Pressure and fps

[69gen1]

Does increasing oal closer to lands raise pressure or lower it and do you gain fps or lose or vice versa deeper in cartridge case raise or lower pressure and fps. I'm thinking closer to lands raises pressure

[Stumpkiller]

MANY variables in your question.

In my narrow and limited 45 years of reloading experience (that is, lots of reloading but of maybe only eight different straight-walled and bottleneck and one wildcat oddball) it has been my observation that the closer you get the bullet to engaging the lands the higher the pressure signs of the fired cases become. There is no practical way for a home/shop reloader to measure pressure. By the time you notice an increase in may be well into unsafe. I do know that playing last year with IMR 4451 a change from 0.020" jump to 0.010" went from OK to opened primer pockets and loose primers!

If you have a bullet seated deep into the case you can hold less of a given powder, but it may still not be compressed. Note also that some sharp-shouldered cartridges have higher pressure with very low loadings than much fuller cases. A concern in fire-forming some brass.

In my .260 Rem I have high S.D. bullets (0.264" x 1.395") with the base set well below the start of the shoulder. Just has to be to achieve good overall length. I like case filling powders and I've had great results with Reloader 19 being slightly compressed with my current pet load. Never a problem or concern with the 52 gr bullets I shoot in my .223 as they don't seat with the base below the neck.

[charlie b]

It is a curve, basically two curves.

When the bullet is deeper in the case it decreases the volume, increasing the pressure (in most cases). But, the bullet has more velocity as it enters the throat and lands so the force needed to engrave the bullet is decreased, meaning less pressure on that end.

As you increase OAL the pressure due to volume is decreased but the pressure necessary to engrave the bullet is raised. The last part of this is a steep part of the curve. When the bullet is seated to touch the lands it has zero velocity to 'assist' in engraving so the pressure will 'spike'.

The peak pressure may not make any difference in muzzle velocity, in fact one load with a higher peak pressure may have a lower muzzle velocity. It is an 'area under the curve' thing where a longer pressure curve can be better than a large pressure spike with a smaller 'tail'.

An added part of this is the ability of the bullet to maintain alignment as it leaves the case and enters the bore. If it has to travel too far there is the possibility of the bullet entering the bore 'crooked'. Which is why some will seat the bullet into the lands and adjust the powder charge to compensate for the increase in pressure. This can also be controlled somewhat by adjusting the diameter of the bullet, chamber neck dia and case neck thickness. Match rifles will do this which is why some folks will take pains to carefully neck turn their brass and have custom case forming dies made.

In the extreme case you can use separately loaded ammunition. Open breech, seat the bullet with a specific amount of pressure. Add case with powder. Close breech and fire. This takes specially designed cases, chambers and loading techniques.

The short answer is, do what many of us do and use the mfg OAL data for specific loads or measure the distance it takes for your bullet to reach the lands. A distance of .020" seems to be a 'standard' for most casual shooters (and is what I use).

One last comment on loading a bullet so it engraves the lands. There is a problem if you want to unload the rifle. Typically the bullet will stick in the throat as the case is extracted, leaving you with a rifle that cannot be fired until the bullet is removed (usually involving a cleaning rod shoved down from the muzzle).

[69gen1]

It is a curve, basically two curves.

When the bullet is deeper in the case it decreases the volume, increasing the pressure (in most cases). But, the bullet has more velocity as it enters the throat and lands so the force needed to engrave the bullet is decreased, meaning less pressure on that end.

As you increase OAL the pressure due to volume is decreased but the pressure necessary to engrave the bullet is raised. The last part of this is a steep part of the curve. When the bullet is seated to touch the lands it has zero velocity to 'assist' in engraving so the pressure will 'spike'.

The peak pressure may not make any difference in muzzle velocity, in fact one load with a higher peak pressure may have a lower muzzle velocity. It is an 'area under the curve' thing where a longer pressure curve can be better than a large pressure spike with a smaller 'tail'.

An added part of this is the ability of the bullet to maintain alignment as it leaves the case and enters the bore. If it has to travel too far there is the possibility of the bullet entering the bore 'crooked'. Which is why some will seat the bullet into the lands and adjust the powder charge to compensate for the increase in pressure. This can also be controlled somewhat by adjusting the diameter of the bullet, chamber neck dia and case neck thickness. Match rifles will do this which is why some folks will take pains to carefully neck turn their brass and have custom case forming dies made.

In the extreme case you can use separately loaded ammunition. Open breech, seat the bullet with a specific amount of pressure. Add case with powder. Close breech and fire. This takes specially designed cases, chambers and loading techniques.

The short answer is, do what many of us do and use the mfg OAL data for specific loads or measure the distance it takes for your bullet to reach the lands. A distance of .020" seems to be a 'standard' for most casual shooters (and is what I use).

One last comment on loading a bullet so it engraves the lands. There is a problem if you want to unload the rifle. Typically the bullet will stick in the throat as the case is extracted, leaving you with a rifle that cannot be fired until the bullet is removed (usually involving a cleaning rod shoved down from the muzzle).

I never was to excited about setting bullits in the lands unless it was by mistake most of the time I just use the mfg.data because I'm usely setting for a detach mag or internal and u only got so much room there to not hinder function. just was curious if a given charge let's say is 45 gr at mfg oal and is set closer to the lands if it raises or lowers pressure and fps or the oppsite... thanks for the input and can understand the spike pressure

[mikeinco]

too many variables to give an answer.
how close to max is the load, how close to the lands are you when you start.
when working on non mag fed target ammo, i start at the lands as that is going to be the high pressure point in a simple ladder test.
find a node or two. confirm and then test off the lands on .005 steps. if nothing shows up off the lands i will try into the lands, but back up the load.


I never was to excited about setting bullits in the lands unless it was by mistake most of the time I just use the mfg.data because I'm usely setting for a detach mag or internal and u only got so much room there to not hinder function. just was curious if a given charge let's say is 45 gr at mfg oal and is set closer to the lands if it raises or lowers pressure and fps or the oppsite... thanks for the input and can understand the spike pressure

[69gen1]

too many variables to give an answer.
how close to max is the load, how close to the lands are you when you start.
when working on non mag fed target ammo, i start at the lands as that is going to be the high pressure point in a simple ladder test.
find a node or two. confirm and then test off the lands on .005 steps. if nothing shows up off the lands i will try into the lands, but back up the load.

Thank you. I think your testing off the lands first is a interesting approach for sure . I dont know how I get myself asking these type questions.the good news is I usely find a
Good load with mfg oal. my 168 amax and is less than .5 at 100 when i do my part and when set to mfg oal is 22 thou from lands the 178hpbt I'm setting up now is .074

[Texas10]

One popular loading technique is to start load development with the bullet seated well into the lands, and work the charge weight. Once you've got the charge narrowed down, starting moving the bullet back to close up the groups.

[mikeinco]

define 'WELL INTO THE LANDS"
generic statements really suck in reloading.
when i say "touch" i mean just that, no jam i use a magnifying glass to inspect with.


One popular loading technique is to start load development with the bullet seated well into the lands, and work the charge weight. Once you've got the charge narrowed down, starting moving the bullet back to close up the groups.