Seating depth

[LD375]

Hi guys, this may sound like a stupid question, as I do not reload
I've tried 2 different factory loads with the 300gr Swift A-Frame out of my .375 H&H, and both shoot very very well out of my Sako rifle, POI is very close with both loads.
Here's my question, on the Norma loaded ammo, the bullet is seated deeper into the case vs the Remington loaded ammo, what effect does this have in regards to velocity, accuracy etc?

Thanks in advance :grin:

Lou

 

[woods]

All else being equal, seating deeper will decrease velocity and pressure. It's effects on accuracy will vary, may be more accurate or less accurate. Depends upon how your rifle likes that seating depth.

 

[DrMike]

If POI is similar and each shoots similar, velocities are likely similar. This could be the result of differing wall thickness, allowing for more powder even though the bullet is seated deeper. It is quite likely that different powders are used so that you obtain similar pressures with attendant velocities being similar. Woods is correct, however, that all else being equal, seating deeper (intruding into the powder column) will decrease velocity.

 

[LD375]

If POI is similar and each shoots similar, velocities are likely similar. This could be the result of differing wall thickness, allowing for more powder even though the bullet is seated deeper. It is quite likely that different powders are used so that you obtain similar pressures with attendant velocities being similar. Woods is correct, however, that all else being equal, seating deeper (intruding into the powder column) will decrease velocity.

Thanks woods, and Mike ..makes sence, the only similar thing in the 2 loads is the bullet, different primers, brass and powders.If I remember correctly, the Remington load shot .5" higher and .5" to the left vs the Norma load.

The Norma load is advertised at 29fps faster at the muzzle.
I well have to chrony them, as I am just curious, thanks again,
Lou

 

[kraky]

I'll swim against the tide and say that seating deeper may cause pressure to go up...or down depending on the throat of the rifle. In my book seating deeper w/a short or normal throat will make pressure go down. A generous throat can actually raise pressure by seating deeper. Either way it would be most likely very little change.
Point of impact changes may be due to velocity diffs...but in most cases is simply barrel harmonics at work.
Working that chrono will shed more light for sure.

 

[ElmerThud]

Here's my question, on the Norma loaded ammo, the bullet is seated deeper into the case vs the Remington loaded ammo, what effect does this have in regards to velocity, accuracy etc? Thanks in advance :grin: Lou

Presumably you're talking of standard spec. factory ammo. so both manufacturers will be producing their ammo to SAAMI spec. It seems to me unlikely that each will have identical MV and so there will be differences in performance.
How the performance changes may be dependent on many things, not least the difference in bearing surface of the bullets, propellant, case dimensions etc etc etc.
As to accuracy, just as Kraky says, it's down to barrel harmonics and which ammo the barrel works best with. Knowing the MV of the ammo you finally end up using is very useful to determine downrange accuracy...it'll also tell you just how much consistency of velocity and give you a clue as to how much consistent accuracy to expect from that 'batch' of ammo.
Pick the ammo that works best & check it out with your Chrono.

Best of luck. ATB

 

[RiverRider]

I'll swim against the tide and say that seating deeper may cause pressure to go up...or down depending on the throat of the rifle. In my book seating deeper w/a short or normal throat will make pressure go down. A generous throat can actually raise pressure by seating deeper. Either way it would be most likely very little change.
Point of impact changes may be due to velocity diffs...but in most cases is simply barrel harmonics at work.
Working that chrono will shed more light for sure.

Kraky is correct.

There are two different things that come into play when seating depth is considered as a pressure factor.

In terms of just the cartridge, seating more deeply will INCREASE pressure. If we shot smoothbores, then this would be evident.

However, we don't shoot smoothbores and proximity to the lands does come into play and because we tend to try and get near the lands if possible, we see pressures increase with longer OALs.

 

[steve4102]

I'll swim against the tide and say that seating deeper may cause pressure to go up...or down depending on the throat of the rifle. In my book seating deeper w/a short or normal throat will make pressure go down. A generous throat can actually raise pressure by seating deeper. Either way it would be most likely very little change.
Point of impact changes may be due to velocity diffs...but in most cases is simply barrel harmonics at work.
Working that chrono will shed more light for sure.

Long throat or short throat, seating deeper in a bottle necked rifle round will reduce pressure.

From John B of Handloader/Rifle Magazine.

Seating deeper decreases peak pressure, for two reasons. The longer "jump" of the bullet to the rifling results in a lower peak pressure, since the bullet engraves more easily the faster it's going when it hits the rifling.

Also involved is the "progressive" burning of almost all modern rifle powders. This means the pressure increases relatively slowly from the time of ignition. Thus peak pressure occurs when the bullet beyond the barrel throat, with very slow-burning powders as much as 3-4 inches.

Charts. As these charts will show, it doesn't matter if the rifle has a long throat or a short throat, it is the distance from the lands that matters. They do not list OAL, they list distance from or to.

 

[RiverRider]

Well, there it is.

I think what John B is saying is essentially what I am saying. We normally work close enough to the lands that the effect of the leade is far more prominent than the effect of the OAL.

 

[ColColt]

Long throat or short throat, seating deeper in a bottle necked rifle round will reduce pressure.

Seems to be a Catch 22 situation in that seating a bullet deeper in the case can also raise pressure from all I've ever read and understood.

 

[RiverRider]

And you are CORRECT. As I've tried to explain, in terms of the cartridge and JUST the cartridge the deeper seated bullet will raise pressure. This does not take into account the effect of the leade. When the leade is factored in the opposite take place. Seating nearer the lands increases pressure because the lands tend to restrict bullet movement to some extent. In that case the overall net pressure is not increased because the case volume under the bullet has increased, it is solely because of the way the leade impedes bullet movement and whatever decreases in pressure the longer OAL tends to cause are vastly overridden by the increases in pressure the engagement with the lands cause. It is TWO TOTALLY DIFFERENT MECHANISMS affecting pressure, and in opposite directions.

 

[steve4102]

Well, there it is.

I think what John B is saying is essentially what I am saying. We normally work close enough to the lands that the effect of the leade is far more prominent than the effect of the OAL.

If you look at the data closely you will see that the pressure did not start to rise until the bullet was seated over .250 off the lands, that enormous, I have no rifles that could be seated that deep.

Also notice that even with the bullet seated .450 (almost a half and inch) deeper into the case the pressure never reached or exceeded the pressures reached when seated .100 or less off the lands.

According to some that have viewed this chart the increase in pressure after seating the bullet over a 1/4 of an inch deeper into the case is not because of reduced case capacity, but the result of extreme compression and distortion of the powder, not case capacity

 

[RiverRider]

I agree with all but the last statement.

 

[ColColt]

When I consider pressure I always think of too much powder or the wrong powder and/or bullets seated into the lands. If you drop a line from the chart at the top left at the peak 64K PSI down you're right at a bit less than .050" from the lands. I normally seat a bullet .015-.030" off the lands-always have and use whichever gives the best accuracy.

Now, you're somewhere around 58,800 PSI at .250" and that just doesn't make sense to me. It's like the deeper you seat a bullet the less the pressure-something that goes against everything I've read over the years. I know of fellows that load for the 45 ACP(me included) and have had some problems when they didn't taper crimp enough and the bullet got shoved back into the case once it hit the feed ramp. I just don't follow that chart's logic.

 

[woods]

There could also be another explanation for the steady increase in pressure after .250". The bullet could be slowing down after the initial pop to the lands and at approx. .500" would lose momentum and have to start again similar to being seated against the lands. That would more readily explain the opposite mirror image of the data comparing 0" to .250" and .250" to approx. .500"

It takes so little pressure to expand brass and push the bullet forward as to be very near negligible.

 

[RiverRider]

The bullet that encounters the lands from .250" from the lands will have less velocity at that time and therefore less momentum than the one that starts out .500" from the lands.

It's pretty universally understood and accepted that you don't want bullets getting lodged deeper into your brass when they hit the feedramp in a semiauto, whether rifle or pistol---but especially small cartridges like the 9mm. The result is skyrocketing pressure.

I really don't understand why it's so damned difficult to grasp the concept of two different effects at work here.

 

[woods]

Nobody is having difficulty grasping anything here. The bar chart and the graph clearly state it is in a 30-06. The pressure trace is in a 6PPC which is still a bottle necked rifle case.

Certainly neither I nor Steve is saying that the same principles apply to a pistol case and it may the situation that semiauto's hitting the feed ramp seat the bullet closer to the .500" mark.

It is not so much a question of how fast the bullet is traveling toward the lands if seated at the deeper depths so much as it is a question of - does the bullet slow down or come to a stop while waiting on pressure to build to push it down the barrel.

 

[RiverRider]

 

[woods]

Having trouble grasping? :grin:

 

[RiverRider]

It's not grasp that's the problem, it's more like acceptance. You can lead the horse to water, but if he doesn't understand the water's for drinking and won't partake, all you can do is drown his ass or let him die of dehydration. Whatever. I can walk.