Effects Of Cartridge Over All Length (COAL) And Cartridge Base To Ogive (CBTO) – Part 1

[ADMIN]

Many shooters are not aware of the dramatic effects that bullet seating depth can have on the pressure and velocity generated by a rifle cartridge. COAL is also a variable that can be used to fine tune accuracy. It's also an important consideration for rifles that need to feed rounds thru a magazine. In this article, we'll explore the various effects of COAL, and what choices a shooter can make to maximize the effectiveness of their hand loads. Read More...

This is a thread for discussion of the article, Effects Of Cartridge Over All Length (COAL) And Cartridge Base To Ogive (CBTO) – Part 1, By Bryan Litz. Here you can ask questions or make comments about the article.

 

[Doug Brownlee]

Your articular is right on. I worked up a load on a 325WSM Browning A Bolt. After going through the load work up, the rifle shot a .57 group at 100 yards with the bullet set .010 off the lands. How ever, the COAL was to long to fit in the Magazine. I had to seat the bullet an additional .033 to achieve a proper feed from the magazine. The three shot group jumped to 1.375 at 100 yards. Factory ammo was set .087 off the lands and shot a 3.278 group at 100 yards. I am looking forward to additional articles. Don't chase velocities load for accuracies.

 

[martyj]

I learned this about 2 years ago on this forum and it is one of the things they should put in a reloading manual.
One of the best things Ive learned about reloading to date.

 

[TrentK16]

So Im trying to understand this better. So if I work up a load to the max COAL that can fit in my magazine then start backing the seating depth off to try to tune the load. Is that gonna cause higher pressures if I dont change the powder charge because Im creating a smaller air space in the cartridge?

thanks,
Trent

 

[Carbine Kid]

G'day all.
This subject has prompted me to my first post.

I have been conscious of COL & it's effect on accuracy for quite a while now, first experimenting on an old 26-06 Rem. I suppose this was when the accuracy bug bit. However to this thread.

My main hunting rifle is a Browning Abolt titanium in 300wsm and has two purposes.
First on pigs, goats, roos, foxes, ie thin skinned targets up to 400m day or night so the Speer 125gn TNT was chosen.
Second same game out to 1000m plus deer donkey & camel out to 200m so the Speer 180 sp.bt. was chosen for its good BC & suitable construction.

The oal of the 125gn posed no mag length problems and .030 jump was chosen with RL15 & Fed GMM-M. Groups varied from 1.1in @ 3621fps to .27in @ 3454fps. OAL did not need to be altered so a lot of components were saved and .27in load was chosen as 3454fps was considered sufficient.

The 180gn'r at mag length was poor. So OAL's from Kiss to .040 were tried before a powder change. Success came at .025 jump reducing groups from 1.6in worst to .59in. best. RL15 was considered to fast but tried anyway (got heaps), AR 2209 & Win760 were tried with 760 & Fed GMM-M getting the nod. This combo raised velocity from 2860 to 3010.
So with this projectile the combination of increased case capacity & reduced jump tuned up nicely.

The McMillan 338 Lapua Ackley Imp. was varied from .010 into the lands to .040 off with groups from 2.4in max to one holers of .28in. The best using 104gn Ar 2217, Rocky Mountain 275gnr's for 3164fps & ES of 18fps. I won't try for single figure ES'S as 18fps equates to only 3in @ 1000yd. It's not worth Bbl life and $$$.

All time's wasted whot's not spent shootin.
Steve

 

[highplains]

Excellent article.

 

[RustyRick]

Can anyone find PART 2?

 

[Crashman1]

Okay so here is a question to Bryan...I recently built a 284 winchester on a Stiller long action, my COAL is over SAAMI specs but I don't think I have reached full potential yet because I still have lots of room in the standard mag box, and the boat tail starts well below the neck/shoulder junction of the case. With this current combination however I am right at the maximum COAL based on my chamber and reamer. I have however purchased a 7mm uni throater to push the throat out further, but I have not muscled up the courage yet to do anything with it.

I think-please correct me if I am wrong-that I want the boat tail to start right at the neck/shoulder junction to get the best combination of useable powder capacity and accuracy. I am currently shooting 180 Lapua Scenars (however a fantastic fellow in your booth at SHOT gave me a healthy sample of your 180 Hybrids to try) and my accuracy is averaging 0.5-0.6 MOA with my best groups hovering in the low 0.3s, velocity however is a paltry 2705fps. Now to clarify, I am an accuracy junky and not really a speed freak, however if I can have both with no additional pressures I think that is the way to go.

Based on this information, do you think it is worth it to push out the throat and get more powder capacity in this case, or should I leave well enough alone?

Other information of loads/components...

Lapua 6.5X284 brass necked up
CCI BR2 primers
H4831SC powder
180 Lapua Scenar L bullet
28" Broughton 9 twist barrel

I know there are powders available that might bump velocity, specifically Reloader 17, but supply of that powder is dismal at best here in Canada, plus I have 10 pounds of H4831SC on hand. And with our temps ranging from -17F yesterday morning to +90F in the summer plus humidity, I would rather stay with the slightly more temperature stable Hodgdon powders.

Excellent article by the way, I copy and paste it into a word document and printed it out and it went directly into my reloading log!

 

[HNDLDR]

I have two Browning X-bolts at my house (25-06, 7mm-08) that lead me to believe that Browning is starting to figure out the whole magazine length thing. Both clips are very generous with their coal's, even with Berger VLDs in the 06.

 

[Carbine Kid]

G'day Crashman1.

You have a good rig there & as you have the reamer I would use it.

When I set up my 6-284 I gave my gunsmith a dummy round with a Lapua Scenar 108 gn loaded with the base of the boatail level with the neck sholder junction specifically to gain max case capacity. At this length there was sufficient bullet shank in the neck to turn the necks (.270 neck in chamber) for a 2 tho grip.
I used an old 25-06 so mag length would not be an issue and checked feed before laying down the $$. As the Sierra 107 HP.BT. MK. was almost identical to the Lapua I could use either.

A nominal jump of .030 was used to work up max loads for AR 2213sc, AR 2217, AR 2225, RL 22 & RL 25. Necked down & turned Lapua 6.5-284 cases & Fed GMM primers for ignition.
Temperatures in Oz vary from 0*c in Sambur country to 45*c (113*f) outback. As this rifle was for outback work I had the same consideration as you re. Temp stability. Ultimately the loads that gave the best average groups were chosen and then OAL's were increased.
I have been told VLD projectiles like to be in the lands and .010 in is where the Lapua's grouped best.

- 47gn AR2213sc gave .35" groups @ 2995fps av & 65fps ES @ 86*f
-50.5gn RL 25 gave 1.1" groups @ 2986fps av & 9fps ES @ 86*f
-50 gn RL 22 gave .66" groups @ 3172fps av & 43fps ES @ 86*f
all shot ay 100m. and RL22 chosen.

Lapua & Berger supplies in Oz are unreliable so Sierra 105 HP.BT.MK. were tried with the same procedure as above with RL22 being chosen. However when increasing OAL everything went smoothly as expected until the last increase to a .010 jam. The bolt was very hard to lift & when the case head was miked I had a 3 tho case head expansion and primers falling out. Hmmm, didn't expect that so suddenly.
The load ultimately chosen to use up the remaining powder was.
- 46.5gn. RL22 gave .95" @ 3081fps av & ES 36fps @ 95*f with a jump of 15tho. ie. 25tho different to the Lapua's

OAL increases for AR2213sc was done due to the "sudden" pressure spike with RL22. Again 15tho was the best jump.
The final load chosen was 47.5gn AR 2213sc @ 2998fps av & ES 22fps @ 106*f. which has given sub 1" @ 300m on a zero wind day.

I have been told by a friend who has some Berger hybrids that OAL is not critical & they shoot well up to .065 off the lands.
IME all Australian Defence Industries powders are very consistent across a large temp range. Just work up loads in the hottest temp you expect to shoot in then there will be no surprises.

The xtra velocity from increased case capacity will be worth wile if you don't mind looking for the new sweet spot. In reality the reason we reload is to get the ultimate from our custom equipment.

Action & stock is x Rem Sendero 25-06 accurised & bedded.
Lapua 6.5-284 necked to 6-284 & turned for a .270 neck
TruFilght 1:8 twist 28" with Sendero profile.
Fed gold medal match primer.
Sierra 107 HP.BT.MK.
ADI. AR 2213sc

Keep well.
Steve.

 

[Crashman1]

Interesting information there. I think I will push the throat out on this rifle to get the extra powder capacity and take advantage of the long action. It will however have to wait for a few weeks as I am in my busy season at work and have very little (really NONE) time for this type of thing likely until mid March or even early April. I would like to have a plan in place though and get it done as soon as time constraints allow.

I will be re-reading your post Steve to see if I can pick up more rid bits for my project. Thanks for the info!

 

[graywolf]

I have a question for you, Brian, that pertains to a point you seem to emphasize:

"you can load your bullets long, which increases the volume for powder and allows you to use more powder and achieve faster MV for the same pressure"


Do you mean what it seems like you are saying- that (as long as you don't jam the lands) that you can achieve higher MV without pressure increase by just making seating further out to make more room in the case and adding more powder? It would be great if true but doesn't this go against conventional wisdom?