longer coal and effect on case pressure?

[dadajack]

I jam all the time! On the way to work, on the way home, at the house, in the shower, at work... always jammin! I never stick it in the grooves though... never.

[Zephhyr]

Just thought I would throw some more data to consider.
IN MY RIFLE, with RL16 42.3gr, Hornady brass, S&B primers, and 147 ELDMs.
I hit the lands at a coal of ~2.909, max magazine is 2.880.
2.800 = 2682fps, SD 6.0, ES 19
2.830 = 2688fps, SD 4.1, ES 16
2.850 = 2696fps, SD 4.2, ES 13
2.860 = 2691fps, SD 5.9, ES 20
2.870 = 2708fps, SD 7.6, ES 31
2.890 = 2721fps, SD 7.2, ES 24 
The 2.860 proved to be the most consistently accurate coal. They were all 5 shot groups and I was rather surprised by the increase in velocity, as I was going on the assumption (you know what they say about that) that a longer oal would decrease the initial pressure spike (as long as you we not jammed into the lands of course) and thus the velocity.

[dnellans]

Just thought I would throw some more data to consider.
IN MY RIFLE, with RL16 42.3gr, Hornady brass, S&B primers, and 147 ELDMs.
I hit the lands at a coal of ~2.909, max magazine is 2.880.
2.800 = 2682fps, SD 6.0, ES 19
2.830 = 2688fps, SD 4.1, ES 16
2.850 = 2696fps, SD 4.2, ES 13
2.860 = 2691fps, SD 5.9, ES 20
2.870 = 2708fps, SD 7.6, ES 31
2.890 = 2721fps, SD 7.2, ES 24 
The 2.860 proved to be the most consistently accurate coal. They were all 5 shot groups and I was rather surprised by the increase in velocity, as I was going on the assumption (you know what they say about that) that a longer oal would decrease the initial pressure spike (as long as you we not jammed into the lands of course) and thus the velocity.

huh - now thats pretty interesting.  thanks for putting some data to it!  how long is your barrel?  did you shoot them in that order going longer?  could speed increase be attributed to change in barrel temp or having started with a clean barrel getting progressively more fouled?

at 43.8gr i'm running them quite a bit hotter than you are (2785-2805 depending on temp) mostly because i couldn't find anything i thought was really good in the 42.x range. next load down that shot well for me was down between 2600 and 2625 which is actually slower than factory loads.  maybe i wasn't looking hard enough in the 42.xgr range but since 43.8 seems to be working ok without issues even at 100F out, i'll stick with it for now.

[Blinkster]

Only time I jam is when I'm fireforming Dasher blue box brass. My match load for said same is .018 off the lands. Consistent laser with Berger 105 Hybrids.

My 6mm BR prefers that everything be jammed ~.010"...105's or 107's all like to be jammed.

[Danbonzo]

My 6mm BR prefers that everything be jammed ~.010"...105's or 107's all like to be jammed.

Yup I'd believe it, some of the best groups I've ever shot were from a .308 and I was about .003" off the lands.

[dnellans]

Was able to seat some bullets last night while watching the Cubs eek out a glorious victory!

Same cases/bullets/charges/primers everything, only difference was seating depth (2.81 versus 2.91).  I combined this with an experiment of using imperial dry lube (graphite) on the necks to see if that would help improve seating dept consistency and accuracy and it definitely did.

20 shots at each COAL,  1 minute rest in between shots,  even warmed the barrel up with 10 shots before starting.  Much to my surprise, and similar to Zephyr's results above the 2.91 COAL did not slow down, in fact it very clearly sped up.

20 shots at 2.81 - avg 2790.15, SD 9.81, ES 36
20 shots at 2.91 - avg 2801.75, SD 15.67, ES 46

The SD on the 2.91 went up and the graph is interesting to me. There is a pretty smooth trend of shots across the spread with the 2.81 seated loads.  the 2.91's have a clear bi-modal thing going on where about half are between 2780-2795 and then there is a big jump and they are 2810-2820. 

(the plot below I sorted the data, there was no trend of the faster shots being at the end of the strings, it would alternate from 2780->2815->2785 without any rhyme or reason)

So the theory that they should slow down due to less cartridge pressure appears to be busted, and clearly I shouldn't add any more powder,  cases looked identical between the two COALs, no more or less signs of pressure on either one, but this jumping of speed from 2780 up to 2820 isn't awesome.  I was shooting for chrono, not for groups so no idea yet if its more/less accurate but had no problem keeping either load on the 2MOA plates at 750 yards. 

[Zephhyr]

huh - now thats pretty interesting.  thanks for putting some data to it!  how long is your barrel?  did you shoot them in that order going longer?  could speed increase be attributed to change in barrel temp or having started with a clean barrel getting progressively more fouled?

at 43.8gr i'm running them quite a bit hotter than you are (2785-2805 depending on temp) mostly because i couldn't find anything i thought was really good in the 42.x range. next load down that shot well for me was down between 2600 and 2625 which is actually slower than factory loads.  maybe i wasn't looking hard enough in the 42.xgr range but since 43.8 seems to be working ok without issues even at 100F out, i'll stick with it for now.

My AR has a 24" Criterion barrel + suppressor.
The barrel / suppressor never got above comfortably warm. I would take a short break between 5 round groups just to keep the temperature in control.
I did fire them short -> long.

[dnellans]

My AR has a 24" Criterion barrel + suppressor.
The barrel / suppressor never got above comfortably warm. I would take a short break between 5 round groups just to keep the temperature in control.
I did fire them short -> long.

getting a bit off topic but can i ask what resolution +-.1gr or +-.02gr you're weighing your charges?  ES under 20 is pretty impressive

[Zephhyr]

getting a bit off topic but can i ask what resolution +-.1gr or +-.02gr you're weighing your charges?  ES under 20 is pretty impressive

Thanks, I've had pretty good luck getting consistent ES of 15-20 with RL16 and H4350.
I use an RCBS Chargemaster to throw the initial charge, .1 under,  then trickle it up on my bald eagle scale to within + - .02.

[dnellans]

i've gotta ask (per another thread i started) - what kind of SD/ES were you seeing if/when you were just using the RCBS with +-.1  before adding the bald eagle to within  +- .02?

I've been wondering lately if my current SD/ES are in fact limited by the chargemaster lite (+-.1) that i'm using.  brian litz's new book makes the claim you will never be able to reliably get an SD below 7-9ish if you're relying on .1gr precision from your scale.

[Zephhyr]

While I would see an occasional sub 10 SD using the Chargemaster alone,  it wasn't consistent enough for my ocd, with my current process is definitely more consistent and repeatable.

[dnellans]

While I would see an occasional sub 10 SD using the Chargemaster alone,  it wasn't consistent enough for my ocd, with my current process is definitely more consistent and repeatable.

thats super interesting for me to hear since I'm basically in the same boat. I can pretty reliably get SD's at 9-11 across 25 shot strings but never below,  maybe i will have to cough up for a gempro or something and start trickling up

[MLN1963]

Maybe the increased speed with increased COAL is due to more complete combustion? If the primer can move around the powder a tad is it possible it is igniting more of it initially?