Is a jam a jam?

[JPickett]

Been reloading for going on 20 years myself but I appear to have come up with a question I can’t quit figure an answer for. Maybe some of you guys have some insights

So is a jamb a jamb regardless of how much?

Here’s why I ask. Worked up a load a while back and must have had my gauge off when I was measuring the lands. Ended up with a great load but turns out I had like 30 thou jamb if not a bit more. Bolt closed, worked great, no pressure signs but I was way deeper in the lands then I thought. So backed it off to 5 thou and zero difference. Shoots the same, ES, FPS, SD whatever you wanna track.

So, got me thinking. If it’s Jammed, already on the lands. Does it change a thing as far as performance goes wether it’s 50 thou or 5? My guess is it could build more pressure but even that isn’t quite penciling out in my mind.

What say you? @Zeke ? You know?

 

[El_Matador]

A jamb goes around your door. A jam goes on toast, between your toes, or into your rifling. I've played with different amounts of jam into the lands and I've determined that the answer to your question is: It depends. If a bullet has a fairly hard jacket and it's shape matches the shape of your chamber's leade then the bullet will stop rather abruptly when it hits the rifling. It will likely seat itself deeper into the case when you close your bolt, so that .050" jam becomes a .010" jam before you fire. If the bullet is soft and/or does not fit the shape of your leade then it will continue to engrave itself further into the rifling before it overcomes your neck tension. In this scenario you may notice a bigger difference between jams of varying amounts. One test I like to do when playing with jam is to seat a bullet long and then chamber it. Hopefully the bullet will not stick in the barrel. If you can successfully extract it then you can measure and determine the maximum jam measurement.

 

[JPickett]

I use a hornady gauge and usually I’m pretty accurate with them. What I did to miss measure the lands the first time I don’t know. But I do know I could extract a round and my over all length ( measured to ogive) would stay the same. Was some marking in the bullet but not grooved if that makes sense. Do run a little tighter neck tension as I use a bushing die with no extractor button. Guess with a jam that’s probably not necessary but most of my loads are off the lands. This one was just too good jammed

So the only practical experience I have with goin from deeper to shallower is it didn’t make a difference . But I don’t know if that’s a proven concept

 

[JPickett]

A jamb goes around your door

Good catch. Carpenter by trade, guess it’s just stuck in my head that way

 

[El_Matador]

Sounds like you have a nice, stable load that is not reacting much to your change in seating depth. I have done seating tests with different loads touching the lands, .005 jam, .010 jam, and "hard jam" which is using the rifling to finish seating my bullets. I do notice a difference most of the time. When you play with powder charges and seating depths you are just fine tuning the pressure curve. That affects the acceleration and timing of the bullet travel as it interacts with the barrel harmonics. If you're in a "node" of stable pressure and harmonics then seating depth won't make much difference compared to if you're outside that node. I'd be curious to hear other opinions, but in my experience a jam is not always the same as another jam.

 

[JPickett]

but in my experience a jam is not always the same as another jam

That is what my gut tells me, just didn’t see it. But increase in pressure would obviously make a difference. Just trying to wrap my head around if a bullets jammed 5 or 30 it’s touching the lands regardless so how much if a pressure increase is there really. I’ve done way more seating froth tests with jumps than jams. Obviously..,

 

[HikeHunt61]

Wondering- did you ever get a chance to measure one after closing the bolt (and then ejecting)? I ask because it's possible you were seating them deeper by closing it.

Not that it affects your question of a jam vs a jam

I've always struggled to get COAL measures to be consistent with my guage. Seems like as hard as I try, I'll get one measurement on a day, then a year later I reverify and its 15 thou different. Swear I use the same technique- a couple different bullets, same pressure and small tap- but it just somehow varies.

 

[JPickett]

I hear ya on that gauge being finicky. I don’t know if it’s bullet dependent but I’ve had the same experience once then real consistent with another rifle.

I did measure chambered cases and was maintaining the coal pretty well. But, not like I measured 100 rounds after chambering so it may have been pushing some back

 

[Bluehair]

I hate these old guy stories, but the way they did it back in the day was to load a long dummy round and use is as a gauge just like you did.

You would “smoke” the bullet with a candle, and seat it to the point the lands didn’t leave a mark in the soot on the bullet.

I vaguely remember doing it, but honestly I never found a rifle that a few thousandths seating depth made that much difference. I have no problem using a dummy round as a gauge.

But I can’t shoot for chit anymore either, so that makes things much easier these days. My only concern these days is whether it’s short enough to chamber

 

[JPickett]

The soot trick is still a good one

 

[HikeHunt61]

Dad taught me to use a magic marker- same idea. Would put the bullet in a case and slightly crimp enough to hold the bullet, which allowed to shift it further out until you started touching the lands.

 

[El_Matador]

I think the reason those gauges are so finicky is that you can push the bullet further into the lands with just a little extra finger pressure. The actual measurement doesn't matter much in my opinion. You're trying to find a sweet spot in the seating depth and it may be .005" off the lands or .010" into the lands. Or maybe its .040" off. The important part is finding your starting point and measuring consistently from that [somewhat arbitrary] length. The tools that measure CBTO are pretty dang accurate and repeatable.
Erik Cortina is a 1000 yd benchrest shooter and here is his method: After measuring approximate distance to lands, make a dummy round thats .050" longer. Apply sizing wax to the bullet and chamber the round. Now measure it again, and it should be shorter. If not, try again with a longer dummy. If it comes out shorter this is your new "jam" measurement. Start your seating depth test .020" shorter than this (still touching the lands in most cases). Shoot groups in .003" increments (-.020, -.023, -.026, etc.). According to Erik the seating depth nodes are only about .006" wide, so if you're testing every .010" then you can easily miss a node. If your magazine length doesn't allow you to touch the lands then just start at magazine length -.020".

Seating depth is definitely a fine-tuning technique. I use it only after finding a good powder node.

 

[HikeHunt61]

Ya- it seems that the bullets these days have longer ogives which certainly challenges the gauges. Clearly, if you find a length that works- you got it. My only concern has been that I may be into the lands by 10-15 thou and in a hunting situation, with temps very different from my testing environment, I might get a bullet stuck when I back one out upon return to vehicle or camp.