jump or jam

[stifffingers]

Chronograph data can answer the extent of pressure variation very well.

For me, its common that I see some variation in pressure/velocity at all increments of seating depth. Going from say .010-OFF to .010-IN typically I won't see much variation, and definitely not an "overpressure". But going from a bigger jump of say .025-OFF to a harder jam like .025-IN, I would expect to see more definite variation in pressure/velocity, but still not an overpressure.

The only times I can attest to experience overpressures came from seating long bullets deeply into a case, and when seating with a hard enough jam that there is excessive resistance in bolt closer. In both scenario's I have seen some dramatic variations.

My 2-Cents
Donovan

When seeking the most advantageous seating depth I jam the bullet completely then back it out 5 thou. then seat a few dummy rounds and cycle them through the action to make sure the bullets won't stay in the barrel when emptying chamber on commands. Then load all the rounds at that depth and head to the range with my arbor press and seating dye and test the load with different seating depths. Simple, right?

 

[Richard Coody]

to each his own

well i shoot jams all the time with my target rifles but i wouldn't load jams in a repeater or hunting rifles or a rifle that i intended to eject a loaded round from. From my experience many of the benchrest shooters also find that the berger VLDs perform best jammed. in fact Walt Berger shot them that way. Now the new berger hybrid i find likes a little jump in my most recent 6BR. Match Kings also perform well for me with a little jump.

i don't go back and forth myself. just asked that question to find out if others had found this dangerous pressure increase going from jump to jam that i have never experienced . it is this unreasonable fear of jamming bullets into the lands that drives me nuts. for example go to the section on this forum for all the different cartridges and look at the recommended loads and just see how many are shooting jams. look at the charge weight and how close to max they are. jamming bullets in the lands is really no big deal. in fact it is a great way to wring that last little bit of accuracy out of your rifle. when the bullet is jammed in the lands concentricity becomes much less of a factor.

 

[Macropod]

Ive found that in one of my 308's that going from touching the lands to a 0.030" jump actually increased velocity. I tried the experiment again and had the same result. I believe shot start pressure and the overall pressure profile doesn't always jive with traditional beliefs about higher pressures with a jam.
A jump does impact on case volume initially and is perhaps the reason for higher velocities noted. Velocities were measured with a Labradar.

 

[USMCDOC]

i really like soft seating when ever possible.. letting it find where it wants to be, that way it chases the lands on it's own accord..

 

[swadiver]

I conduct careful seating depth tests on all my target rifles and select the depth which produces the best performance. Sometimes that's a jam, sometimes it's a little bit of a jump, and sometimes it's a significant jump. The most important thing to realize is that it all depends on many factors and anything I do is unlikely to be the same for your rifle/bullet/powder/primer/caliber/action/barrel combination.

+1

 

[Richard Coody]

i really like soft seating when ever possible.. letting it find where it wants to be, that way it chases the lands on it's own accord..

i too like soft seating for those loads that like a soft touch. just seat them long and let the bolt do the final seating. i use about a .001 of neck tension and the sort by seating effort with my hydro seater

 

[Longtrain]

When I started to load, 35+ years ago for mainly hunting rounds, mag length usually meant jumping to the lands. Once I started shooting XTC competition, function in the rapids was most important and again mag length loads were jumped. Since I only shoot prone now, sling and F Class, OAL is less important, looking for what works. However, I still jump everything and I haven't had to compromise on my accuracy. I'm sure that accuracy is available to both jammers and jumpers, it just depends what works for you.

A case in point, a few years ago I used JLK's 180 VLD's in a 308 for F-TR competition. When I tested that bullet at 600 yards, I came up with a good load using Varget and BR2's. I then played with the seating depth, started 10 in, 5 in, touch, 10 off and 20 off. When I got to the 20 thous off, my buddy called from the pits and asked what was my last 5 shots, I told him 20 off, he said you found it don't change anything.

That load proved to be a winner until that barrel was shot out. That load gave me 2 clean strings at 800 yards of 150-8x and 150-11x shooting F-T/R in conjunction with a Palma match.
Playing now with the Nosler 6mm 105 RDF bullets in a Dasher, which is similar to the Berger hybrid design, my initial accuracy showed at 40 off. More testing will be done before next years matches, but with the BC of those bullets, if they prove to be as accurate as my Berger 108's in the Dasher, they will save me points to the wind.

P.S. If you do jam and need to unload during a match and the bullet gets stuck in the barrel, when you pull that case out of the action, powder will go EVERYWHERE. I have seen this happen to shooters at a match, it is a "Chocolate Mess" to quote a famous commercial.