Jump Vs Jam

B

bdotson

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When I first started reading about benchrest I thought most were jamming there bullets in the lands. Now I am hearing more about jumping? What are the advantages of each? Thanks for Your thoughts.
 
bdotson said:
When I first started reading about benchrest I thought most were jamming there bullets in the lands. Now I am hearing more about jumping? What are the advantages of each? Thanks for Your thoughts.
Click to expand...
To find out whether your gun likes jump or jam you need to run tests with different bullet settings to find out. Start out with the bullets at a hard jam (probably will not shoot the best there). In each 3-shot group, move the bullet seating 0.003" toward deeper bullet seating. Do that till the bullet is jumping.

As you load the different loads, polish the bullet nose of one bullet in each group with fine steel wool. Note the imprint of the rifleing on the bullet nose. Over time you sill notice that most barrel chambering will shoot best in the range of imprint that yours shoots best.
 
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As soon as the weather gets better I am going to try these methods. I am shooting 66 grn flat base in a 6ppc thanks for the advice.
 
Warning Newbie Ahead

Jerry couple of questions for those of us that are not accurately measuring with a bullet comparator etc. Sort of garage Lee Loaders.....

1. If one has an existing accurate load, and wanted to see improvement by jamming the lands, wouldn't it be prudent to back off on the charge to a lower pressure ? I have never seen any mention of that on this board, yet virtually loading manual clearly posts warning high pressure ahead signs regarding seating bullets out too long and contacting the rifling.

2. Don't you mean the rifling marks MAY be evident on the bullet further back from the nose where the projectile reaches full diameter ? The nose or meplat, or the ogive shouldn't be touching the rifling ?

3. By the time rifling marks are evident in the bullet, you are already past just touching the lands. How far into the lands would you estimate this to be as opposed to just touching ?

Thanks for clarifying what is probably a given to most of the shooters on here......
 
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As a ppc shooter,Jerry's comments make perfect sense. Once you do it, you'll get it.
Tuning a ppc is not the same as tuning a .338 win mag. Most ppc shooters like taking their little baby ppc's out for a walk. Let's try this,Let's try that. The mildness and inherent accuracy of the cartridge makes this exercise quite enjoyable. After a while, certain bullets,powders,barrels,etc develop personalities.One learns how things work for this specific chambering.
Early on,if you limit your components to one barrel,one bullet and one powder ( and take your ppc out for a walk),you will learn that combinations preference for seating depth and load.
Components are not interchangeable.A change in powder or bullet will usually result in a change of seating depth in relationship to the lands in a given barrel.
Your results on target will answer any and all questions about what your barrel likes in regard to seating depth and powder charge( with a given bullet).
Joel
 
When checking out a new component, caliber, or rifle, I generally run a pressure series, one shot per load, with steps that are appropriate to the case size, and the bullet seated as long as I can without it being pulled out of the case if I have to unchamber a loaded round. That way I am at a worst case for pressure caused by being into the rifling, and when I determine what my maximum charge is, anything else, with shorter seating and/or smaller charge will be safe, although I have heard of instances where with a small case (BR) that is pretty full, and a heavy bullet is used, that pulling the bullet back into the case significantly, can increase pressure, due to the smaller combustion space.
 
Another theory holds that jamming bullets into the bore increases the pressure curve because there is no space to begin movement. I no longer shoot the ppc's, just 6BR and 30 BR. In the 6BR I prefer VLD bullets and most shooters agree that VLD's shoot better when jammed. On the other hand, Sam Hall recently won nationals with a Dasher and 105 jumped .050".
Bottom line - put your loads together progressively moving them in either direction. You will eventually find the sweet spot.
Yeah, I have jammed bullets at .20" and more and had to spend an hour cleaning out powder granules. Need to think before pulling that bolt rearward.
 
Well
Yesterday we had a beautiful day here in Texas so 3 of us took our PPC's and rail guns out for a walk (or to the range) which ever you prefer. I found out several thing about taking my rail for a walk...More like a slow crawl and found it that it likes Cheek 66gr fb just off the lands just about 1000th back compared to Barts 66 or 68 fb. Barts like a good jump of about 10 to 15k off lands.. Also found out if you dont keep the rail guides clean and slick or as some guys might say "Slippery when wet" just does not perform well if there is any resistance in the slide anywhere...I was really surprised of so many things that can make accuracy difference in a bag gun or even a rail...No one should ever consider that you will shoot small groups by just taking your PPC for walk and setting it on the bench and say sit boo boo sit and it will shoot small groups...Not the case..

It is all trial and error but some things do need to be repetitious in set up and shooting...There are alot of different things to remember before taking the leash off and pulling the trigger...

Still there is nothing like going to the range and having a great day with friends and sending some bullets down range.... One last thing

"GO GREEN BAY" Ha Ha Ha Good shooting to all this season
 
For about a 3-4 year run, I had a phenominol time shooting a specific bullet, jammed so darned hard that the marks were twice as long as they were wide. I ran out of that lot of bullets, (that 1st 10,000 were something else), and began trying others. I settled in on the .790 Barts Boat Tail. But, it took a while to finally figure out that what that Bruno liked was a disaster with the Barts. After some investigating, and trial and error, I figured out that these things liked to be jumped, and I really mean jumped. I find the faintest of makes with a loop, and then move the seating stem out,(bullet into the case), another .020. That's a pretty good jump.

Now I have learned that others have the same results. Why?? I don't have a clue.

Shoot em where they like...........jackie
 
Flouncer said:
Jerry couple of questions for those of us that are not accurately measuring with a bullet comparator etc. Sort of garage Lee Loaders.....

1. If one has an existing accurate load, and wanted to see improvement by jamming the lands, wouldn't it be prudent to back off on the charge to a lower pressure ? I have never seen any mention of that on this board, yet virtually loading manual clearly posts warning high pressure ahead signs regarding seating bullets out too long and contacting the rifling.

2. Don't you mean the rifling marks MAY be evident on the bullet further back from the nose where the projectile reaches full diameter ? The nose or meplat, or the ogive shouldn't be touching the rifling ?

3. By the time rifling marks are evident in the bullet, you are already past just touching the lands. How far into the lands would you estimate this to be as opposed to just touching ?

Thanks for clarifying what is probably a given to most of the shooters on here......
Click to expand...

I'm not Jerry buy I'll opine....

First of all, there's no tool that'll measure your jam any better than your gun. Just seat a bullet long and seat it in the gun (use light neck tension) and measure it. You should order the little Sinclair ogive "nut" to help you be more consistent for your records but you can jam a bullet this way and set your seater from the dummy and never look back.

#1, you can back off on charges if you want but in shooting thousands of rounds over the chrono I've never seen a load spike when the bullet touches the lands..... thousands of rounds.

#2, I have no idea what you're trying to say/ask here

#3, depends.... depending on ogive/leade angle match you can have from as little as .002 play to as much as .075 IME. These numbers reflect the difference between touching and jamming so hard it moves the bullet back into the case.

hth


al
 
A systematic approach as outlined by Jerry and Jackie is the way to find out where your barrel likes its bullets. I start at Jam and load three charges...29.0, 29.5, and 30.0 grains of N133 and Bart's 65 gr. boat tails. I fire one fouler and the 3, 3-shot groups at 200 yards. I do not "hold for the wind, but I do let the flags get to similar positions. Next I seat the bullets .00375" into the case (I have a JLC seater that moves .00125 per click and I move the bullet into the case three clicks).

I clean my rifle and shoot the same 3, 3-shot groups at the new depth. This continues until I jump the bullets a significant amount. After these tests, look at the targets and find a seating depth that shows the least amount of wind deflection and no vertical. The depth that handles all three charges best is probably your best seating depth for that barrel/bullet. Good shooting....James
 
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