Bullet jump vs mag coal.

[doc holiday13]

SAAMI drawings exist for a reason. Its a great starting point that is probably the best bet "blind guess" for optimal accuracy. I'm not saying a bullet can't perform outside the drawing spec or you shouldn't try to get more performance out of the round.

My 7 PRC reloads are setup based on factory loads, which gives me a .120 jump to lands. I tried to load longer(closer to the lands) but the gun wasn't having it. I'm going doing all my 25 creed load dev start based on the spec drawing here in about a month

 

[wind gypsy]

Mag Box Internal Length - .025” = dimension X

COAL with bullet touching lands, - .050” = dimension Y

COAL with boat tail at neck/shoulder junction = dimension Z



If X < Y, and X < Z, COAL set to X.

If X > Y, and Y < Z, COAL set to Y.

If X > Y, and Y > Z, COAL set to Z.


This is my process. Takes a few minutes to figure it out, then I don’t think about it again.

I like this except i'd be a bit more complicated about it - if there is sufficient neck to hold the bullet, I'm happy to keep seating it out to the 0.050"ish jump rather than trying to get the back of the bearing surface close to the shoulder.

 

[wapitibob]

I don't even have a mag box in my rifle, I load .020 off the lands and leave it.

 

[MT762]

Well folks, did it again today with some once fired united brass. I didn't have to force the bolt to close this time. New measurement is .02" for the 6.5 and .03" for the .308 above mag limit

Basically all cartridges are designed so that with the bullet seated at or around magazine length, it has to jump through space to the lands. Otherwise folks would be getting bullets stuck in the rifling every time they chambered and unchambered a round without firing.

.02-.03 jump to the lands from mag length is generally a good place to be. Lots of bullets won't shoot unless you are jumping them .05. And even finicky bullets will shoot well at a large range of jumps. For instance they might shoot well at .025-.035, but also at .075-.085, at .150-.160, etc. It's fairly rare that a bullet only shoots well at one exact jump.

 

[streamerfish]

Lol, isn't that all of reloading? If I didn't want to think then I'd just keep buying the same factory norma ammo that shoots really well for me.

I feel ya. I like to tinker as well.....but, IMHO, especially on the creedmoor, seating depth tinkering is largely futility.

Tinker with bullets, powders, cases etc. Real world differences to play around with.