Optimising powder load and CBTO

[SGK]

Howdy. I'm a newbie and I'm puzzled as to how best to optimise these variables when they both affect pressure. I understand that I should do incremental powder load testing, working up from 5-10% off published maximum load data. I also understand that I need to optimise CBTO length as well. Yet increasing CBTO leads to increasing pressure and so to achieve the same pressure with an increase in CBTO requires less powder. How then to optimise these two variables?

I will be loading .308 Win with 168gr Hornady ELD-Match bullets. I have 'measured', using a loosely seated bullet, callipers and Forster Datum Dial kit, my maximum CBTO for this bullet in my chamber at 2.258". I have not chosen a powder just yet but was planning to select from those listed in the Hornady handbook (recommendations?) and work up from 5% below the maximum listed. I understand that for very low drag bullets I should work up from a CBTO of about 40 thou below my estimated maximum.

Should I incremental test powder load first then back off one increment and walk up CBTO or is there a better way? Guidance much appreciated.

Steve

Post corrected to remove the X from ELD-Match

 

[retired]

buy a tool so you can MEASURE seated bullet ogive length.
if not mag fed start at the lands and work up a load.
increasing length only has the potential for higher pressure
if you started just below the lands and move into them, or at the lands and start jamming.
typically increasing length increase combustion chamber volume and decreases pressure.
consider starting with a known published load and adjust powder/length from there.

 

[Texas10]

Most of the opinions I've read here suggests that seating depth is most important. So I start my load development using a charge that is about mid range of the loading chart, and seat bullets in .005 inch increments from .020 into the lands to .030 off. Find the best of those, and then start the charge development portion of the work up, from min to max in .5 grain (for small charges) to 1 grain for larger charges (like you're using). Narrow down your results and then use .1 grain increments. And then go over seating depth again in .002 increments. Then change primers to fine tune again.

Providing the gun has shown an affinity towards the bullet you've chosen, and of course the powder too , you should hit a winner somewhere along that line of testing.

Be a little cautious when near maximum loads. Note the outside temperature of each range session, and inspect your brass always, every session. You'd be surprised how much a .005 reduction in CBTO and thus less case volume, can make in pressure when you're close to the limits.

 

[Keith Glasscock]

I might be silly, but let's start with the basics.

Pick a powder: Varget works good in 308. There are several others as well, but Varget is very popular (more on the why later).

Find a starting load: You will have to determine what your starting load is based on several factors. The first to consider is which brass you are using. As an example, Winchester brass can take a bit more powder than Federal. Important point: Use load data that uses the same components you use if at all possible.

Work the powder charge up: You really do have to learn what YOUR rifle does pressure-wise with loadings. Start at a safe starting load and work up slowly! If you see pressure signs STOP! You can not safely work in the zone above the point at which you have pressure signs.

Find a node: This is going to seem dumb, but this is done in three sections.

First, work the powder charge. Remember where I pointed out popular components? This is why. Your rifle is an individual and is very special, but it will likely shoot a load quite similar to what others are having success with. At very least, it will probably shoot well at the same velocity others are using (if the barrel lengths are the same).

Second, work the seating depth. Move in small increments (I use .003") toward and away from the lands. The rifle will tell you when you are close. Take advantage of the knowledge and experience of others to determine where to start. I've never shot the ELD-X, so I can't help there.

Third, verify the load. If you are really looking for competition accuracy, a load verification is needed. I do mine by tolerance testing (i.e. move the powder charge up and down .2 grains at a time until the load doesn't shoot well etc.).

More then anything, take advantage of the immense amount of knowledge available here and elsewhere to make your loading and shooting safe and enjoyable.

 

[SGK]

Gotta love it when two consecutive answers emphasise the other of two variables. If anyone has experience with the ELD-X Match regarding seating depth I'd appreciate it.

I'm using PPU brass and so finding loading data with this brass will likely be difficult but - perhaps wrongly - I'd expect variance in volume to 'fit' within the variance of an incremental load test. Maybe I should start 10% down to make sure.

increasing length only has the potential for higher pressure
if you started just below the lands and move into them, or at the lands and start jamming.
typically increasing length increase combustion chamber volume and decreases pressure.

Thanks. I did not realise this subtlety.

I found this by Berger:

http://www.bergerbullets.com/getting-the-best-precision-and-accuracy-from-vld-bullets-in-your-rifle/

Seems they think the optimal band is (a) rather wide and anywhere from about 130 thou off to 10 thou into the lands. I am concerned about how much pressure could vary over this range if I have settled on a powder load using a CBTO somewhere in the middle to start with.

On Varget, I'd love to hear more on why it is good/popular. I understand 168gr bullets aren't particularly heavy for this calibre and the 308 isn't the greatest ballistically. But being 'mid-range' means a lot of powders 'work'. Hornady mention IMR 4064 and VARGET worked best in their load testing and I had planned to pick from those two for no other reason than it seemed a decent place to start. (I plan to load for 223 Rem also. Searching for best powder for that calibre produces a bewildering array of responses for a newbie.)

 

[retired]

you should invest in some quality brass. having a good bullet in average brass is going to yield average results. my opinion is your rifle will benefit from quality brass.

 

[SGK]

Hi. I have purchased 100 of 308 Lapua brass, but wanted to cut my teeth on all this with my large stock of PPU brass (several hundred rounds). I also have a bunch of RWS brass from hunting rounds (1x, 2x and 3x fired now). I noticed the neck wall thickness of the 3F RWS brass is considerably thinner than the PPU brass and so I've largely resigned to disposing of this.

(All my current stock of 223 brass is branded HPS-TR from HPS Target Rifles in the UK. I'm not sure where HPS sourced this brass from. The rounds they loaded shot very well in my rifle however. They used N530 powder but I can't find any published load data for this and either the Sierra Match King bullets they used or the Hornady ELD-X Match I wanted to try and so I will have to work with another powder. The Vihtavuori powders also seem expensive here versus the competition.)

I could also do with finding a local (south Miami) supplier of reloading products. Midway are charging a bomb to ship powder and primers.

 

[Wjesswheel]

Howdy. I'm a newbie and I'm puzzled as to how best to optimise these variables when they both affect pressure. I understand that I should do incremental powder load testing, working up from 5-10% off published maximum load data. I also understand that I need to optimise CBTO length as well. Yet increasing CBTO leads to increasing pressure and so to achieve the same pressure with an increase in CBTO requires less powder. How then to optimise these two variables?

I will be loading .308 Win with 168gr Hornady ELD-X Match bullets. I have 'measured', using a loosely seated bullet, callipers and Forster Datum Dial kit, my maximum CBTO for this bullet in my chamber at 2.258". I have not chosen a powder just yet but was planning to select from those listed in the Hornady handbook (recommendations?) and work up from 5% below the maximum listed. I understand that for very low drag bullets I should work up from a CBTO of about 40 thou below my estimated maximum.

Should I incremental test powder load first then back off one increment and walk up CBTO or is there a better way? Guidance much appreciated.

Steve

I shoot the 178 grain eld-x’s, would be interested in trying the. 168’s but can’t seem to find them on hornadys site. Where did you find them or do you have a item number so I can locate them.

 

[SGK]

My apologies. I meant ELD-Match 168gr. (The ELD X is a hunting round and the nearest weight is 178gr as you mention.)

 

[divingin]

On Varget, I'd love to hear more on why it is good/popular. [snip] Hornady mention IMR 4064 and VARGET worked best in their load testing and I had planned to pick from those two for no other reason than it seemed a decent place to start. (I plan to load for 223 Rem also. Searching for best powder for that calibre produces a bewildering array of responses for a newbie.)

I think those two are popular because a) they work well, and b) they tend to be pretty available. I've had very good luck with both 4064 and Varget in 308. Another you might try is 8208XBR: it works well in 308 and a buddy swears by it for his 223 loads (I use CFE223 for my 223 loads, but use that caliber mostly for plinking rather than precision stuff.)

I've got a couple of 308 loads for VV N-140 (also works well), but as you mentioned, VV powders tend to be pricey, and harder to come by compared to the Hodgdon/IMR offerings.

 

[mike06]

I think those two are popular because a) they work well, and b) they tend to be pretty available. I've had very good luck with both 4064 and Varget in 308. Another you might try is 8208XBR: it works well in 308 and a buddy swears by it for his 223 loads (I use CFE223 for my 223 loads, but use that caliber mostly for plinking rather than precision stuff.)

I've got a couple of 308 loads for VV N-140 (also works well), but as you mentioned, VV powders tend to be pricey, and harder to come by compared to the Hodgdon/IMR offerings.

Just got 8lbs of n140 working with 175 Nosler CC and RDF intested in your N140 results Thanks Mike

 

[Zero333]

Bottom line... Find the Optimal Charge Weight with bullets seated 20 thou away from the lands.

Then once you have a verified optimal charge weight, play with the seating depth towards and away from the lands in 0.005" increments (many on this forum prefer 0.003" increments).

If accuracy is not that good, then it's time to try a different powder or primer.

I recommend VARGET, H-4895 & imr-4064, or if you prefer a ball powder Win-748 is great.

 

[divingin]

Just got 8lbs of n140 working with 175 Nosler CC and RDF intested in your N140 results Thanks Mike

Hi, Mike;

I'd have to look those up (assuming I actually wrote them down.) GRT shows 42.7 right at max, and other nodes at 40.8, 39.7, and 39.1 (if you believe their model - I've found it to be pretty close for the most part.) That puts the load range right alongside Varget (max is about 43.8 grains.)

Revisiting this thread (due to others picking it up), there is one other thing to add: VV powders are now on the cheaper end rather than the more expensive. Not that they've changed their pricing, more that everyone else went up.