Jamming and neck tension

[lazyeiger]

I am having great sucsess with my new 6 x 47 Lapua jamming the Berger 105 Hybrids about .010" shoots better than 1/2" groups at 200 yards. When jumping 0.005" - 0.010" the group noticably opens up to about 1" at 200 yards.

I hate shooting jammed, when I was shooting .308 jammed I had a couple of FTF's and ended up dumping the powder into the action when retracting the bolt, once this cost me my best score in F Class.

I was wondering if jamming is making the start pressure higher and this is what the gun likes, does increasing the neck tension have the same effect?

Jon

 

[BoydAllen]

It might be interesting to see just how much longer than just touching you cal load with that bullet, with the neck tension that you are using. If you work with a dummy round, at least you won't spill powder. My point is that for certain bullet shapes, you can get farther in than others, and if you have something like .003 neck tension, and a decent amount of shank in the neck, I think that you are pretty safe at the depth that you are, but finding out what the max is might tell more of a story.

To get back to your question, you are right about the pressure being higher, but I think that the way that is rises is different than what you get from neck tension alone. One thing to remember is that both jumping and seating into the rifling have sweet spots that can be quite narrow, and they my repeat at intervals. The testing that is needed becomes too cumbersome loading at home, better to do it at the range, with a small number of shots per group, to start with, and a chronograph.

 

[lazyeiger]

Boyd,

I have tested this many times at home with unloaded rounds and everything is just fine where I have the length and tension. Of course as soon as I have an FTF at the range the bullet gets stuck. (I know this because the round with the primer seated upsidown left the bullet in the rifling on ejection) I am guesing that heat in the barrel has a lot to do with this!!!

I also double checked my neck tension and it looks like I made a mistake and only have 0.001" tension (.269" bushing .270" with bullet loaded) I do have a .268" bushing and I loaded up a round with that, it seems like a lot of pressure is needed to seat the bullet? Maybe it just seems high comparing it to the .308 ( I have never reloaded anything else)

Jon

 

[Alex Wheeler]

I would be surprised if you pulled a bullet jamming .010. I think your safe.

 

[lazyeiger]

I would be surprised if you pulled a bullet jamming .010. I think your safe.

I definatly pulled a bullet seated .010" into the rifling, this was the one I seated the primer upsidown! and didn't notice when I loaded the round! I did only have 0.001" neck tension though.

Jon

 

[mikecr]

If you get 0.001" tension with a .269" bushing and .270" loaded, I wonder what your fired necks measure?
Also, do you anneal the necks?

You won't be able to raise pressure equal to touching with neck tension alone.
And you can see clearly that seating depth adjustment from touching is not a fine adjustment to results.
What's your load & MV?

 

[BoydAllen]

Let me back up and furnish a little additional information. When I wrote .003 neck tension, I was operating in .262 neck ppc mode. I should have qualified my remark. If your necks are thicker, you will have more pull per thousandth. Perhaps doubling your number to .002 would be more appropriate. The thickest that I run necks is .0086, and I have gone as low as .0079 playing with more clearance, with a bullet with a big pressure ring.

 

[lazyeiger]

I am using Lapua 6.5 x 47 brass necked down to 6 x 47 and not neck turned, I have 3 firings on these cases they have been full length sized every firing I have not anealed yet.

Chamber is .274" fired rounds measure .2735"

I have found 2 good loads both seated 0.010" into lands

34.8g IMR 4320 MV = 2970 ft/sec
37.2g IMR 4320 MV = 3140 ft/sec

the faster load goups a bit better than the slower ( .250" compared to .400" at 200 yards)

Jon

 

[300 RUM]

Rifle powder is propellant and like all propellants it has a burn rate(this is the exponent) it also has a coefficient (pressure) so as pressure builds the burn rate goes up exponentially. Think of it like this more neck tension more pressure, a bullet jammed into the lands will increase initial pressure causing a regressive burn, a bullet that is being jumped into the lands with cause a progressive burn. Therefore a bullet jammed will have "X" for the amount of pressure induced from the jamb + the neck tension pressure, it would be almost impossible to generate enough neck tension to become equal to the combination.

Jamb or jump? it has always been a topic on it's own. personal I used to always jamb, but recently have switched to a jump. The jump allowed for a border rang of seating depth with excellent group sizes lets say +/- .005. With the bullet jammed I had to hit it right at a +/- of .003


Hope this helps

 

[lpreddick]

i have always been a jumper. picked a bullet, powder and charge that seemed 'right' and proceeded to seat bullets from .005 off the lands to as much as .150 + inches off. usu found a jump that produced a tight group...barrel harmonics etc. more recently i have been seating the bullets into the lands(jamming). i'v chosen .020 thous into the lands as my standard. start low on the powder charge and increase by .2 gr...small cases, .3gr for larger, for 4 or 5 groups. shoot the lightest first and proceed. i'm finding an accurate combo fairly quickly. powder charge adjustment when jamming, seating depth adjustment when using the same charge for all rounds. we are finding the "accuracy node" either way.

 

[300 RUM]

The harmonics are total different from a jamb to jump. The recoil changes from one or the other also. This will effect accuracy a great deal. Either way one goes you need to be able to shoot it.

On my competition AR-10 it jumps when fired so a jamb is easier to hold for 1K shoots,it seem to clam the rifle down a bit, but in a bullet jump condition the rifle still jumps but shoots much much more accurate.

I also get a little better powder burn on the jamb with a 50fps increase in vel.