clearance question / jump-jam

[Steve Ladino]

I haven't seen any discussion of consideration given to a loaded cartridge's for and aft movement in a chamber and its application to determining bullet jump or in the case of jamming maybe not so important because many claim just a .001" bump. Generally do most of you consider it and if not then why ? For example jumping a tangent ogive shaped bullet .010" would have to consider the bump amount of whatever, maybe .003", to really achieve an accurate .010" jump.

 

[rammac]

Maybe I need another coffee.

 

[Raythemanroe]

OP, you calculating headspace? I try not to FL size more than .002-.0025 but I also mostly have rounds that range from .007-.010 jammed.. I don't care what measurements happen after I pull the trigger, all my measurements are from load development and testing.. If it works I don't care..

Ray

 

[powderbrake]

I use the Wheeler method to find the correct sizing for my cases, I don't measure how much I "bump" them. I do record the case length for reference.

I use the Wheeler method to find the lands with each lot of bullets, and record that number. I will recheck about every 200-300 rounds.

I jam all bullets, (including 105 Berger Hybrids) so I do not believe I have any movement of the loaded cartridge in the chamber. All the jams are determined by load development and testing.

So basically all that I measure is my base to ogive length as I am seating bullets.

 

[mike a]

I only shoot at targets that are up hill. Thus keeping the round back against the bolt. I get what your talking about. My Bighorn has control round feed and a mechanical ejector. I think this helps. Mike

 

[jepp2]

I will recheck about every 200-300 rounds.

You don't mention the cartridge you are shooting, but most will change quite a bit in 200-300 rounds.

 

[Uncle Ed]

I haven't seen any discussion of consideration given to a loaded cartridge's for and aft movement in a chamber and its application to determining bullet jump or in the case of jamming maybe not so important because many claim just a .001" bump. Generally do most of you consider it and if not then why ? For example jumping a tangent ogive shaped bullet .010" would have to consider the bump amount of whatever, maybe .003", to really achieve an accurate .010" jump.

The ejector and the firing pin pushes the case forward and removes any fore and aft case movement. All bullet length measurements are actually off the shoulder location and not your bump or head clearance.

With jam the base of the case is held against the bolt face, but the force of the firing pin hitting the primer can push the shoulder back. This is why the reloading manuals tell you to not use cases from reduced loads with full power loads. Meaning the case will get shorter each time the firing pin hit the primer and increase the chance of a case head separation.

 

[47WillysGuy]

I haven't seen any discussion of consideration given to a loaded cartridge's for and aft movement in a chamber and its application to determining bullet jump or in the case of jamming maybe not so important because many claim just a .001" bump. Generally do most of you consider it and if not then why ? For example jumping a tangent ogive shaped bullet .010" would have to consider the bump amount of whatever, maybe .003", to really achieve an accurate .010" jump.

Steve, IMHO, I believe your write up is of two different subjects;
Bump of the case
And
Jump of the bullet

Case bump sets the fore and aft movement of the case in the chamber. For long range precision shooting, I full size and bump my cases to 0.001” less than chamber dimensions. This will be my loaded round dimension. Rounds for hunting should be set I about 0.003”.

Jump of the bullet is determined with a dummy round, no primer and no powder, and seating a bullet long, then chambering and closing the bolt on the dummy round . This process forces the bullet deeper into the neck giving me a hard jam reading, I do this 3-5 times or more until I get a consistent dimensional reading. From that reading, I seat my bullet. 0.020” deeper than my hard jam reading and go to testing for my most accurate load.

If you change bullets, the process starts all over again . Best of luck.

 

[Keith Glasscock]

Why would it be important to have an accurate jump or jam measurement? Put the zero somewhere you can replicate, and adjust from that.

In the end, we are all finding our seating depth experimentally.

One piece of advice: If you have to have really accurate measurements for it to shoot, it doesn't shoot. It isn't unusual for me to find bullet seating depths that allow some significant wiggle room.

 

[WIB3]

In benchrest shooting ,you need to be able to measure two things accurately. One being seating depth and the second is headspace. When I say measure I mean to the .0001 of an inch. That should be you goal. Get 10 rounds as close to that measurement as possible. You will see your groups shrink.

 

[Dusty Stevens]

In benchrest shooting ,you need to be able to measure two things accurately. One being seating depth and the second is headspace. When I say measure I mean to the .0001 of an inch. That should be you goal. Get 10 rounds as close to that measurement as possible. You will see your groups shrink.

Did you mean .0001 or did you add a zero in there accidentally?

 

[mike a]

In benchrest shooting ,you need to be able to measure two things accurately. One being seating depth and the second is headspace. When I say measure I mean to the .0001 of an inch. That should be you goal. Get 10 rounds as close to that measurement as possible. You will see your groups shrink.

Count me out!

 

[Raythemanroe]

I just bump it enough the bolt cycles okay, can't get much tighter than that.. I find neck clearance to be the thing to measure. If you don't have enough you get pressure issues that cause the flyers.. I agree seating is important, verify your numbers so you can repeat them.. Take notes

Ray

 

[mikecr]

Whatever wiggles happen while setting final (best) seating depth will be happening for the history of that chamber. As I think I understand and agree with Keith, defining things beyond that much is not really beneficial.

Also, if your striking is actually altering case dimensions, it's in great excess of that needed to fire primers.

 

[cw308]

powerbrake post#4 I'm with 100% the stripped bolt method I found to be the most accurate in finding closed bolt to datum or shoulder and closed bolt to ogive or rifling . Even with the same make and bullet , I check every lot change . It's pretty simple for me , I'm shooting a Rem.700 .

 

[Steve Ladino]

Steve, IMHO, I believe your write up is of two different subjects;
Bump of the case
And
Jump of the bullet

Case bump sets the fore and aft movement of the case in the chamber. For long range precision shooting, I full size and bump my cases to 0.001” less than chamber dimensions. This will be my loaded round dimension. Rounds for hunting should be set I about 0.003”.

Jump of the bullet is determined with a dummy round, no primer and no powder, and seating a bullet long, then chambering and closing the bolt on the dummy round . This process forces the bullet deeper into the neck giving me a hard jam reading, I do this 3-5 times or more until I get a consistent dimensional reading. From that reading, I seat my bullet. 0.020” deeper than my hard jam reading and go to testing for my most accurate load.

If you change bullets, the process starts all over again . Best of luck.

Thanks Willy's Guy but You read your own interpretation into my question and missed the point of it. The basic question asked was weather or not the bump amount was included in the bullet jump configuration.

 

[Steve Ladino]

The ejector and the firing pin pushes the case forward and removes any fore and aft case movement. All bullet length measurements are actually off the shoulder location and not your bump or head clearance.

With jam the base of the case is held against the bolt face, but the force of the firing pin hitting the primer can push the shoulder back. This is why the reloading manuals tell you to not use cases from reduced loads with full power loads. Meaning the case will get shorter each time the firing pin hit the primer and increase the chance of a case head separation.

Good lesson but not really an answer to my question as I stated it...thanks.

 

[Steve Ladino]

OP, you calculating headspace? I try not to FL size more than .002-.0025 but I also mostly have rounds that range from .007-.010 jammed.. I don't care what measurements happen after I pull the trigger, all my measurements are from load development and testing.. If it works I don't care..

Ray

No , not calculating headspace. Asking a question about compensation for any amount bumped as it relates to a bullets total jump distance.

 

[Steve Ladino]

I only shoot at targets that are up hill. Thus keeping the round back against the bolt. I get what your talking about. My Bighorn has control round feed and a mechanical ejector. I think this helps. Mike

Why would it be important to have an accurate jump or jam measurement? Put the zero somewhere you can replicate, and adjust from that.

In the end, we are all finding our seating depth experimentally.

One piece of advice: If you have to have really accurate measurements for it to shoot, it doesn't shoot. It isn't unusual for me to find bullet seating depths that allow some significant wiggle room.

I actually do prefer to be precise with all aspects of my reloading procedures and do get the best performance by doing so, and yes I agree we start from zero once its found if we are talking about the same zero, that is.

 

[Raythemanroe]

No , not calculating headspace. Asking a question about compensation for any amount bumped as it relates to a bullets total jump distance.

If you do the stripped bolt method (Wheeler) of seating depth measurement be sure to lightly hold the bolt rearward on the extraction so not to be getting a false reading because you are pushing it forward into the lands.. If you fall out of the chair do it four more times.. Find a measurement that repeats and record it, tune from there to what the gun wants.. Record that measurement and call it a day..

Ray