.243 Case Overall length

[Austin.243]

Hi Guys,

 

I'm relatively new to the handloading scene and I have a question to ask.

 

I have recently acquired some Speer 85 grain boattails. I looked on the net and found a COL for the 85 grain bullet of 2.625" so loaded some up (with 40 grains of reloader 15) and went to the range. The group was pretty poor really being about 1.5" @ 100 yrds. I chronographed the load and the standard deviation was about +-10 fps so the poor group is nothing to do with large variations of fps.

 

I usually use a 58 grain V max with a COL of 2.600" which have produced groups around .3". Unfortunately I don’t have the ability to measure the length of the chamber and so I don't really have a point of reference.

 

Therefore I was wondering is there anything you would suggest to try and increase the accuracy eg increase COL- if so would this have any adverse effects?

 

Thanks

[Fishnshoot]

If it were me, I'd do a ocw test first. Then make adjustments to seating depth. That's what I was advised to do when I was developing a load. I presume that you have done some kind of charge weight testing?

You can make your own case length gage by slightly nipping the neck of one of your cases just enough to hold the bullet, leave it long and insert it gently into the chamber. Gently extrAct and measure. Do this half a dozen times and it will give you an good idea of your max seating depth... It's on you tube somewhere as how it's done.

Best of luck

[Austin.243]

Thanks fishnshoot,

 

Unfortunately I've been a bit rushed latley and I haven't done my ussual charge weight testing. I've got some spare time this weekend so I'm going to measure the chamber as you said, Hadn't thought of that! And do some charge weight testing and hopefully one of the combinations will work to better accuracy!

 

Thanks again

[Mick Miller]

You have a bullet that shoots well with the 58gn. Why not buy a comparator and measure from the case Base to ogive, that will give you a more reliable reference point than using the COL.

 

Set your 85gn to this and see how you go. Comparators are relatively inexpensive, but indedpensible when hand loading (thanks Spud for that advice).

[gbal]

Austin,Mick has given excellent advice.

We never know how much 'newcomers' might (not) know-COAL is pretty much only of use for checking a loaded round will fit the SAAMI magazine size. Bullets just differ in their 'ogives'-curved bit at the front,therefore just where they touch the lands etc.

 

You have probably heard of Berger bullets,the best currently readily available.You'd expect their chief ballistics man to be a guru-he is. He's called Bryan Litz,and you should Google as below,and get this issue straight:

 

"Effects of cartridge overall length (COAL) and Cartridge base to ogive (CBTO)"

 

Anything else by him is 'must read' too-see Berger site.

 

Then get a comparator and relevant insert (£30/40) and a good micrometer-the mic is the best bit of extra kit you can have if you go beyond very basic reloading-you may well have one-don't skimp on it.

 

As Mick says,the odds are ,setting the jump (seating depth off lands) about where your rifle clearly very much likes the current bullets wil be at least a good start point for new bullets,but be prepared to fine tune maybe +/- 5 thou either way-it's essentially an individual rifle/bullet thing (you do know bullets from the same box can vary-less so Bergers-another reason COAL is pretty crude...)

 

I'd not do a full ocw test-unless you like doing them. Notwithstanding the individuality just mentioned,it's often a reinvent the wheel -look to see what works for others with that powder and bullet,within the makers data maximum,or failing that start a good grain lower and work up in a few 3/5 round batches of say .2g .There is nearly always a good/optimum accuracy node within a grain of max- the gains from searching on for another node are often only in the mind-and always ask " does it really give me a useable gain"- 50 fps may well make for less accuracy,and have essentially no noticeable effect on trajectory for most distances/purposes.

Velocity is vanity,accuracy is sanity.

 

Anyhow read Litz,and get to work with a comparator to sort optimum seating for that bullet,if your initial loading isn't good enough-there is no guarantee of another .3 load !! Good luck with it,though.

 

gbal

[Austin.243]

Hi gbal,

 

Know you've mentioned the 'jump' into the barrel which the bullet must make, it makes perfect sense! I'll looking into the comparators and when I've got one of those I'll make the CBTO length the same as the 58 grain round and hopefully the group size will reduce!

 

Thanks everyone for the great advise.

 

Austin

[gbal]

Austin-good man.

 

More homework-it should be interesting,and cumulatively it all pays off-Google :

 

Sinclair - determining bullet seatin depth

 

It covers the theory/basics and some of the comparators-Sinclair sell lots of tys for accuracy boys,including Bench reast serious stuff-a must read you don't have everything,but have some spare £ investment capital...:-) But don't go overboard...yet....

 

This article also outlines the old free method-smake a dummy seated cartridge and smake the bullet ,chamber and see if the lands cut into the smoked bit....way back in the day (BC-before Ballistic Coefficients) I used this with my first 243,to gradually ease the bullet out/in until it was just not touching the lands-trial and error,of course. Generally 5 thou off isn't a bad place to be-of course 5 thou was a guestimate-maybe 1/4 turn f the seating die in the press...gets close though-close enough for sub 1/2 moa (though that depends on a rifle /load that can be so precise-never a given. Mine was a Sako varmint-many many rounds later,it's still good enough to resist a rebarrel....but I'm talking to it seriously. There were no 55/58 grain bullets then...mainly 85/87g ....the 58s have trandformed the 243 into a superb velocity varmint machine-as you've noticed. It pretty well outclasses all the 22s etc-velocity( capacity) matters,then after 350 yards,BC matters-and that's gives the 243/6mm the real edge over medium distance.....

I spent my formative years lusting after a 22 middlestead,you should realise how lucky you are (me too) that the 58g class bullets now give that performance in the standard 243 ( 115 g for 1000y do need a fast twist barrel...another story). The 243/6mm is currently an equalfavourite with the precision shooters-moa targets out to 800+ under field conditions. (equal with the 6.5s)-lower recoil,ballistic positives....don't believe 243 denigrators who don't know how it has been transformed-it always was a great dual purpose cartridge,but now there actually isn't much it can't do -at least in UK.

 

oops,got on a soapbox- if you don't know 'Accurate shooter' site,or the 'Precision Rifle blog' site,there is some excellent stuff on both....

 

Let us know how you get on with the 80s- as with all bullets,the heavies have pros and cons-actually,there are differnt 'bests' for different purposes-and of course legal issues if used on sentient critturs. Be guided by the Bryan Litz/Accurate shooter sites ,not the local club pontificator....(unless he's up on Bryan Litz etc too !!) Enjoy.

g

[Austin.243]

Gbal,

 

I've used the technique of pushing a extra long round into the chamber and letting the lands push the bullet back into the case, my measurements came up with 2.752" as an average, so as I said I'll try and get a comparator and hopefully one of the lengths will produce a reasonable group.

 

I've come across the problem with the heaviest end of the spectrum as my 1 in 10 twist tikka wasn't too keen on the 105 grain Amaxes which just tumbled! I wouldn't mind another .243 with a faster tiwst rate for use with heavier bullets but the bank man won't wouldn't let me( as Ive got uni to pay for!)

 

As you say the 58 grains have really transformed the .243 into easily one of the best varmint rounds out there.

 

I'll keep you posted.

 

Austin