OAL vs FPS vs PSI..

[jac74]

I have a SPS tac 308, all that is stock on it is the barrel and action. Everthing else has been changed.
I am loading 175gr SMK, 44.0 gr IMR 4064 powder, CCI BR2 primers, Winchester brass,2.80" OAL. With this load I get 2600 FPS out of my gun, but when the temps here in Fla reach 90+ I start to see slight pressure signs of soot on my primers.This load is very accurate .5 MOA if I do my part.
I loaded up a batch of the same load I listed above but the OAL was 2.925", in order for the bullet to touch the lands 2.935".This load to my supprise was easier to shoot accuratelly but my FPS were 2490 avg, so getting out to 1000 yards is out of the question.
My question is:Inorder for me to get this round back up to 2600 FPS I need more powder, but am I going to see the same pressure signs when I try to get to 2600 FPS? I am loading the bullet out farther, will the pressure needed to get this round up to "speed" be the same or does this change the entire characteristics of this load when it is related to pressure and the signs of over pressure?

 

[McCrazy]

Re: OAL vs FPS vs PSI..

I'm not a reloading guru so I may get corrected here but I will try to answer your question.

As far as I know you will have to add more of the same powder in order to increase the pressure back to what it was when you were getting the velocities you achieved earlier. This will probably result in the pressure signs you saw at those velocities and conditions however.

Have you considered using a different powder with a slower burn rate? This can allow you to achieve the same velocity but have a slower pressure increase and a lower peak pressure.

HTH

 

[ChrisGarrett]

Re: OAL vs FPS vs PSI..

You have a few issues going on. By getting closer to the lands, you can, in theory, increase pressure, as the bullet has no room to jump, allowing that gap to release some 'gas pressure'.

The flip side to the above is that you have more room in the case for that gas pressure to develop and this, in essence, lowers pressure a bit.

You have something akin to a 'proportional valve' and saying that the increase is going to be 26%, but more internal room in the case is going to knock that back 22%, is kind of hard to guesstimate.

I've clocked rounds where they were mag length and then seated quite long and didn't see the swing that you saw...110 fps, but all things are not equal.

I would seat long and trickle up from a minimum, or just above, looking for pressure signs and let the ammo tell you what's happening.

Trying other powders is always a safe bet...IMR 4895, RL-15, VV-150 and Varget all worthy substitutes with the 175s and 308s in general.

Chris

 

[Brain]

Re: OAL vs FPS vs PSI..

You might also look to the chronograph as a possible source of error. Unless you have confirmed the velocities under varying conditions or through data at distance, I'd be skeptical of 110fps reduction.

 

[MitchAlsup]

Re: OAL vs FPS vs PSI..

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: jac74</div><div class="ubbcode-body"> My question is:Inorder for me to get this round back up to 2600 FPS I need more powder, but am I going to see the same pressure signs when I try to get to 2600 FPS? I am loading the bullet out farther, will the pressure needed to get this round up to "speed" be the same or does this change the entire characteristics of this load when it is related to pressure and the signs of over pressure? </div></div>

By moving the bullet forward, you have increased the volume behind the bullet and thus altered (lowered) the expansion ratio (analagous to the compression ration in an automobile engine). As this part of the pressure regimine is adiabatic (both volume and pressure are changing) there is no linear way to get from point A to point B.

So, you start where you are, walk up until you see pressure signs, drop back and look for the accuracy node.

The pressure does not start to rise until the bullet is closer than 0.005 to the lands, so stick at 0.010 to 0.015 for jump distance.

My guess is that you will see 2600 fps with just a touch less pressure at 2.925 than when seated to 2.800. But do run the experiment.

 

[Fuzzball]

Re: OAL vs FPS vs PSI..

First, leaking primers indicates more than slightly elevated pressures.

Unlike with handguns, seating deeper in rifles tends to lower peak pressures because it allows the bullet to get a running start for engraving the rifling. Actual velocity is much less dependant on peak pressure than it is on the timeressure curve.

Try a less temp sensitive powder in the same burn range, Varget is very close to IMR 4064 and it measures well too. Try maybe 30 thou off the lands and see if you can't ease the charge up enough to get back to your original speed safely.