U-die effects on pressure

[Sarge]

I am at a crossroads with my newly aquired U-die for 9mm minor. I am only currently using the U-die on certain brands of cases. I am also not sizing all the way down because they fit fine in my 34's chamber and I am primarily using the die to increase neck tension. I posted a while back that some brass was prone to very slight setback if I pushed hard enough.

The die works perfectly in the capacity I am using it. My question is, since it sizes the brass a little smaller (one 1000th?) will that effect capacity or pressure enough to potentially effect velocity and therefore PF?

I am trying to figure out if I should just use it for everything since I am in the process of developing a new load with n320 and would like to take as many variables as possible out of the formula.

If this is a dumb question feel free to answer with, "Dumb Question". I can take it.

[Jman]

My question is, since it sizes the brass a little smaller (one 1000th?) will that effect capacity or pressure enough to potentially effect velocity and therefore PF?

Highly unlikely Sarge. The amount of "fuel" + or - will dictate pressure levels. Well, primarily at least.

Jim

[CocoBolo]

Sarge - If you are looking for consistancy then be consistent. Reducing case volume could potentially increase both pressure and velocity, but in a 9 minor its not going to cause any real issues.

I am a little concerned about pressure when putting 10gr of AA#7 in a 9 mm and compressing the powder to get the bullet in, but they shoot fine. I don't however give them a second time to fail.

[XRe]

Yes, you're reducing case volume slightly, and that will have some effect - it doesn't seem like it should be super huge, though. However - you're also increasing bullet pull (that's the tension on the bullet that cases it to resist moving while it's in the case). That will also affect pressure - pressure will spike higher before the volume in the system begins to change (ie, before the bullet starts moving).

So - yes, you should be cautious when starting to use it, and the stock answer would be to back off your load 10% and work it back up to be sure you don't have any issues. But, barring that, you won't encounter any safety issues with one in any of your loads.

I developed all my .40 loads using a U-die, so I can't give you any real idea how much the effect might be from personal experience... (ie, I didn't use a regular die first, and then switch)

[G-ManBart]

Nerd alert...yes, I have done a comparison on this very topic

I ran off a batch of .38SC using the U-die for some and a standard Dillon die for others. Other than that, they were loaded sequentially using my old, tired, practice brass (wanted the worst case scenario). I shot one string right after the other on 8/20/08, 75*f, slightly overcast weather. I ran 15 rounds of each over the chrono.

The rounds loaded using the Dillon die gave me an average of 1,471fps.

The rounds loaded with the U-die gave me an average of 1,470fps.

Granted, that's only one load in one gun, but I was actually surprised a little. I expected the additional bullet pull to maybe help the U-die rounds have a bit more complete burn with such a slow powder (N105). With essentially identical results I've pretty much discounted it as a significant factor. Some day I'll try it again with new or once-fired brass and see if that makes any difference, but I'm not worrying about it. R,

Edit to add: I think this is proof that no matter how odd a question you might have, somebody has probably tested it already!

Edited May 15, 2010 by G-ManBart

[G-ManBart]

Yes, you're reducing case volume slightly, and that will have some effect

I'm not being a wiseguy here, but are you sure? Wouldn't squeezing the case down more make the case longer? The material has to go somewhere...if you squeeze it, it should get longer, and keep the volume the same, just in a different place. If the OAL would wind up identical with either I'd expect a smaller case volume, but it seems that case length slightly changes OAL settings, so it might all even out in the end...just a thought. R,

[Sarge]

Thanks Bart. I knew if anyone did a test it would be you!

To Coco and the others, Thanks as well. I am sure what I am loading to is safe. My main concern was, if I want to stay as consistant as possible will I see a difference switching back and forth from u die to dillon die. So far it looks like the effects are minor to say the least.

Thanks again all!

[XRe]

but it seems that case length slightly changes OAL settings, so it might all even out in the end...just a thought. R,

I've never seen that happen on my press (that is, OAL changing when case length changes... cause case lengths do change over the lifetime of a case, as you indicate). I suppose it could, but it'd be very counterintuitive - and I haven't observed any change in OAL here that couldn't be attributed to user error (ie, die was loose... or I was using MG 121gr IFPs...).

So... I guess I'd have to see that in action and see it proven that case length was adjusting seating depth somehow to believe it?

BTW - I'm not super surprised that N105 would show no difference between the two, actually. The rise in pressure on that powder is so slow, it hasn't peaked until the bullet is basically out of the case anyway, if I'm remembering correctly... I would expect that, if a difference were able to be seen, it would be more pronounced with a faster powder. However, the difference may be so small that we couldn't detect it without more sensitive pressure monitoring gear or something like that - it might not show up at the chrono at all, either way... Still, I'd be cautious, either way, if I were messing with a fast powder... Just my opinion, of course

Edited May 15, 2010 by XRe

[G-ManBart]

but it seems that case length slightly changes OAL settings, so it might all even out in the end...just a thought. R,

I've never seen that happen on my press (that is, OAL changing when case length changes... cause case lengths do change over the lifetime of a case, as you indicate). I suppose it could, but it'd be very counterintuitive - and I haven't observed any change in OAL here that couldn't be attributed to user error (ie, die was loose... or I was using MG 121gr IFPs...).

So... I guess I'd have to see that in action and see it proven that case length was adjusting seating depth somehow to believe it?

BTW - I'm not super surprised that N105 would show no difference between the two, actually. The rise in pressure on that powder is so slow, it hasn't peaked until the bullet is basically out of the case anyway, if I'm remembering correctly... I would expect that, if a difference were able to be seen, it would be more pronounced with a faster powder. However, the difference may be so small that we couldn't detect it without more sensitive pressure monitoring gear or something like that - it might not show up at the chrono at all, either way... Still, I'd be cautious, either way, if I were messing with a fast powder... Just my opinion, of course

Yeah, I only asked because what I've seen I haven't gotten around to really sorting through yet. When I load with new brass I get a resulting OAL that is .05" shorter than when I load my old practice cases. It's pretty consistent. I leave the sizing die in place for both, so that doesn't change and everything else stays the same.. I would have thought it would work the other way...new brass being slightly thicker at the mouth would resist the bullet more and make a tiny change in the OAl. Then again, it could be that the used brass is more brittle, having work hardened somewhat, and it actually resists the bullet better, or flares just a tiny bit differently. Heck, it could be something entirely different, but I know I've gotten that same difference over time

You're probably right about the fact that N105 is so slow that it probably lessens the impact of bullet pull, but I did see a slight (15fps or so) increase from FSPM to FSR primers and thought maybe it was due to a more complete burn...of course it just could be the extra oomph that the FSR primers give the bullet.

Either way, it's not a bad idea to drop 10% and work back up with any sort of change as you suggested...and probably moreso with the fast powders as they'll go over the edge pretty quickly in certain combos. R,

[XRe]

When I load with new brass I get a resulting OAL that is .05" shorter than when I load my old practice cases. It's pretty consistent.

What's even weirder about that is that the fired cases should be shorter than the new cases - usually by a few thou. At least, that's been my experience with the measurements. After the first firing, it seems to stabilize, and I don't see cases growing or shrinking over time - at least, not enough to really measure any kind of real trend on. I got bored after tracking some cases for several firings with nothing changing - just a big PITA to take all of those measurements... I didn't think about that earlier, but it's an interesting point.

I leave the sizing die in place for both, so that doesn't change and everything else stays the same.. I would have thought it would work the other way...new brass being slightly thicker at the mouth would resist the bullet more and make a tiny change in the OAl. Then again, it could be that the used brass is more brittle, having work hardened somewhat, and it actually resists the bullet better, or flares just a tiny bit differently. Heck, it could be something entirely different, but I know I've gotten that same difference over time

Brass of different lengths will definitely flare/expand differently - but, the expander and shoulder of the powder die should still end up at the same spot relative to the shell plate (and, hence, bottom of the case). That should mean that a bullet being seated to the same point will meet resistance at the same depth, but... ???

More germaine to the topic of this thread, though - barring the difference between new and once+ fired cases, I would imagine that your seat depth is the same between fired cases sized with the Dillon die, and fired cases sized with the U-die? New brass is kind of a strange beastie, in a way - it's never the same thing after that first firing. I can't explain that behavior you see, but what's more interesting to me is what happens after that first firing - and, in this case, the difference between the two sizing dies....

You're probably right about the fact that N105 is so slow that it probably lessens the impact of bullet pull, but I did see a slight (15fps or so) increase from FSPM to FSR primers and thought maybe it was due to a more complete burn...of course it just could be the extra oomph that the FSR primers give the bullet.

Now you're talking a different variable, though... I haven't compared FSPM or FSR to WSPM and WSR over the chrono, yet, so I don't know what to expect out of them, that way - wouldn't surprise me, though, if the FSR had a little more flame to it, which would imply quicker and more complete burn to the powder charge... I'm beginning to think that FSRs might be a tad hotter than WSRs, at least judging by the gunk in the gun - I seem to have a little more unburned junk with WSRs...

[G-ManBart]

Brass of different lengths will definitely flare/expand differently - but, the expander and shoulder of the powder die should still end up at the same spot relative to the shell plate (and, hence, bottom of the case). That should mean that a bullet being seated to the same point will meet resistance at the same depth, but... ???

Yeah, that's what I would think too...which is exactly why I asked...just doesn't make sense!

More germaine to the topic of this thread, though - barring the difference between new and once+ fired cases, I would imagine that your seat depth is the same between fired cases sized with the Dillon die, and fired cases sized with the U-die? New brass is kind of a strange beastie, in a way - it's never the same thing after that first firing. I can't explain that behavior you see, but what's more interesting to me is what happens after that first firing - and, in this case, the difference between the two sizing dies....

I didn't notice a difference in OAL between the two sizing dies and I don't recall a difference between once-fired and multi-fired. I haven't loaded any once-fired in a while, but I've got a lot of it accumulated (like 6K) so I'll start using it soon and will try to remember to compare.

Now you're talking a different variable, though... I haven't compared FSPM or FSR to WSPM and WSR over the chrono, yet, so I don't know what to expect out of them, that way - wouldn't surprise me, though, if the FSR had a little more flame to it, which would imply quicker and more complete burn to the powder charge... I'm beginning to think that FSRs might be a tad hotter than WSRs, at least judging by the gunk in the gun - I seem to have a little more unburned junk with WSRs...

Oh great, one more thing for me to think about...LOL. I have been planning on some more comparisons between FSPM, FSR and WSR, but don't have any WSPM...I'll keep my eye out for them at the one place locally I could pick up just 1K since I've got lots of primers stocked up now. R,

Edited May 15, 2010 by G-ManBart