adjustment to accuracy (far more than powder).

Thanks for responding and sharing your thoughts and experiences Mike. It was an interesting read. lightbulb