finding seating depth questions???

[186thFCo]

1) What is the furthest distance you can jump a .308 Win round into the grooves while still being safe?

2) How much do you change seating depth between each string of fire? I have heard of changing it .005 per string and I have heard that I should change it up to .02 per string

I have already determined what charge weight to use that gives me the best SD and is still safe to shoot if the round is jammed into the grooves. I am using 44.5gr of Varget CCI BR-2 primers and federal brass and 175gr SMK

 

[canezach]

I think I'm getting confused on your wording when you say "jump the bullet into the lands". Do you really mean jump the bullet, or are you asking about jamming the bullet, since that would be into the lands.

When you mention changing seating depths, I'm assuming you mean during testing. If so, then 0.005" is fine. Personally, I think 0.020" is a bit too much of a change and you'll likely still have to go back and do smaller increments anyway. Let's say you find 0.040" jump gives you good results, you'll likely test 0.030" or 0.035: to see how they perform

 

[lash]

1) There is no furthest distance to the lands unless maybe you are talking negative distances.

2) I assume that this question is regarding how big a change in jump per each set of test rounds. The answer, as is usual, is not necessarily cut and dried. I usually have a good idea up front roughly where a particular projectile will be comfortable and tend towards 0.005" changes. I have heard of those that use 0.020" changes when they are not sure where a bullet will perform best.

Choices have to be made based upon a particular bullet style, typically. Secant ogive style bullets seem to be more jump sensitive as a generality, so with them some research helps to get an idea where to start and then use smaller increments in your test loads.

Tangent ogive bullets are generally more seating depth forgiving, to a point. The 175 SMKs are a tangent ogive design.

 

[186thFCo]

I think I'm getting confused on your wording when you say "jump the bullet into the lands". Do you really mean jump the bullet, or are you asking about jamming the bullet, since that would be into the lands..

No I'm talking about jumping the bullet. My current load has the bullet seated .010 jammed into the grooves now. I'm trying to tighten up my groups now by changing my seating depth.

 

[JoeBUtah]

Not sure I understand your question: "... jump a round into the grooves" is a bit confusing to me. If you actually mean jump, I think that depends on your chamber dimensions. For example my .308 has such a long throat if I try to get close to the lands I won't have enough bullet in the case to hold the finished round together. The rule of thumb I've typically seen is s minimum of one bullet diameter into the case so .30 inches of bullet inside the case neck.

If you mean jam a bullet into the lands, I'll let someone else address that since I've never had the occasion to do so. All I do know is the more you jam, the better the likelihood of preassure spikes.

The second part of your question is a bit fuzzy to me as well. I don't think of seating a bullet in terms of depth, but rather in terms of length. And by length I mean cartridge base to ogive of bullet. If you are crowding the lans or jamming them, the ogive is where the jump or jam is.

 

[spife7980]

1) What is the furthest distance you can jump a .308 Win round into the grooves while still being safe?

Gonna try to break this down to its base level generalities for you to conceptualize in your mind but know that there are expectations to everything. You have your distance to the lands. Think of that as your zero mark on a number line. You can jump (negative numbers) or you can jam (positive numbers). If youre thinking about jumping causing pressure issues as you get the bullet further back from the lands you will only run into that once you start crushing powder generally as that can change the powders burn characteristics. Up until that point the further the jump should give you less pressure. Thats because there is a little space and some of that pressure is able escape around the bullet. If you started way back off of the lands and worked closer to the lands then your pressures will rise from that as there is less room for the gases to be able to escape around. Once you get to the lands (0) you can have pressure spikes easily as there is zero space for gas to escape, its starts as a sealed vessel. The further you push into the lands the higher your pressures will be. When jumping you have a bit of momentum going into the lands. Starting jammed in begins static and there is an uphill battle to get the bullet moving. An object at rest wants to stay at rest so it will take more energy to get it moving than an object that is already moving.

So I think you mostly understand this but I just want to make sure.

2) How much do you change seating depth between each string of fire? I have heard of changing it .005 per string and I have heard that I should change it up to .02 per string

I will typically start .015-.02 off the lands myself. I try to avoid the secant bullets as they have a sharper point where the ogive turns into the bearing surface, the tangents are a smoother more gradual transition. This means that a secant will be more particular where it is seated in relation to the lands than the tangent profiles which have are a bit more forgiving based on their construction. A smooth angle is smoother than a sharp angle and will thus slide in a bit more smoothly. Hybrids take the pointy sharp tips and then taper that back to portray a more tangent profile near the bearing surface. Many secant shooters like to be right at or just barely off the lands (or possibly jammed but this forum is more field oriented vs bench rest oriented where jamming is more common). Many tangent shooters find that the bullets like to be a bit further off of the lands. Thats not to say that a secant bullet cant shoot well from a bunch of jump or that a tangent bullet wont shoot well at or jammed into the lands. For a tangent which is more forgiving I think the .02 (or .01 as I do in my tests) increments are just fine. For the more persnickety secant profiles a smaller increment may provide a better result.

I have already determined what charge weight to use that gives me the best SD and is still safe to shoot if the round is jammed into the grooves. I am using 44.5gr of Varget CCI BR-2 primers and federal brass and 175gr SMK

If you know that your charge will safely shoot jammed in the lands then I would feel confident giving it any amount of jump but again in my initial disclaimer, thats not a given. Work it up carefully monitoring your pressure signs. Personally I try to stay away from jamming bullets because a grain of sand or just simply being jammed to far can leave a bullet stuck in your bore with powder pouring out of the case as you try to extract an unfired round. Just seems less flexible to me for any of the varying issues that might arise requiring the unloading of your rifle thus a practice I try to avoid.

Many people like to load close to the lands (.005-.015 off) but Ive found that a bunch of my bullets like a fair amount of jump and if they still shoot well I have zero issue giving it to them. Most of mine live somewhere between .03-.06 off the lands. I tested the 70 rdf (which my 223 didnt really like at all but I wanted to wring it out and test every possible combination) in .005 increments from at the lands to .025 off and it didnt really like any of them. So I went from .010-.080 off the lands in .015 increments and it really seemed to like being .050 off the lands. Still shot like crap but compared to all the other combinations it was leaps and bounds better.

 

[eicas]

I have never loaded jammed, but have always jumped. My best .308 load using the 175gr SMK and RE15 has a jump of .010 in my factory 700 chamber. Since you have already jammed (makes me nervous) you can safely start backing out using the same load. I'd try .010, .020, .040 for starts. Every gun is different. I also have a 175smk load that is a duplicate of the MK118 loading that I load magazine length but jumps a bit more. I dont recall the number off my head. You can keep jumping more, creating a smaller COAL in the process until you start compressing the charge with the bullet. Pressures tend to go back up again then.


Edit: The above post ^^^^ is much more helpful and in depth than I can type from the middle seat on airplane wifi over Kansas. He beat me to it with much greater info!

 

[Darkside-Six]

I usually push mine back in .01 increments until I get where I like them and then may go up .005 and down .005 from there to see what happens.


Sent from my iPhone using Tapatalk

 

[186thFCo]

lots of good stuff thanks for the help guys

 

[Jimgal62]

I always find the nodal points with a 0.02"jump.

Then I perform seating depths from 0.06" to touching the lands in 0.01"increments.

I do not jam my bullets into the lands due to excessive pressure.

 

[186thFCo]

ok one last thing. (hopefully this isn't to ridiculous)

When I first started shooting this rifle (the rifle was made by GAP) I was shooting factory 175gr FGM and it shot pretty well around .65 +/- MOA. I shot maybe 150 rounds of FGM through it before I started my load development. The first thing I did is I got my COAL to the ogive of my rifle using the method that is shown in the sticky here in the reloading section. My COAL to the tip of the bullet is 2.721, and to the ogive with the comparator gauge on is 3.129. (I'm using 175gr SMK's) I compared that to a round from one of the boxes of FGM I still had left and I found that the FGM was much longer. The COAL of those are measuring 3.231 with the comparator gauge, a full .102 JAMMED!!! into my lands. I thought that it was strange that my chamber seemed to be really short so thinking I might have made a mistake measuring somewhere, I repeated the process to find my chambers COAL to the ogive but came up the the same numbers (2.721").

I cant help but feel like something is off, weather it be the method I used to find OAL or what. I never had any signs of pressure firing the FGM and I never had any extraction issues with an unfired round even though its .1 jammed. I hope I'm making sense, but should I care that my chamber seems to be really short?

 

[Brux]

Did you have GAP chamber the rifle to a certain bullet? If not I would get with them on this situation.

 

[spife7980]

Hmm, not sure what exactly to say about that. It should be the same smk but I have a hard time seeing a full tenth difference just based on lot. Does the factory measure the same off of the ogive? Color one and see if it leaves marks, with that much jam it would be very apparent even without marker I'd imagine.
If so it is what it is.

edit: try this instead of jamming a bullet with a cleaning rod.

 

[186thFCo]

Did you have GAP chamber the rifle to a certain bullet? If not I would get with them on this situation.

They did not I'll have to call tomorrow

 

[JoeBUtah]

You say you jammed a round a tenth of an inch into the lands? And A) You were able to close the bolt and B) your gun didn't blow up when you touched it off? Wow....