Jam vs Jam

[BATattack]

hi all!

Yep that's rite jam vs jam!

I have seen significant difference in varying JUMP to achieve accuracy gains but for myself not so much with jam.

Do those in the know have any statistical evidence to show varying JAM have any meaningful effect on accuracy?

Speaking with the prophet he has suggested jam is jam and little can be gained by varying from .010 to .050 for instance. From what I've seen I'm thinking along the same lines but I'm interested to hear others though and experiences.

Regards

Adam

[Longranger]

One thing I've noticed is that when running slightly jammed (0.005" or a little less) that varying neck tension makes little difference. One less variable to worry about.

Running more than 0.010" may incur the risk of a projectile lodged in the leade and subsequent messy powder dump if you have to unload.

[DenisA]

Adam, I like the theory of jamming and therefore develop jammed loads for all of my cartridges. I do however run my tests out to a .015 jump in curiosity. I start all of my VLD development powders tests at .015" jam. I test seating depth in .003" increments with 5 shot groups.
I've found that the larger VLD's such as 7mm's and 30cals have shot best with a light jam, usually around .003" - .005". The 6mm's just seem to be so tight at .015" that there's no point looking anywhere else.

I have on many occasions run seating depth tests past .015" and never found groups to tighten with any sized bullet. The groups have always opened. With VLD and Hybrid jams past .020" I also worry that I might dump loads of powder in the chamber on the mound in the case of a cease fire or un-chambering a round.

Despite the published advice that you won't see a difference in .015" increments, in my experience the difference in group size and shape of small incremental changes has been quite obvious. That said, I don't load test with 30 round groups for a meaningful SD.

I run light neck tensions and have never been able to jam a tangent bullet more than .005" without pushing it back in to the case.

[Steve N]

Comparisons of jam length can be very difficult because of the various methods used to determine it. I remove the firing pin from the bolt and gradually seat the projectile deeper until there is no feel when closing the bolt very gently. The case has to be sized so that the bold is loose closing without a projectile.
Others have told me they seat a projectile by closing the bolt on it and that is their jam length.
Split neck method, Hornady or Sinclair methods all give different results.
One person's 15 thou jam could easily be anothers 10 thou off!
Steve

[Brad Y]

Barrels will like what ever they like I reckon. Won a queens on a 40 thou jumped VLD load. Generally if I try to develop a load with a VLD now I will go for a 10 thou jam straight away. Hybrids I jump 20 thou currently and they shoot brilliant at long range. In 6.5mm I could jam a vld and a hybrid both 10 thou and could shoot in the same group at 900m with my old 260 improved. Barrels will like what they will like.

[ned kelly]

G'Day All,
some years ago a test was done in the now defunct Precision Shooting magazine that as I recall tested varying Jam OAL but the consistent thing was that the projectile always was pushed back into the case to the same amount for a given neck tension. The more consistent results came from light neck tension i.e. 2-3 thou. Heavier tension and I'm assuming variable case hardness were not as consistent. Heavier tension also gave heavier engraving of the bullet into the lands and greater risk of a stuck bullet if you needed to remove a loaded case.
I personally run a bushing 2 thou smaller than the loaded case and run 10 thou jam. Seems to work regardless of barrel, bullet or application.
Hope this helps.
Cheerio Geoff

[johnk]

Earlier this century, Boots Obemeyer concluded that if you were moly coating projectiles, better consistency was achieved with up to .060" jump. His contention was any amount of jam resulted in a certain amount of the coating being removed by the lands but not necessarily consistently land to land, resulting in variable start times. I corresponded with him on this, but never got round to asking whether he was speaking of hard or soft jamming.