Will changing my seating depth throw off my numbers... a lot?

[Lefty Trigger]

I was always told to find a load you like that gives consistent groups and low ES / SD numbers. Well I found such a load then started playing with seating depths and it all went to shit! To start with its a load for my .308 rifle. The load I settled on is Win brass / CCI 200 primer / 39.5 gr of IMR4895 / Nosler 175gr Custom Comp bullet seated at factory length of 2.800. This will yield me 1/2 to 3/4 inch 10 shot groups at 100yds with really impressive numbers,.. velocity 2442 ES 7.7 SD 3.2. I decided to "close up" the gap and reduce the seating depth by .005 each time and keeping the load exactly the same. Nothing really happened except group size fell apart by opening up until I got to around .020 / .015 jump and the numbers went wonkers, the velocity actually went up a tad to 2464 but the ES went to 28.5 and the SD went to 12.4 with a 1 1/4 group. Looks like I need to go back to the 2.800 length.

 

[mikecr]

You never had a reason to choose 2.800 & load developed at that. Right?
And you gotta know that after load development, any change affects that as well.

For instance, say you want to try different primers after load development. This would affect whatever load you had worked to identify as best. Then things get better or worse, and you really learn nothing either way, because it's not a system. It's an abstract. Chances are, you'll go back to the primers you developed with.
But would you assume the chosen primer is best then? Many folks actually do..

You could have tested seating before powder development. This, using Berger's recommended testing.
You can still do it afterwards, by backing off on powder -clear of whatever powder node you've found.
This provides you with a really bad shooting gun for testing. The advantage in this is that the only thing improving it is best seating.
And with best seating determined you move on to powder testing.
With best seating, and best powder load, you move into fine tweaking of seating (within it's window) for best/tightest group shaping.

 

[mike06]

Start adding more powder .01 to .02 tenths at a time they should tighten back up.

 

[Dusty Stevens]

Do what works best on the target- unless youre in a chronograph match

 

[Lefty Trigger]

Do what works best on the target- unless youre in a chronograph match

Well this is a pickle then because I stopped when the numbers fell apart but it appeared that maybe if I kept going the groups might have started closing back up. Is that even possible? The group at .015 (where I quit) was better than the group at .020 and .025, .030

 

[Lefty Trigger]

Start adding more powder .01 to .02 tenths at a time they should tighten back up.

So do I continue on and find the best group then tailor the load back up to improve my numbers??

 

[Dusty Stevens]

Well this is a pickle then because I stopped when the numbers fell apart but it appeared that maybe if I kept going the groups might have started closing back up. Is that even possible? The group at .015 (where I quit) was better than the group at .020 and .025, .030

You can find a good tune even at .100+ off

 

[Lefty Trigger]

Personally I can't tell ES from BS
I only look at the target.
one thing Jumps out to me is the low speed you mentioned, I would be looking for 2600 FPS at least .

Why?? Both Nosler and Sierra have their accuracy loads well below 2600fps, Nosler at 2462fps and Sierra at 2400fps

 

[allen529]

One of the most accurate loads for me in 308 is 40.0 grains of varget. I dont know the velocity but this absolutely hammers in my rifle.

 

[Lefty Trigger]

One of the most accurate loads for me in 308 is 40.0 grains of varget. I dont know the velocity but this absolutely hammers in my rifle.

I have shot 43gr of Varget which is a National champion load with a 175gr bullet and it is hard on the rifle and me. Made up 10 of them and only shot 4 because the recoil was so harsh I ended up two targets to the right in the scope. Did not have my Labradar that day but would like to have known the data, can tell you that 43.7gr of IMR4064 netted 2765fps and it was harsh as well!

 

[711rollerdog]

All those good SD's and ES's are likely from good/consistent neck tension...

 

[Ned Ludd]

Upon reading this thread, several things come to mind, in no particular order:

1) The OP never mentioned barrel length. However, his velocities might still be reasonable for a 16"-18", maybe even 20" barrel. Otherwise, they're much too slow.

2) You mentioned both "closing up the gap", which usually means seating bullets longer to me, and reducing the seating depth. I'm a little confused as to what you actually did. Can you clarify exactly what you did to the seating depth in terms of starting/ending COAL, or starting/ending distance from the lands? At 2.800" COAL, exactly how far off the lands is a 175 CC bullet actually seated? In your testing, did you seat them farther out in the neck toward the lands, or seat them deeper down in the neck?

3) 43 gr Varget under a 175 is a fairly light load, even in a rifle that weighs in at less than 12-14 lb. If that feels like "harsh" recoil to you, you have a pretty critical issue with recoil management that needs to be solved; above and beyond any load development concerns. If a strap-on shoulder recoil pad or different buttstock pad doesn't help, I'd suggest that you may be wasting your time shooting a .308. Shooting 175s at low velocity is not really a recipe for success in a .308, anyhow. You could easily load up 80-90 gr bullets in a .223 Rem rifle and markedly surpass the performance of slow 175s, with a significant reduction in felt recoil. Something to consider, at any rate.

4) Changing seating depth, especially .005" at a time, which is a relatively coarse increment, should be anticipated to change group spread, perhaps significantly. Clearly, the seating depth where you originally had the 175s seated was working. If you want to find a new seating depth optimum, you may potentially need to test over a fairly wide range to find the next adjacent seating depth optimum. If you have to move the bullets more than about .020" to .030" in either direction, expect your velocity numbers to start to change as you increase/decrease the effective case volume and change pressure. It may also be necessary to optimize powder charge weight in order to fully re-tune the load at a different seating depth.

 

[Lefty Trigger]

Upon reading this thread, several things come to mind, in no particular order:

1) The OP never mentioned barrel length. However, his velocities might still be reasonable for a 16"-18", maybe even 20" barrel. Otherwise, they're much too slow.

2) You mentioned both "closing up the gap", which usually means seating bullets longer to me, and reducing the seating depth. I'm a little confused as to what you actually did. Can you clarify exactly what you did to the seating depth in terms of starting/ending COAL, or starting/ending distance from the lands? At 2.800" COAL, exactly how far off the lands is a 175 CC bullet actually seated? In your testing, did you seat them farther out in the neck toward the lands, or seat them deeper down in the neck?

3) 43 gr Varget under a 175 is a fairly light load, even in a rifle that weighs in at less than 12-14 lb. If that feels like "harsh" recoil to you, you have a pretty critical issue with recoil management that needs to be solved; above and beyond any load development concerns. If a strap-on shoulder recoil pad or different buttstock pad doesn't help, I'd suggest that you may be wasting your time shooting a .308. Shooting 175s at low velocity is not really a recipe for success in a .308, anyhow. You could easily load up 80-90 gr bullets in a .223 Rem rifle and markedly surpass the performance of slow 175s, with a significant reduction in felt recoil. Something to consider, at any rate.

4) Changing seating depth, especially .005" at a time, which is a relatively coarse increment, should be anticipated to change group spread, perhaps significantly. Clearly, the seating depth where you originally had the 175s seated was working. If you want to find a new seating depth optimum, you may potentially need to test over a fairly wide range to find the next adjacent seating depth optimum. If you have to move the bullets more than about .020" to .030" in either direction, expect your velocity numbers to start to change as you increase/decrease the effective case volume and change pressure. It may also be necessary to optimize powder charge weight in order to fully re-tune the load at a different seating depth.

Reading your post and some others above I now feel I'm working this all wrong. Looking into this a little farther I found that the "optimum" velocity for the bullet I'm shooting and my barrel length (24 inch) I need to rethink and crank it up a notch. I found that the Nosler 175gr Custom Comp likes to run at about 2610-2625 speed, I have some loads that will get me there but need to tune them to get the numbers down for improved consistency.
And yes "closing the gap" means moving the bullet forward towards the lands. My gun at COAL of 2.800 is a .042 jump with this bullet. I was moving out .005 at a time to see if the groups would hold then tune the depth in smaller amounts. My 6.5 Creed really likes one bullet at a weird length of .017 off and will shoot a large bug hole at 100yds with that depth, so I change nothing for those loads in that gun.

 

[Uthink Uknow]

Is there a reason why you can't or don't go back to where you started?

 

[Laurie]

But not near what the Varget is, RL-15 is worse.
If I were looking for a 308 175gr load it would most likely be 4064 the recoil feels more of a push than jolt.

An old reloaded told me that my rifle didn't read that book, ya gotta test.

I shot the 175gn SMK in year one of FTR - like a lot of other foolish people who believed the BS written about long-range 308 performance before Bryan Litz started to publish external ballistics books and produce bullet by bullet G7 data to let us know what really happens out there beyond 600 yards. IMR-4064 was the outstanding performer for me too in conventional large primer Lapua brass with this bullet weight. As a short-distance number it was an outstanding combination.

 

[Laurie]

On the original thread question about effects of COAL on ES/SD, quite coincidentally I was out testing just this metric on the 100 yard benchrest range in ideal test conditions yesterday.

The cartridge and load were 284 Win with the very long, high BC 183gn Sierra MK, a really aggressive VLD bullet form with a Litz measured Rt/R ratio of 0.37. (1.0 = true tangent ogive; 0.5 = normal secant ogive VLD form; lower than 0.5 = trouble usually.)

I'd done an initial try last year with rising Viht N165 charges and the bullets around 15-20 thou' 'in' (as per traditional VLD practice), but every group had excessive vertical, some straight vertical stringing. Reports on the AS forum subsequently suggested they like some jump, recommended values ranging from 5 thou' to large figures of 30 or more.

So I went back yesterday with the best charge weight of the original bunch, three loaded to my original 'in' COAL as a control, and five each at 20, 30, 40, 60 thou' off that original value, all settings by comparator readings. One N165 charge throughout of 53.8gn (Lapua brass / Murom KVB-7 primer / 30-inch Bartlein 1:8).

The 3 x 'ins' actually did OK, but only a 3-round group; minus 20 0.9" vertically strung; minus 30 0.37" nice shape; minus 40 0.65" with excessive vertical; minus 60 0.27" and a nice shape and I'll try that one further.) The first reduction of minus 20 from the original setting sometimes produced just the faintest feel of bolt closure resistance, so I believe the bullet was barely kissing the lands at this setting.

So far, no surprises as this was what I'd expected / hoped would happen. I was expecting possible POI and MV changes though especially from a load with the bullet 15-20 thou' in the lands to the final one likely 30-40 'out'. Here are the MVs:

'In' ....................... 2,738
Barely kissing ........ 2,741
5-10 out .............. 2,740
15-20 out ............ 2,738
35-40 out ............ 2,728

So, a 10 fps drop with the shortest COAL, inconsequential, the rest remarkably consistent. ES/SDs were consistent and acceptable for all five batches bar the 'barely kissing' quintet with 17 ES / 6.3 SD, the others with 9-12 ES values.

POI elevation was entirely unchanged throughout and its lateral position likewise bar the control 'in-trio' whose group centre was apparently just a shade (under 1/10 inch) left of the other four, but with varying group shapes that was possibly misleading.

So .......... the conventional wisdom that the bullet in the lands has a big effect on peak chamber pressure / MV and COAL changes affect both MV and POI? In this particular cartridge / chamber / bullet apparently not! (But .... I have seen these effects with other cartridges, albeit I believe that for many high-BC bullet shapes being 10 or 15 or so thou' in has little if any effect on pressure / MV, however a real 'jam' position is a very different matter. Likewise, traditional blunt 7-calibre ogive form tangent bullets sometimes produce noticeable pressure increases from being seated 'in'.)

 

[Laurie]

I should add to the previous post that MVs / ESs / SDs were from a Labradar - the second chronograph only out of the many I've had the misfortune to own over the years that I would trust as being accurate / consistent enough to produce meaningful figures in this kind of test. (The other model being the MagnetoSpeed V3.)

 

[Ned Ludd]

Laurie - I recently tested some Juggernauts in a .308 load at various distances off the lands, touching, and into the lands. As compared to jumped bullets, all loads seated at touching or into the lands exhibited a consistent increase in velocity of 15-17 fps (also measured with a LabRadar). Not a huge increase in velocity/pressure, but detectable nonetheless. Perhaps the much larger ogive radius and less abrupt ogive to bearing surface transition of the 183 SMK might to some degree mitigate the pressure effects?

Lefty - I think you will find over the long run that getting a little more velocity out of your load will be an improvement, but you will likely need to re-tune it from scratch. I would suggest using Varget or H4895 as a powder. IMR4895 is a little on the fast side for 175s, even faster than H4895, and somewhat less temperature-stable. Because it sounds as though you aren't limited to loading to mag length for this endeavor, I'd start the charge weight testing at about .015" or .020" off the lands. Once you have found the desired charge weight window, you can go back and optimize the seating depth window as you originally described in this thread. Based on the measurements you provided, starting them at .015" to .020" off the lands should be close to about halfway between where they used to be seated and touching the lands. You should be able to cover that region in a seating depth test with only minimal effects on velocity due to changing the effective (usable) case volume.

 

[Laurie]

Laurie - I recently tested some Juggernauts in a .308 load at various distances off the lands, touching, and into the lands. As compared to jumped bullets, all loads seated at touching or into the lands exhibited a consistent increase in velocity of 15-17 fps (also measured with a LabRadar). Not a huge increase in velocity/pressure, but detectable nonetheless. Perhaps the much larger ogive radius and less abrupt ogive to bearing surface transition of the 183 SMK might to some degree mitigate the pressure effects?

That's my take on it too Ned. It looks like the longer the bullet nose and gentler its curvature as seen in terms of calibres radius, the less pressure change it generates in terms of being in or out of the lands.

Even your 15-17 fps Juggernaut increase is fairly modest in pressure terms whilst if one applies the QuicklOAD 'shot-start' pressure guidelines, there is a much larger computed Pmax increase. (To get 15-17 fps out of a 'jumped Juggernaut' with VarGet in a 30-inch barrel, QL says FWIW that it's the equivalent of a 0.3gn powder increase producing another 1,240 psi PMax. Not to be ignored if working on the pressure limit, but not that great. Conversely, increasing the 'shot start pressure' by the amount QL recommends for a bullet seated 'in' sees a more than 9,000 psi PMax increase in this hypothetical load. Maybe so for a true jammed bullet of course!)

I've got to say I was really surprised by the nil MV change results I got though, also if to a lesser extent with the consistent POI on the paper.

I've never used Berger's recommendations on VLD COALs - ie start 'in', then change to jumping them by increasing amounts in large steps. I have a fair few 168gn 7mm Berger VLDs that won't shoot for toffee seated 'in' in my 7-08 long-freebore F/O rifle, and I mean to try Berger's recommendations exactly next year to see if I can rectify this. It'll be interesting to see how / if MVs change here too. (I've also invested in a couple of hundred 162gn Hornady ELD-Ms for this rifle despite the 162 Amax never having done much in it. Bryan Litz confirms the ELD's significant ballistic improvement over the AMax to an impressive 0.877 G7 form factor / 0.327 average BC from barely noticeable shape changes, but it is a true VLD with a 17.29 calibre radius secant nose form and 0.50 Rt/R value.)

 

[Ned Ludd]

That's my take on it too Ned. It looks like the longer the bullet nose and gentler its curvature as seen in terms of calibres radius, the less pressure change it generates in terms of being in or out of the lands.

Even your 15-17 fps Juggernaut increase is fairly modest in pressure terms whilst if one applies the QuicklOAD 'shot-start' pressure guidelines, there is a much larger computed Pmax increase. (To get 15-17 fps out of a 'jumped Juggernaut' with VarGet in a 30-inch barrel, QL says FWIW that it's the equivalent of a 0.3gn powder increase producing another 1,240 psi PMax. Not to be ignored if working on the pressure limit, but not that great. Conversely, increasing the 'shot start pressure' by the amount QL recommends for a bullet seated 'in' sees a more than 9,000 psi PMax increase in this hypothetical load. Maybe so for a true jammed bullet of course!)

I've got to say I was really surprised by the nil MV change results I got though, also if to a lesser extent with the consistent POI on the paper.

I've never used Berger's recommendations on VLD COALs - ie start 'in', then change to jumping them by increasing amounts in large steps. I have a fair few 168gn 7mm Berger VLDs that won't shoot for toffee seated 'in' in my 7-08 long-freebore F/O rifle, and I mean to try Berger's recommendations exactly next year to see if I can rectify this. It'll be interesting to see how / if MVs change here too. (I've also invested in a couple of hundred 162gn Hornady ELD-Ms for this rifle despite the 162 Amax never having done much in it. Bryan Litz confirms the ELD's significant ballistic improvement over the AMax to an impressive 0.877 G7 form factor / 0.327 average BC from barely noticeable shape changes, but it is a true VLD with a 17.29 calibre radius secant nose form and 0.50 Rt/R value.)

I also was surprised by the very modest increase in pressure/velocity. I did adjust the Start Pressure setting in QuickLoad to account for the velocity increase and found the increase necessary was also modest. The Jugs that exhibited this increase were seated at touching, .003", .006", and .009" into the lands. I suspect a "hard jam" of .020" to .025" into the lands might have produced the more significant pressure spike and change in velocity I had been expecting based on existing anecdotal information. However, I had no reason to seat them that far into the lands and so did not test it.