155.5 jump

[AussieMike]

This may not be the place to ask and if it is not then please say so. I am beginning the process of getting the 155.5s to shoot in a new 5R Krieger, 1:12 .3070 .298 barrel. With 46.2 of Varget (AR2208) in Lapua brass I get about 3009 fps @ 10 feet from the muzzle. This is the barrel and setup I want to use in the upcoming WLRC Teams matches at Camp Perry in August. I need somewhere to start as I have not used these before.

 

[TMiller]

In a 12 twist Bartlein barrel with the same components you are using I used .015 off the lands and had excellent accuracy with the 155.5 Fullbores.

 

[spclark]

0.020" off, 0.010" off in any of three barrels I have (two 1:12 Bartleins, one 1:13 Obermeyer) & they shoot just as good from 600 to 1,000 yards.

45.7 grains IMR 8208XBR, Lapua Palma brass, KVB-223M primers = 2,975 - 3,025 fps (Bartleins slower) SD's < 8.

My conclusion is the 155.5 Fullbore bullets are very forgiving of seating depth.

(If I'm not shooting with you this August I'll be pulling targets!)

 

[Keith Glasscock]

.015" off the lands here. Shoots really well in my F/TR rifle. 200-10X @ 600 yards and the 10's were my fault...

 

[gsg5pk]

Just wonder those who setup .015" off land, how many rounds will you recheck the throat erosion and seat out new seatdepth?

 

[XTR]

My last 308 barrel that got to 1000 rounds w/o me sticking a throater into it moved about .010 in 1000 rounds. That is shooting F class strings.

 

[Keith Glasscock]

Mine doesn't seem to want to be chased. I run a seating check every 400 rounds or so. Still likes the same depth. Barrel has 2000 rounds on it.

 

[Bryan Litz Ballistics]

The 155.5 FULLBORE is a very forgiving bullet. The common approach is to seat 0.015" off, but they do well with jumps up to at least 0.060". So you can seat all your ammo at the same length, and as the throat erodes and the jump increases, the bullets still shoot just as good. It's a very forgiving bullet.

-Bryan

 

[AussieMike]

Thank you to everyone for your honest and informative replies and especially to Brian for his input. It would seem that these projectiles are pretty forgiving regards the amount of jump to the lands. I have had some fine groups at 45 thou off the lands. I am not fussed about having them into the lands as a breach full of powder during a cease fire is not very convenient.

The role of interior ballistics versus exterior ballistics is quite fascinating, as you would think that by varying the amount of jump the projectile has to cover should influence the barrel exit time but the foregoing discussions are saying this may not lead to group dispersion. Then something else is at play. I wonder what that is?

Hope to see some of you in August.

Best of shooting to all
Mike

 

[AussieMike]

Sorry, Bryan!