Bullet jump, accuracy and velicity

[Qld4390]

I just got back from my last round of load development at the local range and have some questions. The load data is as follows:

Lapua 308 brass
46.0 grains of Varget (OCW charge)
168 SMK bullet
CCI BR2 primer

Today was testing bullet jump to see if I could fine tune things a bit. I loaded 6 rounds of each jump distance (as measured to the o-give)

2.205 (duplicate jump to Federal GMM 168)
2.215
2.225
2.235
2.245

Shooting from a rest, all groups were within 1/4" maximum spread of each other, with 2.205 being exactly the same as Federal GMM 168.

So now to my dumb question. Would a longer bullet jump (less case capacity) result in slightly higher velocities due to the pressures in the case being higher? Would there be any logical reason to run a longer OAL if the shortest OAL is just as accurate?

By they way, no pressure signs found in any of the loads.

 

[5R Milspec]

Re: Bullet jump, accuracy and velicity

you are right sir.the case will end short for room and pressure will rise.but if the loaded round shows great accuracy with out showing pressure signs then run with it.ACCURACY is what you want right.so either one will work for ya.the longer bullet will just allow you more space for powder if needed.but when loading them long and useing the space can run into pressure real fast.so be careful.

 

[Dog Doc]

Re: Bullet jump, accuracy and velicity

Interesting to see someone else who had similar results as I did. I won't post all the numbers but did something very similar with 175gr Sierra HPBT. Measured ogive from a start of OAL of 2.800 and increased in .005" increments. I didn't see any change other than slightly increased speed as I increased OAL (10-15fps per .005" jump). Accuracy was not any better than the starting load..actually less consistant.
I figured it was me having a bad day...

 

[Fuzzball]

Re: Bullet jump, accuracy and velicity

<span style="font-style: italic">"Would a longer bullet jump (less case capacity) result in slightly higher velocities due to the pressures in the case being higher?"</span>

Rifles are not handguns; seating deeper (within reason) actually reduces rifle chamber pressures and, usually, velocities.


<span style="font-style: italic">"Would there be any logical reason to run a longer OAL if the shortest OAL is just as accurate?"</span>

No. Seating close to the lands is a BR method that doesn't even work all the time for them. Few factory sporter rifles shoot best that way and nothing is lost with a longer jump.

 

[arnie19]

Re: Bullet jump, accuracy and velicity

So what was your actual jump to the lands from the ogive of the bullet touching? Arnie

 

[Qld4390]

Re: Bullet jump, accuracy and velicity

Touching the lands is 2.265. Federal GMM is a 0.06 jump. I loaded 0.06 through 0.02. I did a previous test from 0.02 all the way up to 0.005 off the lands, but the accuracy was crap. It looks like 0.06 jump and 0.02 are the most accurate jump distances with the above load. I think I'll try to track down a chronograph tomorrow and test a batch of 10 rounds of each jump to see what I get.

I forgot to mention earlier, the barrel is not factory, but is a PacNor 5 groove.

 

[tcasexx]

Re: Bullet jump, accuracy and velicity

what was the velocity change in the diff seating depth

 

[Qld4390]

Re: Bullet jump, accuracy and velicity

Ok, I went back to the range today and tested 2.205 and 2.245. I loaded 10 rounds of each jump distance. I fired 5 foulers to get things warmed up and fouled up a bit.

2.205 velocities were :
2569 (low), 2585, 2584, 2591, 2622 (high), 2609, 2578, 2583, 2586, 2598
ES = 53
Average = 2590
SD = 15

2.245 velocities were :
2595, 2616 (high), 2586 (low), 2607, 2596, 2601, 2602, 2613, 2587, 2602
ES = 30
Average = 2600
SD = 9

So, all in all, the longer OAL (0.020 jump to lands) ended up being faster and more consistent. Not by a huge margin, but enough to make a difference.

Oh, and for anyone thinking to themselves "Damn, those are low velocities for 46.0 grains of Varget!!" it should be noted that this is all from a 16" barrel.