308 jump

[simong]

Evening peeps,

Just invested in a Hornady OAL gauge ( starting to take "roll 'ure own a bit more seriously ) any ideas about how much jump to start with ? 155grn Sierra hpbt . I don't understand the relationship between jump/jam and how straight the boolit flies ?

Any advice or guidance will be greatfully received

Ttfn
The greek

[Steve E]

Anything from 5 to 50 thou of an inch dependant on barrel and bullet type. I Load the SMK 2156 to 2.234" base of case to ogive and shoot sub .75 moa at 1000yds out of my target rifles.

You will have to try different amounts of jump to see what works for you. Both the Sierra 2155 and the 2156 are very tolerant of jump and you may not see much difference if any no matter how far you jump them.
Load the 2155 to a col of 2.8" and start fro there. RUAG 155 is loaded to 2.75" and shoots well in all rifles with a decent barrel.

[WelshShooter]

Don't forget that there's a chance your overall length might exceed your magazine length, so you'll have to single load your rounds. The theory is that by reducing the jump you reduce the variation involved for bullet orientation while engaging the rifling. With my 308 I've found myself getting good groups on paper at 100m using almost the max mag length. I have no paper results at 1,000m like Steve so can't correlate this at long range.

[WelshShooter]

PS this article is quite useful.
http://www.bergerbullets.com/effects-of ... to-part-1/

[Mr_Logic]

I start with 20thou for initial testing and then try from 10 thou back. I don't jam unless nothing else works, and even then I'd rather just use something else. Light neck tension and a stop order would mean a breech full of powder. No ta!

[simong]

Morning peeps

Thanks for the info. it seems that the further i get into this the more there is to experiment with/ learn.
Oh dear, my poor brain ! what with all of this and what my tribe are going to do about the euro/new drachma

ttfn

the greek

[Maggot]

It depends on a lot of things mate

Personally I would try 15tho then go back in 5 thou increments. I say that purely because that si what has always worked with the 2156 but who says it will with your rifle? It does seem to work for a lot of folk though.

As Steve says (going back to 50) some bullets can shoot particularly well deep seated. my 2156 and HBC loads work best around 15-16thou off.

But...when I used Berger Hybrids they were seated around 50 off. I tried this at the advice of Russel Simmonds who also knows his way round precision 308 loads.

Funilly enough the HBCs hit the spot almost immediately with 15thou although the are way off the 2156, so is it the rifle of the bullet....?

Once you get it in the ball park try much smaller increments either way with both OAL and load, you will be surprised what difference a small change can make for the better. Seriously, I had trouble duplicating a load with different brass (slightly larger capacity). It just needed 0.2gr more and in it came.

One thing you soon learn with load dev is that there are few free rides, and it improves as you learn (you soon learn what is you and what is the rifle as your test regimen refines....if that makes sense?)

I would test at 200 as well if you can, I find the differences are more stark.

[Dannywayoflife]

It's a hugely interesting subject and as Maggot says you have to find what works in your gat. My 155 HBC load in my dta didn't get below an inch till I moved back from 15 thou to -20 - -30 thou. Now it shoots bug holes.

Until Davy altered my maga I was running my 208 Amax at may length which worked out at -65thou they still shot though! I'm no -10 and they are slightly more accurate but that could be me having a better day behind the butt.