The UK's Finest Independent Forum for Shooters and Gun Owners. Est 2010.

The whole issue of bullet seating depth has always had me puzzled as it seems to fly in the face of one of fundamental load development rules, only change on thing at a time so that you know what made the improvement or otherwise.

When we change the seating depth it is typically reported as about the amount of bullet jump to the lands and how much jump each load/rifle combination likes and is most accurate with.

However when we change seating depth we are also changing case volume and consequntly increasing or decreasing pressure/velocity, especially in cartridges loaded with a powder to or near case capacity and close to maximum pressure for that cartridge.

So how do we know whether it is the bullet jump distance effect on the bullet or the change in pressure/velocity that has given us our most accurate combination.

Is bullet jump/seating depth just another way of fine tuning our pressure/velocity?

Perhaps it doesn't matter so long as you get the best accuracy for the velocity you want, however enquiring minds must know.

I always play with seating depth dromia , when i do i am looking to drop my es's in velocity and try and to tighten my groups at distance , some bullet types prefer to jump some dont but depends on barrel/bullet combo. By experimenting you can find which do and which dont. Its just an advancement to load dev along with different powder weights and neck tensions.

So do I and so it is.

But my question is. Is the bullets preference the jump to the rifling that makes the difference or is it the change in case capacity and consequently pressure/velocity? Both are effected when we adjust the bullet seating depth.

I see what you mean now.

In theory if we seat the bullet longer with the same charge weight, internal case presssure will drop. I also think that the length of jump depends on the bearing suface. What i mean by that is, the bearing suface will still stay within contact of the neck walls longer whilst pressure builds and a longer lasting seal is made behind the bullet to aid the pressure curve to build. This is wh we get our custom chambers throated specificlyy for certain bullet types to aid the jump/pressure curve dilemma.


personally i think its the bullets bearing suface contact in this area that makes the biggest difference to seating depth with realtion to the pressure curve for the above reasons.

dromia wrote:So do I and so it is.

But my question is. Is the bullets preference the jump to the rifling that makes the difference or is it the change in case capacity and consequently pressure/velocity? Both are effected when we adjust the bullet seating depth.

I guess if you really want to get under the skin of this rather than just accept a good end result you could (down) load a standard COL round to match the Chronographed velocity of one which has an extended COL to minimise jump and see which gives best accuracy?

I am asuming here that extending the COL and therefore increasing the case capacity will reduce pressure a tad for a given powder charge.... unless you are jamming into lands that is.

I am just happy with the end result be it minimised jump or actual load value and would normally start with a good accurate load and then fiddle with COL/Jump to see if I could better it.

spud wrote:I see what you mean now.

In theory if we seat the bullet longer with the same charge weight, internal case presssure will drop. I also think that the length of jump depends on the bearing suface. What i mean by that is, the bearing suface will still stay within contact of the neck walls longer whilst pressure builds and a longer lasting seal is made behind the bullet to aid the pressure curve to build. This is wh we get our custom chambers throated specificlyy for certain bullet types to aid the jump/pressure curve dilemma.


personally i think its the bullets bearing suface contact in this area that makes the biggest difference to seating depth with realtion to the pressure curve for the above reasons.

If so and I have no idea how much bullet retention varies with the amount of bearing surface there is with the case, although with cast boolits and fast rifle powder at around 50% case capacity or under there is a minumum amount of bearing surface needed to give optimum burning of the powder, once this is reached then any extra bullet retention/bearing surface makes no difference. I think we might be surprised at how little of the bullet you may need inside your sized case to get optimium powder burn.

But if extra bearing surface beyond that needed for good powder burning does have an effect then that is a third variable that happens to the load when you adjust COL.

Well I had thought that there might have been thoughts on this from more than just spud, dave and myself. With all you long range, gnats gonads shooting hand loaders being out there.

Or is it just too arcane a subject for Full-Bore UKers?

No idea how I missed this thread

Pressures spikes... I think the optimal seating depth and accordingly the jump/jam run on a rifle is as much about minimising the pressure spikes as anything and I do agree the length of bearing surface is going to be key to the results you see. Internal ballistics are hellishly complex with so many variables and I would be as interested in the impact of varying case fill capacities which are a product of the bullet seated depth.

Getting an accurate measurement as a base is the key thing and casting the chamber is a great aid for this to understand what you are working with, also I build my comparators to as close to the measurement of the lands as possible. This means once I have determined the maximum OAL when just touching the lands I can measure this distance accurately and work back from it.

Hmm... I am struggling to explain this, sadly my brain is a bit fried this morning from a cocktail of Anti Virals and Co-codamol, with luck I will wake up properly in a while and come up with something more meaningful.

This change in seating depth can definitley take a good load out of the node. If I am experimenting between jam and jump, I cronograph the loads if possible. With my 284, I had to add .5-.6gn to keep the same speed when switching to bergers jammed versus jumped. If I am making .010" changes in bullets with a bit of jump, or anywhere between .010" and .020" jam with VLDs, the speed doesnt change much, but changing from jammed to jumped, always needs more powder to keep in the node.

RDavies wrote:This change in seating depth can definitley take a good load out of the node. If I am experimenting between jam and jump, I cronograph the loads if possible. With my 284, I had to add .5-.6gn to keep the same speed when switching to bergers jammed versus jumped. If I am making .010" changes in bullets with a bit of jump, or anywhere between .010" and .020" jam with VLDs, the speed doesnt change much, but changing from jammed to jumped, always needs more powder to keep in the node.

I always thought that you had to REDUCE a load when Jamming due to high start pressures IE no 'run-up' as when jumping a little.