Seating Depth Pressure Change

[Bobr]

Started to get some bullets loaded for the Berger seat depth test of reducing the seating depth by .040-.080-.120 on a 6.5 load that was right at max pressure while being seated at the lands. According to Quick load by seating them back into the case the Pressure will be + max. My question is will the effect of moving the bullet back off the land and back into the case actually raise the pressure. I do not believe I have ever seen anywhere in Quick Load a place to specify bullet to land measurement but would think a jumped bullet would tend to have less pressure than one jammed. Am I missing something. As always thanks for all your help. Bob

 

[necchi]

but would think a jumped bullet would tend to have less pressure than one jammed. Am I missing something.

The bullet that's farther away from the lands, is also further into the neck and case right?

The more bullet in the case, the more it reduces case capacity,, that's where the pressure increase comes from.

If I was at max pressure already, I'd go a little slower with the jump,, more like 010 off then .020,,, then 030,,,

You might be surprised how much your groups will change and nowhere to go with the powder charge than down. I usually work the opposite than what you've done in this example.
I'll start at sammi OAL or a long jump, find the charge, then work towards the lands with the seating. If there's an improvement, I'll select the best seating and re-work the charge +/- in small increments

 

[Nomad47]

Hover over "Shot Start Pressure" on QL and a pop up note will appear saying to add 7200 psi for cases with bullets seated at or in the lands.
As Necchi points out, as you seat deeper, you decrease case capacity. And QL will indicate that as you change the COL dimension on QL. But the same load load in the lands will NOT be over pressure seated .120" off the lands if it was not over pressure when in the lands because of this 7200 psi difference.

 

[fullersson]

As necchi stated the futher back in the case you are, you have less case capacity. All this is true and it makes sense when you think about it, the only problem is that it doesn't work when you test it (at least for me). When I have a load that is showing pressure signs in the lands and I back off the lands I always show a drop in pressure. I've done this 4-5 times to check it because it just didn't seem right, but the results have been consistent. In my experience if a load is jammed into the lands you will develop more pressure than the same load not jammed. Brian Brown.

 

[deadlyswift]

I tend to believe that jumping or jamming can spike pressures. I always shoot over a chronograph when doing any load development. AS I change seating depths. I watch Vel go up and down. So to me as pressures go up or down so does vel. This is why we work in small increments. I always keep in mind that I'm searching for that pressure point that will put me in a particular node and noting any pressure signs. I have and do find pressure signs with jumping and jamming.

 

[Shynloco]

I also agree with what deadlyswift has said. Now I'm no "expert" and haven't been reloading as much as others. But I have also been taught, through reading and comments by long time reloaders, that "jamming" or gettinbg close thereto, will cause a rise in pressure. Now I understand the theory that necchi is saying that the closer the bullet in the case, the higher the pressure. But in practicality, I've never experienced a rise in pressure possibly because I never go deeper than the SAMMI spec for bullet depth (using the OAL measurement approach.) SO my variable or total distance for any bullet seating is the minimum (SAMMI spec) all the way to the jam. Because my some of my rifles are Savages that have that God aweful Sear Block Accurtrigger, I never can get much more than maybe .005 - .010 into the jam. So I always pay close to attention to ANY signs of pressure, while seeking the best accuracy in terms of performance. Thats because the last thing I want to see is any of my rifles blown up because the pressure has gone off the charts and the chamber is so pressurized that an explosive BOOM goes wild. Funny thing, is the only such incident I've ever seen happen (a friend's rifle at the range) was ironically a .223 bolt gun that had a load that was way to light and the absence of sufficient powder in the casing caused such as explosion that it literally blew the rifle into four pieces with the top strap looking like a banana peel. Lucky the shooter wasn't hurt, but his ego got hit petty hard, not to mention his wallet.

 

[Bobr]

Thank you all for taking the time to help out
Bob

 

[fdshuster]

I had the same seating depth/pressure experience years ago with a 223 Rem. Had been seating them with .020" of jump & decided to try touching. Kept everything else the same. With the first shot fired I had hard bolt lift & an ugly primer, not ruptured, but close. Stopped using those loads, seated them with .020" jump & all was once again normal.

So, seems to me that in certain instances seating depth alone can effect pressure. But, push the bullet back far enough into the case & it will also raise pressure because of the now smaller case capacity that has been created.

Gotta find that "ideal" seating depth for pressure and accuracy. Too much in either direction ( into the lands or into the case) will cause problems.

 

[Nomad47]

Once again, QuickLoad states that jamming causes an increase of 7200 PSI to the shot start pressure. So once you back off the lands, you can increase the charge a little.
Walt Berger (of Berger Bullets) says that as you seat deeper in the case you do decrease case capacity, but you increase the "jump" of the bullet. And during this "jump" some gas (pressure) can escape around the bullet. Walt says this pretty much balances things out. In other words, as you seat deeper in the case, case capacity decreases but jump increases.

 

[queen_stick]

Pressure increases/decreases work both ways...

For example, keeping the primer and powder charge the same, and jumping .020 as a starting point. If you seat the bullet to touch the lands, the pressure will increase. If you seat the bullet deeper, say .100 jump, then pressure will increase. Seating deeper causes a pressure increase due to reduced case capacity. Seating longer to touch the lands creates a pressure increase because the bullet doesn't have a "running start" into the lands.

This is why tuning a load can be a vicious cycle. Say you do a ladder test to find the 'sweet spot' for bullet seating depth at 1K yards. If you keep the powder charge the same for rounds that have a jump, and rounds that are jammed, then you've really created two different velocities (because of pressure differences between the jumped and jammed). If one shoots better than the other, then which aspect do you focus on? The bullet seating depth, or the increase/decrease in pressure?

With that said... some very respectable and successful BR shooters have told me that bullet seating depth is the number one factor in creating the most accurate load for your barrel.

 

[spclark]

With that said... some very respectable and successful BR shooters have told me that bullet seating depth is the number one factor in creating the most accurate load for your barrel.

I'd have to agree with that but would add some bullet profiles are more forgiving of seating depth than others.

Too, accurate loads can often be found for more than one charge weight. While looking for velocity "nodes" that offer good potential it's important to keep an eye on pressure signs: unexpected dimensional changes to cases, fired primer appearance, "sticky bolt lift', obvious noise changes when rounds are fired, etc.

Changing any one factor / component of a working load's recipe is usually worth a return to careful testing to learn what, if any, the change may impact.

 

[spclark]

...as you seat deeper in the case you do decrease case capacity, but you increase the "jump" of the bullet. And during this "jump" some gas (pressure) can escape around the bullet.

Logically this may be true but I'd also have to think any particular chamber's neck clearance would be a significant factor here, as would the amount of freebore and the length of bullet bearing surface being used.

Minimum neck clearance would be far more prone to pressure spikes while longer freebore used with shorter bullets might be more forgiving?

 

[SDDasher]

Wow, this question sure has raised some interesting comments/theories/suggestions without a definitive answer to the question of which is most beneficial. No wonder it takes so much time (read loading/shooting) to find the perfect load for your gun/barrel. I suppose it is an over simplification to say that accuracy will drive all of this in the end. The bullet jumped/jammed with the most accuracy wins eh?

 

[spclark]

The bullet jumped/jammed with the most accuracy wins eh?

Accuracy is what counts. Little point shooting at anything if you can't count on hitting it where you're aiming....

 

[Shynloco]

Wow, this question sure has raised some interesting comments/theories/suggestions without a definitive answer to the question of which is most beneficial. No wonder it takes so much time (read loading/shooting) to find the perfect load for your gun/barrel. I suppose it is an over simplification to say that accuracy will drive all of this in the end. The bullet jumped/jammed with the most accuracy wins eh?

IMHO, the "most definitive answer to the question is" you must test to find out which combination YOUR PARTICULAR rifle likes the best. From my readings by most noted BR experts is that best accuracy is determined by the rifle (its bullet seating depth) and chances are very good that IN MOST CASES, jammed bullets into the lands will results in best accuracy. Now from personal experience using two different rifles (made by different manufacturers) have resulted in best accuracy at opposite bullet seating depths. In other words, one loves the jam and the other hates the jam and loves the long jump. My 6mmbr loves the jam (if only .010) but my .308 hates it and loves the jump for best accuracy and grouping. So you cannot ASSUME that what works in one rifle will work equally as well in another, even if made by the same manufacturer and even same caliber. But when I test, the most common denominator I look for in the early stages is any signs of high pressure. A poor combination may lead to bad groups, but high pressure can cause REAL grief and damage to your rifle. Guess I'd rather opt to be safe than sorry, even though I might try out what someone on this blog says works in his rifle.

 

[necchi]

Well said Shynloco

 

[SDDasher]

" My 6mmbr loves the jam (if only .010) but my .308 hates it and loves the jump for best accuracy and grouping. "

Hence my comment that there is no "definitive" (read; universal) answer to the question of which is best; jumped or jammed. Most of us that have been reloading for a while understand every barrel has a favorite load including jumping/jamming. It is interesting that if one believes that bullet seating into the case and how it addresses the lands is "critical" to accuracy there is not much info out there on which is better one way or the other sans, go out and half shoot out your barrel to find out.
As a 6.5x.284 shooter as well as 6BR it would be very helpful to be able to work up a load with all the parameters including different bullets, powders,primers,brass,etc, etc to arrive at the optimal load to keep the rounds down range count to a reasonable number. To me that would be the holy grail of bullet info. Just saying.......

 

[Hedge]

Saw a graph, somewhere, where they tested seating depth vs. pressure in a 30-06. As the depth increased, the pressure dropped off...to a point. Then, it rose again. Seemed to be a lot of room for depth before the pressure started going up.

 

[spclark]

If you think about it a little I'd have to believe seating bullets deeper into larger cases will have relatively less effect than in smaller cases. 30-06 ought to be more forgiving than, say 6BR or 223.

 

[Hedge]

Rgr, that. I'm thinking the trend would be the same but with less leeway in the smaller rounds. Especially if your powder charge is close to case capacity.